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  • 1. Abazov, V. M.
    et al.
    Abbott, B.
    Abolins, M.
    Acharya, B. S.
    Adams, M.
    Adams, T.
    Aguilo, E.
    Ahn, S. H.
    Ahsan, M.
    Alexeev, G. D.
    Alkhazov, G.
    Alton, A.
    Alverson, G.
    Alves, G. A.
    Anastasoaie, M.
    Ancu, L. S.
    Andeen, T.
    Anderson, S.
    Andrieu, B.
    Anzelc, M. S.
    Arnoud, Y.
    Arov, M.
    Arthaud, M.
    Askew, A.
    Asman, B.
    Jesus, A. C. S. Assis
    Atramentov, O.
    Autermann, C.
    Avila, C.
    Ay, C.
    Badaud, F.
    Baden, A.
    Bagby, L.
    Baldin, B.
    Bandurin, D. V.
    Banerjee, S.
    Banerjee, P.
    Barberis, E.
    Barfuss, A. -F
    Bargassa, P.
    Baringer, P.
    Barreto, J.
    Bartlett, J. F.
    Bassler, U.
    Bauer, D.
    Beale, S.
    Bean, A.
    Begalli, M.
    Begel, M.
    Belanger-Champagne, C.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Astronomy and Space Physics.
    Bellantoni, L.
    Bellavance, A.
    Benitez, J. A.
    Beri, S. B.
    Bernardi, G.
    Bernhard, R.
    Bertram, I.
    Besancon, M.
    Beuselinck, R.
    Bezzubov, V. A.
    Bhat, P. C.
    Bhatnagar, V.
    Biscarat, C.
    Blazey, G.
    Blekman, F.
    Blessing, S.
    Bloch, D.
    Bloom, K.
    Boehnlein, A.
    Boline, D.
    Bolton, T. A.
    Borissov, G.
    Bose, T.
    Brandt, A.
    Brock, R.
    Brooijmans, G.
    Bross, A.
    Brown, D.
    Buchanan, N. J.
    Buchholz, D.
    Buehler, M.
    Buescher, V.
    Bunichev, V.
    Burdin, S.
    Burke, S.
    Burnett, T. H.
    Buszello, C. P.
    Butler, J. M.
    Calfayan, P.
    Calvet, S.
    Cammin, J.
    Carvalho, W.
    Casey, B. C. K.
    Cason, N. M.
    Castilla-Valdez, H.
    Chakrabarti, S.
    Chakraborty, D.
    Chan, K. M.
    Chan, K.
    Chandra, A.
    Charles, F.
    Cheu, E.
    Chevallier, F.
    Cho, D. K.
    Choi, S.
    Choudhary, B.
    Christofek, L.
    Christoudias, T.
    Cihangir, S.
    Claes, D.
    Coadou, Y.
    Cooke, M.
    Cooper, W. E.
    Corcoran, M.
    Couderc, F.
    Cousinou, M. -C
    Crepe-Renaudin, S.
    Cutts, D.
    Cwiok, M.
    da Motta, H.
    Das, A.
    Davies, G.
    De, K.
    de Jong, S. J.
    De la Cruz-Burelo, E.
    Martins, C. De Oliveira
    Degenhardt, J. D.
    Deliot, F.
    Demarteau, M.
    Demina, R.
    Denisov, D.
    Denisov, S. P.
    Desai, S.
    Diehl, H. T.
    Diesburg, M.
    Dominguez, A.
    Dong, H.
    Dudko, L. V.
    Duflot, L.
    Dugad, S. R.
    Duggan, D.
    Duperrin, A.
    Dyer, J.
    Dyshkant, A.
    Eads, M.
    Edmunds, D.
    Ellison, J.
    Elvira, V. D.
    Enari, Y.
    Eno, S.
    Ermolov, P.
    Evans, H.
    Evdokimov, A.
    Evdokimov, V. N.
    Ferapontov, A. V.
    Ferbel, T.
    Fiedler, F.
    Filthaut, F.
    Fisher, W.
    Fisk, H. E.
    Ford, M.
    Fortner, M.
    Fox, H.
    Fu, S.
    Fuess, S.
    Gadfort, T.
    Galea, C. F.
    Gallas, E.
    Galyaev, E.
    Garcia, C.
    Garcia-Bellido, A.
    Gavrilov, V.
    Gay, P.
    Geist, W.
    Gele, D.
    Gerber, C. E.
    Gershtein, Y.
    Gillberg, D.
    Ginther, G.
    Gollub, N.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Astronomy and Space Physics.
    Gomez, B.
    Goussiou, A.
    Grannis, P. D.
    Greenlee, H.
    Greenwood, Z. D.
    Gregores, E. M.
    Grenier, G.
    Gris, Ph.
    Grivaz, J. -F
    Grohsjean, A.
    Grunendahl, S.
    Grurnewald, M. W.
    Guo, J.
    Guo, F.
    Gutierrez, P.
    Gutierrez, G.
    Haas, A.
    Hadley, N. J.
    Haefner, P.
    Hagopian, S.
    Haley, J.
    Hall, I.
    Hall, R. E.
    Han, L.
    Hansson, P.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Astronomy and Space Physics.
    Harder, K.
    Harel, A.
    Harrington, R.
    Hauptman, J. M.
    Hauser, R.
    Hays, J.
    Hebbeker, T.
    Hedin, D.
    Hegeman, J. G.
    Heinmiller, J. M.
    Heinson, A. P.
    Heintz, U.
    Hensel, C.
    Herner, K.
    Hesketh, G.
    Hildreth, M. D.
    Hirosky, R.
    Hobbs, J. D.
    Hoeneisen, B.
    Hoeth, H.
    Hohlfeld, M.
    Hong, S. J.
    Hossain, S.
    Houben, P.
    Hu, Y.
    Hubacek, Z.
    Hynek, V.
    Iashvili, I.
    Illingworth, R.
    Ito, A. S.
    Jabeen, S.
    Jaffre, M.
    Jain, S.
    Jakobs, K.
    Jarvis, C.
    Jesik, R.
    Johns, K.
    Johnson, C.
    Johnson, M.
    Jonckheere, A.
    Jonsson, P.
    Juste, A.
    Kajfasz, E.
    Kalinin, A. M.
    Kalk, J. R.
    Kalk, J. M.
    Kappler, S.
    Karmanov, D.
    Kasper, P. A.
    Katsanos, I.
    Kau, D.
    Kaur, R.
    Kaushik, V.
    Kehoe, R.
    Kermiche, S.
    Khalatyan, N.
    Khanov, A.
    Kharchilava, A.
    Kharzheev, Y. M.
    Khatidze, D.
    Kim, T. J.
    Kirby, M. H.
    Kirsch, M.
    Klima, B.
    Kohli, J. M.
    Konrath, J. -P
    Korablev, V. M.
    Kozelov, A. V.
    Krop, D.
    Kuhl, T.
    Kumar, A.
    Kunori, S.
    Kupco, A.
    Kurca, T.
    Kvita, J.
    Lacroix, F.
    Lam, D.
    Lammers, S.
    Landsberg, G.
    Lebrun, P.
    Lee, W. M.
    Leflat, A.
    Lehner, F.
    Lellouch, J.
    Leveque, J.
    Li, J.
    Li, Q. Z.
    Li, L.
    Lietti, S. M.
    Lima, J. G. R.
    Lincoln, D.
    Linnemann, J.
    Lipaev, V. V.
    Lipton, R.
    Liu, Y.
    Liu, Z.
    Lobodenko, A.
    Lokajicek, M.
    Love, P.
    Lubatti, H. J.
    Luna, R.
    Lyon, A. L.
    Maciel, A. K. A.
    Mackin, D.
    Madaras, R. J.
    Maettig, P.
    Magass, C.
    Magerkurth, A.
    Mal, P. K.
    Malbouisson, H. B.
    Malik, S.
    Malyshev, V. L.
    Mao, H. S.
    Maravin, Y.
    Martin, B.
    McCarthy, R.
    Melnitchouk, A.
    Mendoza, L.
    Mercadante, P. G.
    Merkin, M.
    Merritt, K. W.
    Meyer, J.
    Meyer, A.
    Millet, T.
    Mitrevski, J.
    Molina, J.
    Mommsen, R. K.
    Mondal, N. K.
    Moore, R. W.
    Moulik, T.
    Muanza, G. S.
    Mulders, M.
    Mulhearn, M.
    Mundal, O.
    Mundim, L.
    Nagy, E.
    Naimuddin, M.
    Narain, M.
    Naumann, N. A.
    Neal, H. A.
    Negret, J. P.
    Neustroev, P.
    Nilsen, H.
    Nogima, H.
    Novaes, S. F.
    Nunnemann, T.
    O'Dell, V.
    O'Neil, D. C.
    Obrant, G.
    Ochando, C.
    Onoprienko, D.
    Oshima, N.
    Osta, J.
    Otec, R.
    Otero y Garzon, G. J.
    Owen, M.
    Padley, P.
    Pangilinan, M.
    Parashar, N.
    Park, S. -J
    Park, S. K.
    Parsons, J.
    Partridge, R.
    Parua, N.
    Patwa, A.
    Pawloski, G.
    Penning, B.
    Perfilov, M.
    Peters, K.
    Peters, Y.
    Petroff, P.
    Petteni, M.
    Piegaia, R.
    Piper, J.
    Pleier, M. -A
    Podesta-Lerma, P. L. M.
    Podstavkov, V. M.
    Pogorelov, Y.
    Pol, M. -E
    Polozov, P.
    Pope, B. G.
    Popov, A. V.
    Potter, C.
    da Silva, W. L. Prado
    Prosper, H. B.
    Protopopescu, S.
    Qian, J.
    Quadt, A.
    Quinn, B.
    Rakitine, A.
    Rangel, M. S.
    Ranjan, K.
    Ratoff, P. N.
    Renkel, P.
    Reucroft, S.
    Rich, P.
    Rieger, J.
    Rijssenbeek, M.
    Ripp-Baudot, I.
    Rizatdinova, F.
    Robinson, S.
    Rodrigues, R. F.
    Rominsky, M.
    Royon, C.
    Rubinov, P.
    Ruchti, R.
    Safronov, G.
    Sajot, G.
    Sanchez-Hernandez, A.
    Sanders, M. P.
    Santoro, A.
    Savage, G.
    Sawyer, L.
    Scanlon, T.
    Schaile, D.
    Schamberger, R. D.
    Scheglov, Y.
    Schellman, H.
    Schliephake, T.
    Schwanenberger, C.
    Schwartzman, A.
    Schwienhorst, R.
    Sekaric, J.
    Severini, H.
    Shabalina, E.
    Shamim, M.
    Shary, V.
    Shchukin, A. A.
    Shivpuri, R. K.
    Siccardi, V.
    Simak, V.
    Sirotenko, V.
    Skubic, P.
    Slattery, P.
    Smirnov, D.
    Snow, J.
    Snow, G. R.
    Snyder, S.
    Soldner-Rembold, S.
    Sonnenschein, L.
    Sopczak, A.
    Sosebee, M.
    Soustruznik, K.
    Spurlock, B.
    Stark, J.
    Steele, J.
    Stolin, V.
    Stoyanova, D. A.
    Strandberg, J.
    Strandberg, S.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Astronomy and Space Physics.
    Strang, M. A.
    Strauss, M.
    Strauss, E.
    Stroehmer, R.
    Strom, D.
    Stutte, L.
    Sumowidagdo, S.
    Svoisky, P.
    Sznajder, A.
    Talby, M.
    Tamburello, P.
    Tanasijczuk, A.
    Taylor, W.
    Temple, J.
    Tiller, B.
    Tissandier, F.
    Titov, M.
    Tokmenin, V. V.
    Toole, T.
    Torchiani, I.
    Trefzger, T.
    Tsybychev, D.
    Tuchming, B.
    Tully, C.
    Tuts, P. M.
    Unalan, R.
    Uvarov, S.
    Uvarov, L.
    Uzunyan, S.
    Vachon, B.
    van den Berg, P. J.
    van Kooten, R.
    van Leeuwen, W. M.
    Varelas, N.
    Varnes, E. W.
    Vasilyev, I. A.
    Vaupel, M.
    Verdier, P.
    Vertogradov, L. S.
    Verzocchi, M.
    Villeneuve-Seguier, F.
    Vint, P.
    Vokac, P.
    Von Toerne, E.
    Vorwerk, V.
    Voutilainen, M.
    Wagner, R.
    Wahl, H. D.
    Wang, L.
    Wang, M. H. L. S.
    Warchol, J.
    Watts, G.
    Wayne, M.
    Weber, M.
    Weber, G.
    Welty-Rieger, L.
    Wenger, A.
    Wermes, N.
    Wetstein, M.
    White, A.
    Wicke, D.
    Wilson, G. W.
    Wimpenny, S. J.
    Wobisch, M.
    Wood, D. R.
    Wyatt, T. R.
    Xie, Y.
    Yacoob, S.
    Yamada, R.
    Yan, M.
    Yasuda, T.
    Yatsunenko, Y. A.
    Yip, K.
    Yoo, H. D.
    Youn, S. W.
    Yu, J.
    Zatserklyaniy, A.
    Zeitnitz, C.
    Zhao, T.
    Zhou, B.
    Zhu, J.
    Zielinski, M.
    Zieminska, D.
    Zieminski, A.
    Zivkovic, L.
    Zutshi, V.
    Zverev, E. G.
    Search for excited electrons in p(p)over-bar collisions at root s=1.96 TeV2008In: Physical Review D, ISSN 1550-7998, E-ISSN 1550-2368, Vol. 77, no 9, article id 091102Article in journal (Refereed)
    Abstract [en]

    We present the results of a search for the production of an excited state of the electron, e(*), in proton-antiproton collisions at root s = 1.96 TeV. The data were collected with the D0 experiment at the Fermilab Tevatron Collider and correspond to an integrated luminosity of approximately 1 fb(-1). We search for e(*) in the process p (p) over bar -> e(*)e, with the e(*) subsequently decaying to an electron plus photon. No excess above the standard model background is observed. Interpreting our data in the context of a model that describes e(*) production by four-fermion contact interactions and e(*) decay via electroweak processes, we set 95% C.L. upper limits on the production cross section ranging from 8.9 to 27 fb, depending on the mass of the excited electron. Choosing the scale for contact interactions to be Lambda = 1 TeV, excited electron masses below 756 GeV are excluded at the 95% C.L.

  • 2. Abia, C.
    et al.
    de laverny, P.
    Wahlin, Rurik
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Astronomy and Space Physics.
    Chemical analysis of carbon stars in the Local Group2008In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 481, no 1, p. 161-168Article in journal (Refereed)
    Abstract [en]

    Aims. We present new results of our ongoing chemical study of carbon stars in Local Group galaxies to test the critical dependence of s-process nucleosynthesis on the stellar metallicity. Methods. We collected optical spectra with the VLT/UVES instrument of two carbon stars found in the Carina Dwarf Spheroidal (dSph) galaxy, namely ALW-C6 and ALW-C7. We performed a full chemical analysis using the new generation of hydrostatic, spherically symmetric carbon-rich model atmospheres and the spectral synthesis method in LTE. Results. The luminosities, atmosphere parameters and chemical composition of ALW-C6 and ALW-C7 are compatible with these stars being in the TP-AGB phase undergoing third dredge-up episodes, although their extrinsic nature ( external pollution in a binary stellar system) cannot be definitively excluded. Our chemical analysis shows that the metallicity of both stars agree with the average metallicity ([Fe/H] similar to -1.8 dex) previously derived for this satellite galaxy from the analysis of both low resolution spectra of RGB stars and the observed colour magnitude diagrams. ALW-C6 and ALW-C7 present strong s-element enhancements, [s/Fe] = + 1.6, + 1.5, respectively. These enhancements and the derived s-process indexes [ls/Fe], [hs/Fe] and [hs/ls] are compatible with theoretical s-process nucleosynthesis predictions in low mass AGB stars (similar to 1.5 M(circle dot)) on the basis that the (13)C(alpha, n) (16)O is the main source of neutrons. Furthermore, the analysis of C(2) and CN bands reveals a large carbon enhancement (C/O similar to 7 and 5, respectively), much larger than the values typically found in galactic AGB carbon stars ( C/O similar to 1-2). This is also in agreement with the theoretical prediction that AGB carbon stars are formed more easily through third dredge-up episodes as the initial stellar metallicity drops. However, theoretical low-mass AGB models apparently fail to simultaneously fit the observed s-element and carbon enhancements. On the other hand, Zr is found to be less enhanced in ALW-C7 compared to the other elements belonging to the same s-peak. Although the abundance errors are large, the fact that in this star the abundance of Ti ( which has a similar condensation temperature to Zr) seems also to be lower than those of others metals, may indicate the existence of some depletion into dust-grains in its photosphere.

  • 3.
    Daldorff, L. K. S.
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Astronomy and Space Physics.
    Eliasson, B.
    Parallelization of a Vlasov-Maxwell solver in four-dimensional phase space2009In: Parallel Computing, ISSN 0167-8191, E-ISSN 1872-7336, Vol. 35, no 2, p. 109-115Article in journal (Refereed)
    Abstract [en]

    We present a parallelized algorithm for solving the time-dependent Vlasov–Maxwell system of equations in the four-dimensional phase space (two spatial and velocity dimensions). One Vlasov equation is solved for each particle species, from which charge and current densities are calculated for the Maxwell equations. The parallelization is divided into two different layers. For the first layer, each plasma species is given its own processor group. On the second layer, the distribution function is domain decomposed on its dedicated resources. By separating the communication and calculation steps, we have met the design criteria of good speedup and simplicity in the implementation.

  • 4.
    Daldorff, L. K. S.
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Astronomy and Space Physics.
    Pécseli, H. L.
    Trulsen, J.
    Eliasson, B.
    Stenflo, L.
    Nonlinearly generated plasma waves as a model for enhanced ionacoustic lines in the ionosphere2007In: New Journal of Physics, ISSN 1367-2630, E-ISSN 1367-2630Article in journal (Other academic)
  • 5.
    Daldorff, Lars Kristen Selberg
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Astronomy and Space Physics.
    Numerical Simulation as a Tool for Studying Waves and Radiation in Space2008Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Plasma physics governs the area of interactions between charged particles. As 99% of the visible universe is in a plasma state, it is an important topic in astronomy and space physics, where we already at an altitude of 60 km reach the plasma environment surrounding our planet in the form of the ionosphere. The search for fusion, the source of power for the sun, as well as industrial use have been the main topics for earth bound plasma reasurch.

    A plasma is composed of charged particles which interact by the electromagnetic force. In the kinetic description, via the Vlasov-Maxwell equations, the system is described in terms of probability distribution functions for each particle species, expressed in terms of particles position and velocity. The particles interact via self-consistent fields as determined by Maxwell's equations. For understanding the complex behaviour of the system, we need numerical solvers. These come in two flavours, Lagrangian methods, dealing with the moving around of synthetic particles, and Eulerian methods, which solve the set of partial differential, Vlasov and Maxwell equations. To perform the computations within reasonable time, we need to distribute our calculations on multiple machines, i.e. parallel programming, with the best possible matching between our computational needs and the need of splitting algorithms to adapt to our processing environment.

    Paper I studies electron and ion beams within a Lagrangian and fluid model and compare the results with experimental observations. This is continued with studies of a full kinetic system, using an Eulerian solver, for a closer look at electron-ion interactions in relation to ionospheric observations, (Papers II and IV). To improve the performance of the Eulerian solver it was parallelised (Paper III). The thesis is ending with the possibility to observe ultrahigh energy neutrinos from an orbiting satellite by using the Moon's surface as a detector Paper V.

    List of papers
    1. Phase space vortices in collision-less plasmas
    Open this publication in new window or tab >>Phase space vortices in collision-less plasmas
    Show others...
    2003 (English)In: Nonlinear processes in geophysics, ISSN 1023-5809, E-ISSN 1607-7946, Vol. 10, no 1/2, p. 75-86Article in journal (Refereed) Published
    Abstract [en]

    Results on the formation and propagation of electron phase space vortices from laboratory experiments are summarized. The electron phase space vortices were excited in a strongly magnetized Q-machine plasma by applying a pulse to a segment of a waveguide surrounding the plasma. Depending on the temporal variation of the applied pulse, one or more phase space vortices can be excited, and their interaction can be followed in space and time. We were able to demonstrate, for instance, an irreversible coalescence of two such vortices. These results are extended by numerical simulations, showing how electron phase space vortices can also be formed by beam instabilities. Furthermore, a study of ion phase space vortices is performed by numerical simulations. Both codes allow for an externally applied magnetic field in three spatial dimensions. Ion phase space vortices are formed by the nonlinear saturation of the ion-ion two-stream instability, excited by injecting an ion beam at the plasma boundary. By following the evolution of the ion distribution of the velocity perpendicular to the direction of propagation of the injected ion beam, we find a significant ion heating in the direction perpendicular to the magnetic field associated with the ion phase space vortices being formed. The results are relevant, for instance, for the interpretation of observations by instrumented spacecraft in the Earth's ionosphere and magnetosphere.

    National Category
    Physical Sciences
    Identifiers
    urn:nbn:se:uu:diva-97954 (URN)
    Available from: 2009-01-01 Created: 2009-01-01 Last updated: 2017-12-14Bibliographically approved
    2. Nonlinearly generated plasma waves as a model for enhanced ion acoustic lines in the ionosphere
    Open this publication in new window or tab >>Nonlinearly generated plasma waves as a model for enhanced ion acoustic lines in the ionosphere
    2007 (English)In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 34, no 18, p. L18101-Article in journal (Refereed) Published
    Abstract [en]

    Observations from the EISCAT Svalbard Radar, for instance, demonstrate that the symmetry of the naturally occurring ion line can be broken by an enhanced, non-thermal, level of fluctuations, i.e., Naturally Enhanced Ion-Acoustic Lines (NEIALs). In a significant number of cases, the entire ion spectrum can be distorted, with the appearance of a third line, corresponding to a propagation velocity significantly below the ion acoustic sound speed. By numerical simulations, we consider one possible model accounting for the observations, suggesting that a primary process can be electron acoustic waves excited by a cold electron beam. Subsequently, an oscillating two-stream instability excites electron plasma waves which in turn decay to asymmetric ion lines. Our code solves the full Vlasov equation for electrons and ions, with the dynamics coupled through the electrostatic field derived from Poisson's equation.

    National Category
    Physical Sciences
    Identifiers
    urn:nbn:se:uu:diva-97955 (URN)10.1029/2007GL031513 (DOI)000249683400008 ()
    Available from: 2009-01-01 Created: 2009-01-01 Last updated: 2017-12-14Bibliographically approved
    3. Parallelization of a Vlasov-Maxwell solver in four-dimensional phase space
    Open this publication in new window or tab >>Parallelization of a Vlasov-Maxwell solver in four-dimensional phase space
    2009 (English)In: Parallel Computing, ISSN 0167-8191, E-ISSN 1872-7336, Vol. 35, no 2, p. 109-115Article in journal (Refereed) Published
    Abstract [en]

    We present a parallelized algorithm for solving the time-dependent Vlasov–Maxwell system of equations in the four-dimensional phase space (two spatial and velocity dimensions). One Vlasov equation is solved for each particle species, from which charge and current densities are calculated for the Maxwell equations. The parallelization is divided into two different layers. For the first layer, each plasma species is given its own processor group. On the second layer, the distribution function is domain decomposed on its dedicated resources. By separating the communication and calculation steps, we have met the design criteria of good speedup and simplicity in the implementation.

    Keyword
    Vlasov–Maxwell system, Spectral method, Transpose method
    National Category
    Physical Sciences
    Identifiers
    urn:nbn:se:uu:diva-97956 (URN)10.1016/j.parco.2008.11.002 (DOI)000263659600004 ()
    Available from: 2009-01-01 Created: 2009-01-01 Last updated: 2017-12-14Bibliographically approved
    4. Nonlinearly generated plasma waves as a model for enhanced ionacoustic lines in the ionosphere
    Open this publication in new window or tab >>Nonlinearly generated plasma waves as a model for enhanced ionacoustic lines in the ionosphere
    Show others...
    2007 (English)In: New Journal of Physics, ISSN 1367-2630, E-ISSN 1367-2630Article in journal (Other academic) Submitted
    Keyword
    Ionosphere: Plasma waves and instabilities (2772), Ionosphere: Ionospheric irregularities, Radio Science: Waves in plasma (7867)
    National Category
    Physical Sciences
    Identifiers
    urn:nbn:se:uu:diva-97957 (URN)
    Available from: 2009-01-01 Created: 2009-01-01 Last updated: 2017-12-14Bibliographically approved
    5. Prospects for Lunar Satellite Detection of Radio Pulses from Ultrahigh Energy Neutrinos Interacting with the Moon
    Open this publication in new window or tab >>Prospects for Lunar Satellite Detection of Radio Pulses from Ultrahigh Energy Neutrinos Interacting with the Moon
    Show others...
    2007 (English)In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 98, no 7, p. 071103-Article in journal (Refereed) Published
    Abstract [en]

    The Moon provides a huge effective detector volume for ultrahigh energy cosmic neutrinos, which generate coherent radio pulses in the lunar surface layer due to the Askaryan effect. We report systematic Monte Carlo simulations which show that radio instruments on board a Moon-orbiting satellite can detect Askaryan pulses from neutrinos with energies above 10^{19} eV, i.e. near and above the interesting GZK limit, at the very low fluxes predicted in different scenarios.

    National Category
    Physical Sciences
    Identifiers
    urn:nbn:se:uu:diva-26236 (URN)10.1103/PhysRevLett.98.071103 (DOI)000244250300016 ()
    Available from: 2007-02-15 Created: 2007-02-15 Last updated: 2017-12-07Bibliographically approved
  • 6.
    Davidsson, Björn J. R.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Astronomy and Space Physics.
    Comet Knudsen layers2008In: Space Science Reviews, ISSN 0038-6308, E-ISSN 1572-9672, Vol. 138, no 1-4, p. 207-223Article in journal (Refereed)
    Abstract [en]

    This paper reviews some important results about Knudsen layers obtained in theoretical gas kinetics research in the last few decades, focusing on the weak and strong evaporation problems in two-surface, half-space, and spherical geometries. Furthermore, the application of such results in cometary science is reviewed. In order to illustrate some properties of the half-space evaporation problem for water ice surfaces at temperatures relevant for active comets, a number of numerical Direct Simulation Monte Carlo calculations are presented.

  • 7. DeMeo, Francesca E.
    et al.
    Binzel, Richard P.
    Lockhart, Matthew
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Astronomy and Space Physics.
    Mars encounters cause fresh surfaces on some near-Earth asteroids2014In: Icarus (New York, N.Y. 1962), ISSN 0019-1035, E-ISSN 1090-2643, Vol. 227, p. 112-122Article in journal (Refereed)
    Abstract [en]

    All airless bodies are subject to the space environment, and spectral differences between asteroids and meteorites suggest many asteroids become weathered on very short (<1 Myr) timescales. The spectra of some asteroids, particularly Q-types, indicate surfaces that appear young and fresh, implying they have been recently been exposed. Previous work found that Earth encounters were the dominant freshening mechanism and could be responsible for all near-Earth object (NEO) Q-types. In this work we increase the known NEO Q-type sample of by a factor of three. We present the orbital distributions of 64 Q-type near-Earth asteroids, and seek to determine the dominant mechanisms for refreshing their surfaces. Our sample reveals two important results: (i) the relatively steady fraction of Q-types with increasing semi-major axis and (ii) the existence of Q-type near-Earth asteroids with Minimum Orbit Intersection Distances (MOID) that do not have orbit solutions that cross Earth. Both of these are evidence that Earth-crossing is not the only scenario by which NEO Q-types are freshened. The high Earth-MOID asteroids represent 10% of the Q-type population and all are in Amor orbits. While surface refreshing could also be caused by Main Belt collisions or mass shedding from YORP spinup, all high Earth-MOID Q-types have the possibility of encounters with Mars indicating Mars could be responsible for a significant fraction of NEOs with fresh surfaces.

  • 8.
    Edvardsson, Bengt
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Astronomy and Space Physics.
    Observed versus predicted stellar flux distributions of solar-type stars2008In: Physica Scripta, ISSN 0031-8949, E-ISSN 1402-4896, Vol. T133, p. 014011-Article in journal (Refereed)
    Abstract [en]

    I have compared the low-resolution spectrophotometric fluxes in the visible spectral region of F- and G-type MARCS 2008 model atmospheres to six spectrophotometric databases of the Sun and stars. These observational databases disagree with each other concerning the overall red/blue flux balances on the several percent scale for the same objects. There is, however, no systematic overall trend between MARCS model fluxes and these observations taken together and therefore no reason to suspect any problem with the MARCS overall spectrum balance. The results strongly suggest, however, that there are systematic errors in the ultraviolet and blue opacities used in the construction of the model atmospheres. These errors appear in wavelength regions with widths of some 50-150 angstrom. Similar uncertainties are found also in other independent libraries of synthetic model atmosphere fluxes. I also highlight a number of unidentified spectral features with unusual shapes in spectra of the Sun and solar-type stars.

  • 9.
    Eriksson, Kjell
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Astronomy and Space Physics.
    CNO abundances in carbon star atmospheres-a progress report2008In: Physica Scripta, ISSN 0031-8949, E-ISSN 1402-4896, Vol. T133, p. 014029-Article in journal (Refereed)
    Abstract [en]

    A brief progress report is given on an on-going project to estimate the C, N and O abundances in carbon stars of the Magellanic Cloud clusters NGC 1846 and NGC 419. The changes in these abundances as a function of position along the asymptotic giant branch (AGB) are sought for.

  • 10. Folsom, C. P.
    et al.
    Wade, G. A.
    Kochukhov, Oleg
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Astronomy and Space Physics.
    Alecian, E.
    Catala, C.
    Bagnulo, S.
    Landstreet, J. D.
    Hanes, D. A.
    Magnetic, chemical and rotational properties of the Herbig Ae/Be binary system HD 721062008In: Contributions of the Astronomical Observatory Skalnaté Pleso, ISSN 1335-1842, Vol. 38, no 2, p. 245-250Article in journal (Refereed)
    Abstract [en]

    Recently, strong, globally-ordered magnetic fields have been detected in some Herbig Ae and Be (HAeBe) stars, suggesting a possible evolutionary connection to main sequence magnetic chemically peculiar Ap and Bp stars. We have undertaken a detailed study of the binary system HD 72106, which contains a B9 magnetic primary and a HAeBe secondary, using the ESPaDOnS spectropolarimeter mounted on the CFHT. A careful analysis of the very young primary reveals that it has an approximately dipolar magnetic field geometry, strong chemical peculiarities, and strong surface chemical abundance inhomogeneities. Thus the primary is very similar to an Ap/Bp star despite having completed less then 1.5% of its main sequence life, and possible still being on the pre-main sequence. In contrast, a similar analysis of the secondary reveals solar chemical abundances and no magnetic field.

  • 11. Fossati, L.
    et al.
    Bagnulo, S.
    Monier, R.
    Khan, S. A.
    Kochukhov, Oleg
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Astronomy and Space Physics.
    Landstreet, J. D.
    Wade, G. A.
    Weiss, W. W.
    Chemical evolution of A- and B-type stars in open clusters: observed abundances vs. diffusion models2008In: Contributions of the Astronomical Observatory Skalnaté Pleso, ISSN 1335-1842, Vol. 38, no 2, p. 123-128Article in journal (Refereed)
    Abstract [en]

    We have decided to address the problem of how abundances and peculiarities change during main sequence evolution. We have setup a program to measure the atmospheric abundance patterns from tens of A-type star members of clusters of different ages, and compare the results with theory predictions. In this paper we present the overall project and we focus on the results obtained for a sample of Am stars of the Praesepe cluster (log t = 8.85 +/- 0.15; Gonzalez-Garcia et al., 2006). We have obtained spectra for eight Am stars, two normal A-type stars and one blue straggler, that are probable members of the Praesepe cluster. For all of these stars we have determined fundamental parameters and photospheric abundances for a large number of chemical elements. For seven stars we also obtained spectra in circular polarisation and applied the LSD technique to measure the mean longitudinal magnetic field. We have found good agreement between abundance predictions of diffusion models and measured abundances, except for Na and S. Li appears to be overabundant in three stars of our sample. No magnetic field was detected in any of the analysed stars.

  • 12.
    Freytag, Bernd
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Astronomy and Space Physics.
    Höfner, Susanne
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Astronomy and Space Physics.
    Three-dimensional simulations of the atmosphere of an AGB star2008In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 483, no 2, p. 571-583Article in journal (Refereed)
    Abstract [en]

    Context. Winds of asymptotic giant branch stars are assumed to be driven by radiation pressure on dust. Previously, this process has been modeled with detailed time-dependent simulations of atmospheres and winds assuming spherically symmetric flows. In such models kinetic energy is injected by a variable inner boundary ("piston") simulating the effects of stellar pulsation. However, the dynamical processes in these atmospheres - convection and pulsations - are actually three-dimensional. Aims. We present and analyze first 3D radiation hydrodynamics simulations of the convective interior and the atmosphere of a typical AGB star. In particular, we check whether the piston description in the 1D wind models is compatible with the 3D results. Methods. We used two different RHD codes, one (CO5BOLD) to produce 3D models of the outer convective envelope and the inner atmosphere of an AGB star, the other to describe the atmosphere and the wind acceleration region, including dust formation and non-grey radiative transfer, but assuming spherically symmetric flows. From the movements of stellar surface layers in the 3D models, we derived a description for the variable inner boundary in the 1D models. Results. The 3D models show large convection cells and pulsations that give rise to roughly spherically expanding shock waves in the atmosphere, levitating material into regions which are cool enough to allow for dust formation. The atmospheric velocity fields have amplitudes and time scales close to the values that are necessary to start dust formation in the 1D wind models. Conclusions. The convection cells in the 3D simulations are so large that the associated shock fronts appear almost spherical, justifying the assumption of spherical symmetry and the use of a piston boundary condition in the context of wind models. Nevertheless, certain non-radial structures exist in the dust shell developing in the 3D simulations which should be detectable with current interferometric techniques.

  • 13. Guio, P.
    et al.
    Børve, S.
    Daldorff, L. K. S.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Division of Astronomy and Space Physics.
    Lynov, J. P.
    Michelsson, P.
    Pécseli, H. L.
    Rasmussen, J. J.
    Saeki, K.
    Trulsen, J.
    Phase space vortices in collision-less plasmas2003In: Nonlinear processes in geophysics, ISSN 1023-5809, E-ISSN 1607-7946, Vol. 10, no 1/2, p. 75-86Article in journal (Refereed)
    Abstract [en]

    Results on the formation and propagation of electron phase space vortices from laboratory experiments are summarized. The electron phase space vortices were excited in a strongly magnetized Q-machine plasma by applying a pulse to a segment of a waveguide surrounding the plasma. Depending on the temporal variation of the applied pulse, one or more phase space vortices can be excited, and their interaction can be followed in space and time. We were able to demonstrate, for instance, an irreversible coalescence of two such vortices. These results are extended by numerical simulations, showing how electron phase space vortices can also be formed by beam instabilities. Furthermore, a study of ion phase space vortices is performed by numerical simulations. Both codes allow for an externally applied magnetic field in three spatial dimensions. Ion phase space vortices are formed by the nonlinear saturation of the ion-ion two-stream instability, excited by injecting an ion beam at the plasma boundary. By following the evolution of the ion distribution of the velocity perpendicular to the direction of propagation of the injected ion beam, we find a significant ion heating in the direction perpendicular to the magnetic field associated with the ion phase space vortices being formed. The results are relevant, for instance, for the interpretation of observations by instrumented spacecraft in the Earth's ionosphere and magnetosphere.

  • 14.
    Gustafsson, Bengt
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Astronomy and Space Physics.
    Is the Sun unique as a star - and if so, why?2008In: Physica Scripta, ISSN 0031-8949, E-ISSN 1402-4896, Vol. T130, p. 014036-Article in journal (Refereed)
    Abstract [en]

    The question whether the Sun is peculiar as compared with other stars in its neigbourhood is revisited. It is concluded that although the Sun is rather normal from many points of view, it departs in several respects from most stars of similar age and galactic orbit. Thus, it is more massive, and the amplitude of the micro-variability of the Sun at visual wavelengths seems unusually small. It also departs from most stars in being a single star, and it may have an unusual planetary system. There are some tentative indications that its chemical composition departs from those of most solar-type stars of similar age. This is discussed and the departures are found not to be significant. I discuss here to what extent these peculiarities may be understood in terms of it being a planet host.

  • 15.
    Gustafsson, Bengt
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Astronomy and Space Physics.
    Sanctions against scientists threaten progress2009In: Nature, ISSN 0028-0836, E-ISSN 1476-4687, Vol. 461, no 7265, p. 723-723Article in journal (Refereed)
  • 16.
    Gustafsson, Bengt
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Astronomy and Space Physics.
    Edvardsson, Bengt
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Astronomy and Space Physics.
    Eriksson, Kjell
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Astronomy and Space Physics.
    Jorgensen, U. G.
    Nordlund, A.
    Plez, B.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Astronomy and Space Physics.
    A grid of MARCS model atmospheres for late-type stars2008In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 486, no 3, p. 951-970Article, review/survey (Refereed)
    Abstract [en]

    Context. In analyses of stellar spectra and colours, and for the analysis of integrated light from galaxies, a homogeneous grid of model atmospheres of late-type stars and corresponding flux spectra is needed. Aims. We construct an extensive grid of spherically-symmetric models (supplemented with plane-parallel ones for the highest surface gravities), built on up-to-date atomic and molecular data, and make it available for public use. Methods. The most recent version of the MARCS program is used. Results. We present a grid of about 104 model atmospheres for stars with 2500K <= T-eff <= 8000 K, -1 <= log g = log (GM/R-2) <= 5 (cgs) with various masses and radii, -5 <= [Me/H] <= + 1, with [alpha/Fe] = 0.0 and 0.4 and different choices of C and N abundances. This includes "CN-cycled" models with C/N=4.07 (solar), 1.5 and 0.5, C/O ranging from 0.09 to (normally) 5.0 to also represent stars of spectral types R, S and N, and with 1.0 <= xi(t) = 5km s(-1). We also list thermodynamic quantities (T, P-g, P-e, rho, partial pressures of molecules, etc.) and provide them on the World Wide Web, as well as calculated fluxes in approximately 108 000 wavelength points. Underlying assumptions in addition to 1D stratification (spherical or plane-parallel) include hydrostatic equilibrium, mixing-length convection and local thermodynamic equilibrium. We discuss a number of general properties of the models, in particular in relation to the effects of changing abundances, of blanketing, and of sphericity. We illustrate positive and negative feedbacks between sphericity and molecular blanketing. We compare the models with those of other available grids and find excellent agreement with planeparallel models of Castelli & Kurucz (if convection is treated consistently) within the overlapping parameter range. Although there are considerable departures from the spherically-symmetric NextGen models, the agreement with more recent PHOENIX models is gratifying. Conclusions. The models of the grid show considerable regularities, but some interesting departures from general patterns occur for the coolest models due to the molecular opacities. We have tested a number of approximate "rules of thumb" concerning effects of blanketing and sphericity and often found them to be astonishingly accurate. Some interesting new phenomena have been discovered and explored, such as the intricate coupling between blanketing and sphericity, and the strong effects of carbon enhancement on metal-poor models. We give further details of line absorption data for molecules, as well as details of models and comparisons with observations in subsequent papers.

  • 17.
    Heiter, Ulrike
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Astronomy and Space Physics.
    Luck, R. E.
    How unique is the local region of the Galaxy disk?2008In: Physica Scripta, ISSN 0031-8949, E-ISSN 1402-4896, Vol. T133, p. 014040-Article in journal (Refereed)
    Abstract [en]

    We present the results of a spectroscopic study of over 500 dwarf and giant stars within 100 pc of the Sun. Using stellar metallicity data, the goal of this study is to determine whether there are any significant variations of metallicity related to stellar characteristics or location in the Galaxy. The abundance data are based on high-resolution spectroscopy and correlated with kinematical properties based on Hipparcos data. The spectroscopic data are made available to the public through an online database. We find that most elements behave as expected from Galactic chemical evolution and are consistent with standard stellar evolution. Interesting trends are found for some elements, e. g. Mn and probably Ba. There is evidence for systematic carbon abundance variations in the red giant clump. To determine the uniqueness of the local region, in particular at its galactocentric radius, one should consider in-depth spectroscopic and kinematic studies of dwarfs and giants in similar-sized volumes in other regions of the disc. This will require accurate positions, distances and space motions out to hundreds of parsecs from space missions such as Gaia and next-generation telescopes for high-resolution spectroscopy.

  • 18. Holopainen, Janne
    et al.
    Zackrisson, Erik
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Astronomy and Space Physics.
    Knebe, Alexander
    Nurmi, Pasi
    Heinamaki, Pekka
    Flynn, Chris
    Gill, Stuart
    Riehm, Teresa
    An analytical model of surface mass densities of cold dark matter haloes - with an application to MACHO microlensing optical depths2008In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 383, no 2, p. 720-728Article in journal (Refereed)
    Abstract [en]

    The cold dark matter (CDM) scenario generically predicts the existence of triaxial dark matter haloes which contain notable amounts of substructure. However, analytical halo models with smooth, spherically symmetric density profiles are routinely adopted in the modelling of light propagation effects through such objects. In this paper, we address the biases introduced by this procedure by comparing the surface mass densities of actual N-body haloes against the widely used analytical model suggested by Navarro, Frenk and White (NFW). We conduct our analysis in the redshift range of 0.0-1.5. In cluster-sized haloes, we find that triaxiality can cause scatter in the surface mass density of the haloes up to sigma(+) = +60 per cent and sigma(-) = -70 per cent, where the 1 sigma limits are relative to the analytical NFW model given value. Subhaloes can increase this scatter to sigma(+) = +70 per cent and sigma(-) = -80 per cent. In galaxy-sized haloes, the triaxial scatter can be as high as sigma(+) = +80 per cent and sigma(-) = -70 per cent, and with subhaloes the values can change to sigma(+) = +40 per cent and sigma(-) = -80 per cent. We present an analytical model for the surface mass density scatter as a function of distance to the halo centre, halo redshift and halo mass. The analytical description enables one to investigate the reliability of results obtained with simplified halo models. Additionally, it provides the means to add simulated surface density scatter to analytical density profiles. As an example, we discuss the impact of our results on the calculation of microlensing optical depths for massive astrophysical compact halo objects in CDM haloes.

  • 19.
    Höfner, Susanne
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Astronomy and Space Physics.
    Winds of AGB stars: does size matter?2008In: Physica Scripta, ISSN 0031-8949, E-ISSN 1402-4896, Vol. T133, p. 014007-Article in journal (Refereed)
    Abstract [en]

    Asymptotic giant branch (AGB) stars are showing clear signs of significant mass loss through cool stellar winds. These outflows are attributed to the combined effects of pulsation-induced shocks and radiation pressure on dust grains formed in the outer atmospheric layers. This paper gives an overview of the current status of radiation-hydrodynamical modelling of these processes, and presents a toy model that allows analysis of certain features of detailed models, such as the influence of grain size dependent opacities and basic differences in winds of C- and M-type AGB stars.

  • 20. Jucker, Martin
    et al.
    Pavlenko, Vladimir
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Astronomy and Space Physics.
    On the kinetic stability of magnetic structures in electron drift turbulence2007In: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 14, no 3, p. 032303-Article in journal (Refereed)
    Abstract [en]

    The generation of large-scale magnetic fields in magnetic electron drift mode turbulence is investigated. The mechanism of magnetic Reynolds stress is elucidated with the help of kinetic theory, and a sufficient criterion in the form of Nyquist's criterion for the generation of zonal magnetic fields is developed. This criterion is then applied to a narrow wave packet, where an amplitude threshold due to finite width of the wave spectrum in k space is found.

  • 21. Jucker, Martin
    et al.
    Pavlenko, Vladimir
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Astronomy and Space Physics.
    On the modulation stability of magnetic structures in electron drift turbulence2007In: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 14, no 10, p. 102313-102317Article in journal (Refereed)
    Abstract [en]

    The generation of large-scale magnetic fields in magnetic electron drift mode turbulence is investigated. In particular, the mechanism of modulational instability caused by three-wave interactions is elucidated and the explicit increment is calculated. Also, a stability criterion similar to the known Lighthill criterion is found.

  • 22.
    Kochukhov, Oleg
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Astronomy and Space Physics.
    Magnetic fields, spots and weather in chemically peculiar stars2008In: Contributions of the Astronomical Observatory Skalnaté Pleso, ISSN 1335-1842, Vol. 38, no 2, p. 191-200Article in journal (Refereed)
    Abstract [en]

    New observational techniques and sophisticated modelling methods have led to dramatic breakthroughs in our understanding of the interplay between the surface magnetism, atomic diffusion and atmospheric dynamics in chemically peculiar stars. Magnetic Doppler images, constructed using spectropolarimetric observations of Ap stars in all four Stokes parameters, reveal the presence of small-scale field topologies. Abundance Doppler mapping has been perfected to the level where distributions of many different chemical elements can be deduced self-consistently for one star. The inferred chemical spot structures are diverse and do not always trace underlying magnetic field geometry. Moreover, horizontal chemical inhomogeneities are discovered in nonmagnetic CP stars and evolving chemical spots are observed for the first time in the bright mercury-manganese star a And. These results show that in addition to magnetic fields, another important non-magnetic structure formation mechanism acts in CP stars.

  • 23.
    Kochukhov, Oleg
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Astronomy and Space Physics.
    Zeeman split lines in CP stars: a discovery of low-contrast fields in slow rotators?2008In: Contributions of the Astronomical Observatory Skalnate Pleso, ISSN 1335-1842, Vol. 38, no 2, p. 421-422Article in journal (Refereed)
    Abstract [en]

    We show that high-resolution observations of resolved Zeeman split lines can be used to obtain new constraints on the stellar magnetic field geometry. In particular, the contrast of the field strength distribution over the stellar surface can be deduced from differential measurement of the second moment of the pi and sigma Zeeman components. Our analysis of the triplet lines in slowly rotating cool magnetic CP stars uncovers a surprisingly homogeneous field structure, inconsistent with any low-order multipolar geometry.

  • 24.
    Kochukhov, Oleg
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Astronomy and Space Physics.
    Ryabchikova, T.
    Bagnulo, S.
    Lo Curto, G.
    A search for very low amplitude magnetoacoustic pulsations with HARPS2008In: Contributions of the Astronomical Observatory Skalnate Pleso, ISSN 1335-1842, Vol. 38, no 2, p. 423-424Article in journal (Refereed)
    Abstract [en]

    We have obtained time-resolved spectroscopic observations for a sample of 10 cool Ap stars using the ultra-stable spectrograph HARPS at the ESO 3.6-m telescope. The aim of our study was to search for low-amplitude oscillations in Ap stars with no or inconclusive evidence of pulsational variability. Here we report initial results of our investigation. We confirm the presence of approximate to 16-min period pulsations in beta CrB (HD 137909) and demonstrate multi-periodic character of oscillations in this star. Furthermore, we discovered very low amplitude 9-min pulsations in HD 75445 - an object spectroscopically very similar to known roAp stars.

  • 25.
    Kochukhov, Oleg
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Astronomy and Space Physics.
    Shulyak, D.
    New generation model atmospheres for chemically peculiar stars2008In: Contributions of the Astronomical Observatory Skalnate Pleso, ISSN 1335-1842, Vol. 38, no 2, p. 419-420Article in journal (Refereed)
    Abstract [en]

    The atmospheric structure of chemically peculiar stars deviates from that of normal stars with similar fundamental parameters due to unusual chemistry, abundance inhomogeneities and the presence of strong magnetic field. These effects are not considered in the standard model atmospheres, possibly leading to large errors in stellar parameter determination and abundance analysis. To resolve this problem we used the state-of-the-art opacity sampling model atmosphere code LLmodels to calculate a comprehensive grid of new generation model atmospheres for magnetic CP stars. This grid covers the whole parameter space occupied by SrCrEu and Si-peculiar stars, taking into account the characteristic temperature dependence of the chemical abundances. Here we present the first results of our model atmosphere calculations.

  • 26. Leyser, T. B.
    et al.
    Norin, L.
    Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Physics, Department of Physics and Astronomy, Division of Astronomy and Space Physics.
    McCarrick, M.
    Pedersen, T. R.
    Gustavsson, B.
    Radio pumping of the ionosphere with orbital angular momentumIn: Physical Review Letters, ISSN 0031-9007Article in journal (Refereed)
  • 27. Lueftinger, T.
    et al.
    Kochukhov, Oleg
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Astronomy and Space Physics.
    Ryabchikova, T.
    Piskunov, Nikolai
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Astronomy and Space Physics.
    Weiss, W. W.
    Ilyin, I.
    3D atmospheric structure of the prototypical roAp star HD 24712 (HR1217)2008In: Contributions of the Astronomical Observatory Skalnate Pleso, ISSN 1335-1842, Vol. 38, no 2, p. 335-340Article in journal (Refereed)
    Abstract [en]

    The first analysis of the structure of the surface magnetic field of a rapidly oscillating Ap (roAp) star is presented. We obtain information about abundance distributions of a number of chemical elements on the surface of the prototypical roAp star HD 24712 and about magnetic field geometry. Inverting rotationally modulated spectra in Stokes parameters I and V obtained with the SOFIN spectropolarimeter attached to the NOT, we recover surface abundance structures of sixteen different chemical elements, including Mg, Ca, Sc, Ti, Cr, Fe, Co, Ni, Y, La, Ce, Pr, Nd, Gd, Tb, and Dy. Our analysis reveal a pure dipolar structure of the stellar magnetic field and surprising and unexpected correlations of the various elemental surface abundance structures to this field geometry. Stratification analysis at phases of both magnetic extrema enable us to obtain the vertical dimension in the atmosphere of HD 24712. High time resolved spectroscopic data and observations obtained with the MOST space photometer give us the possibility to compare (Luftinger, 2007) our results to detailed pulsational analysis.

  • 28.
    Mattsson, Lars
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Astronomy and Space Physics.
    On the Winds of Carbon Stars and the Origin of Carbon: A Theoretical Study2009Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Carbon is the basis for life, as we know it, but its origin is still largely unclear. Carbon-rich Asymptotic Giant Branch (AGB) stars (carbon stars) play an important rôle in the cosmic matter cycle and may contribute most of the carbon in the Galaxy.

    In this thesis it is explored how the dust-driven mass loss of these stars depends on the basic stellar parameters by computing a large grid of wind models. The existence of a critical wind regime and mass-loss thresholds for dust-driven winds are confirmed. Furthermore, a steep dependence of mass loss on carbon excess is found. Exploratory work on the effects of different stellar metallicities and the sizes of dust grains shows that strong dust-driven winds develop also at moderately low metallicities, and that typical sizes of dust grains affect the wind properties near a mass-loss threshold.

    It is demonstrated that the mass-loss rates obtained with the wind models have dramatic consequences when used in models of carbon-star evolution. A pronounced superwind develops soon after the star becomes carbon rich, and it therefore experiences only a few thermal pulses as a carbon star before the envelope is lost. The number of dredge-up events and the thermal pulses is limited by a self-regulating mechanism: each thermal pulse dredges up carbon, which increases the carbon excess and hence also the mass-loss rate. In turn, this limits the number of thermal pulses.

    The mass-loss evolution during a thermal pulse (He-shell flash) is considered as an explanation of the observations of so-called detached shells around carbon stars. By combining models of dust-driven winds with a stellar evolution model, and a simple hydrodynamic model of the circumstellar envelope, it is shown that wind properties change character during a He-shell flash such that a thin detached gas shell can form by wind-wind interaction.

    Finally, it is suggested that carbon stars are responsible for much of the carbon in the interstellar medium, but a scenario where high-mass stars are major carbon producers cannot be excluded. In either case, however, the carbon abundances of the outer Galactic disc are relatively low, and most of the carbon has been released quite recently. Thus, there may neither be enough carbon, nor enough time, for more advanced carbon-based life to emerge in the outer Galaxy. This lends some support to the idea that only the mid-part of the Galactic disc can be a “Galactic habitable zone”, since the inner parts of the Galaxy are plagued by frequent supernova events that are presumably harmful to all forms of life.

    List of papers
    1. Dust driven mass loss from carbon stars as a function of stellar parameters I: A grid of solar-metallicity wind models
    Open this publication in new window or tab >>Dust driven mass loss from carbon stars as a function of stellar parameters I: A grid of solar-metallicity wind models
    2010 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 509, no 1, p. 13-Article in journal (Refereed) Published
    Abstract [en]

    Context. Knowing how the mass loss of carbon-rich AGB stars depends on stellar parameters is crucial for stellar evolution modelling, as well as for the understanding of when and how circumstellar structures emerge around these stars, e.g., dust shells and so-called detached shells of expelled gas.

    Aims. The purpose of this paper is to explore the stellar parameter space using a numerical radiation hydrodynamic (RHD) model of carbon-star atmospheres, including a detailed description of dust formation and frequency-dependent radiative transfer, in order to determine how the mass loss of carbon stars changes with stellar parameters.

    Methods. We have computed a grid of 900 numeric dynamic model atmospheres (DMAs) using a well-tested computer code. This grid of models covers most of the expected combinations of stellar parameters, which are the stellar temperature, the stellar luminosity, the stellar mass, the abundance of condensible carbon, and the velocity amplitude of the pulsation.

    Results. The resultant mass-loss rates and wind speeds are clearly affected by the choice of stellar temperature, mass, luminosity and the abundance of available carbon. In certain parts of the parameter space there is also an inevitable mass-loss threshold, below which a dust-driven wind is not possible. Contrary to some previous studies, we find a strong dependence on the abundance of free carbon, which turns out to be a critical parameter. Furthermore, we have found that the dust grains that form in the atmosphere may grow too large for the commonly-used small-particle approximation of the dust opacity to be strictly valid. This may have some bearing on the wind properties, although further study of this problem is needed before quantitative conclusions can be drawn.

    Conclusions. The wind properties show relatively simple dependences on stellar parameters above the mass-loss threshold, while the threshold itself is of a more complicated nature. Hence, we chose not to derive any simplistic mass-loss formula, but rather provide a mass-loss prescription in the form of aeasy-to-use FORTRAN routine. Since this mass-loss routine is based on data coming from an essentially self-consistent model of mass loss, it may therefore serve as a better mass-loss prescription for stellar evolution calculations than empirical formulae. Furthermore, we conclude that there are still some issues that need to be investigated, such as the role of grain-sizes.

    Keyword
    stars: AGB and post-AGB; stars: atmospheres; stars: carbon; circumstellar matter; stars: evolution; stars: mass-loss
    National Category
    Astronomy, Astrophysics and Cosmology
    Research subject
    Astronomy; Physics
    Identifiers
    urn:nbn:se:uu:diva-99592 (URN)10.1051/0004-6361/200912084 (DOI)000274159400026 ()
    Available from: 2009-03-16 Created: 2009-03-16 Last updated: 2017-12-13Bibliographically approved
    2. Intense mass loss from C-rich AGB stars at low metallicity?
    Open this publication in new window or tab >>Intense mass loss from C-rich AGB stars at low metallicity?
    2008 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 484, no 1, p. L5-L8Article in journal (Refereed) Published
    Abstract [en]

    We argue that the energy injection of pulsations may be of greater importance to the mass-loss rate of AGB stars than metallicity, and that the mass-loss trend with metallicity is not as simple as sometimes assumed. Using our detailed radiation hydrodynamical models that include dust formation, we illustrate the effects of pulsation energy on wind properties. We find that the mass-loss rate scales with the kinetic energy input by pulsations as long as a dust-saturated wind does not occur, and all other stellar parameters are kept constant. This includes the absolute abundance of condensible carbon (not bound in CO), which is more relevant than keeping the C/O-ratio constant when comparing stars of different metallicity. The pressure and temperature gradients in the atmospheres of stars, become steeper and flatter, respectively, when the metallicity is reduced, while the radius where the atmosphere becomes opaque is typically associated with a higher gas pressure. This effect can be compensated for by adjusting the velocity amplitude of the variable inner boundary (piston), which is used to simulate the effects of pulsation, to obtain models with comparable kinetic-energy input. Hence, it is more relevant to compare models with similar energy-injections than of similar velocity amplitude. Since there is no evidence for weaker pulsations in low-metallicity AGB stars, we conclude that it is unlikely that low-metallicity C-stars have lower mass-loss rates, than their more metal-rich counterparts with similar stellar parameters, as long as they have a comparable amount of condensible carbon.

    Keyword
    stars: AGB and post-AGB, stars: atmospheres, stars: carbon, stars:, mass-loss, hydrodynamics, radiative transfer
    National Category
    Astronomy, Astrophysics and Cosmology
    Research subject
    Astronomy; Physics
    Identifiers
    urn:nbn:se:uu:diva-99594 (URN)10.1051/0004-6361:200809689 (DOI)000256309400002 ()
    Available from: 2009-03-16 Created: 2009-03-16 Last updated: 2017-12-13Bibliographically approved
    3. Dust Driven Mass Loss from Carbon Stars as Function of Stellar Parameters II: Effects of Relaxing the Small Particle Approximation
    Open this publication in new window or tab >>Dust Driven Mass Loss from Carbon Stars as Function of Stellar Parameters II: Effects of Relaxing the Small Particle Approximation
    (English)Manuscript (Other academic)
    Abstract [en]

    Context. It is well-established that the winds of carbon-rich AGB stars (carbon stars) can be driven by radiation pressure on grains of amorphous carbon and collisional transfer of momentum to the gas. This has been demonstrated by different numerical wind models including time-dependent dust formation, where it has been assumed that the dust grains that form never grow to sizes comparable to wavelengths around the stellar flux maximum (or beyond), which simplifies the treatment of grain opacities considerably. It is not clear, however, if this small particle approximation (SPA) is always valid.

    Aims. In this paper we explore the effects of relaxing the SPA by considering a few less severe approximations for the radiation pressure efficiency,  which include the effects of grain sizes. The purpose of the study is mainly to establish when the SPA can be applied and to quantify the possible errors that may occur when it does not hold.

    Methods. We have computed wind models with time-dependent dust formation and grain-size dependent opacities, where (1) the radiation pressure efficiency is approximated using grain sizes based on various means of the grain size distribution, and (2) where the problem is simplified by assuming a single dust-grain size.

    Results. It is shown that in critical cases, the effect of grain sizes can be significant. Mass-loss rates may increase by a factor of two, or more, and wind speeds may be an order of magnitude higher. Furthermore, all models with grain-size dependent opacities that have resultant winds appear to have much lower degrees of dust condensation, compared to corresponding SPA models. Consequently, the "dust-loss rates" are much lower in the new models. However, for well-developed dust-driven winds, where the dust formation has saturated, the effect of grain sizes on the mass-loss rate and wind speed is almost negligible.

    Conclusions. We conclude that the SPA is, under many circumstances, a reasonable simplification in models of carbon star mass loss. However, critical cases do exist, where especially the effects of momentum transfer due to scattering become significant. It is therefore uncertain whether previous results for winds in the transition regions (between no dust-driven mass loss and well-developed winds) are quantitatively correct. However, we argue that the SPA is a reasonable simplification for strong dust-driven winds. Furthermore, we note that there are other effects (not yet included in the model) than those of grain-size dependent opacities, which may become important in the critical wind regime.

    National Category
    Astronomy, Astrophysics and Cosmology
    Research subject
    Astronomy; Physics
    Identifiers
    urn:nbn:se:uu:diva-99591 (URN)
    Available from: 2009-03-16 Created: 2009-03-16 Last updated: 2010-01-14
    4. Mass loss evolution and the formation of detached shells around TP-AGB stars
    Open this publication in new window or tab >>Mass loss evolution and the formation of detached shells around TP-AGB stars
    2007 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 470, no 1, p. 339-352Article in journal (Refereed) Published
    Abstract [en]

    Context: The origin of the so called “detached shells” around AGB stars is not fully understood, but two common hypotheses state that these shells form either through the interaction of distinct wind phases or an eruptive mass loss associated with a He-shell flash. We present a model of the formation of detached shells around thermal pulse asymptotic giant branch (TP-AGB) stars, based on detailed modelling of mass loss and stellar evolution, leading to a combination of eruptive mass loss and wind interaction. Aims: The purpose of this paper is first of all to connect stellar evolution with wind and mass loss evolution and demonstrate its consistency with observations, but also to show how thin detached shells around TP-AGB stars can be formed. Previous attempts to link mass loss evolution with the formation of detached shells were based on approximate prescriptions for the mass loss and have not included detailed modelling of the wind formation as we do here. Methods: Using stellar parameters sampled from an evolutionary track for a 2 ~M_ȯ star, we have computed the time evolution of the atmospheric layers and wind acceleration region during a typical thermal pulse with detailed radiation hydrodynamical models including dust formation. Based on these results, we simulate the subsequent circumstellar envelope (CSE) evolution using a spherical hydrodynamic model. Results: We find that existing simple mass loss prescriptions all suggest different mass loss evolutions and that they differ from our detailed wind modelling. The most important factor for the formation of a detached shell is the wind velocity evolution which has a strong impact on the wind interaction and the resulting pile-up of matter. Our CSE model shows that a thin shell structure may be formed as a consequence of a rather short phase of intense mass loss in combination with a significant variation in the wind velocity, as obtained by our wind models. This situation can only be obtained for a limited range of amplitudes for the piston boundary used in the dynamic atmosphere models. Conclusions: The combined mass loss eruption and wind interaction scenario for the formation of detached shells around AGB stars (suggested by previous work) is confirmed by the present modelling. Changes in mass loss rate and wind velocity due to a He-shell flash are adequate for creating distinct wind phases and a “snow plow effect” that is necessary to form a geometrically thin detached shell. The derived properties of the shell (i.e. radius, thickness and density) are more or less consistent with existing observational constraints.

    National Category
    Astronomy, Astrophysics and Cosmology
    Identifiers
    urn:nbn:se:uu:diva-14027 (URN)10.1051/0004-6361:20066368 (DOI)000247977400034 ()
    Note
    Appendix A is only available in electronic form at http://www.aanda.orgAvailable from: 2008-01-29 Created: 2008-01-29 Last updated: 2017-12-11Bibliographically approved
    5. Effects of Carbon-excess dependent Mass Loss and Molecular Opacities on Models of C-star Evolution
    Open this publication in new window or tab >>Effects of Carbon-excess dependent Mass Loss and Molecular Opacities on Models of C-star Evolution
    Show others...
    (English)Manuscript (Other academic)
    Abstract [en]

    We present models of stellar evolution for a 2Msun-star of initial metallicity Z = 0.01 (scaled solar), with focus on the carbon-star phase, the effects of increasing carbon excess on mass loss and molecular opacities. We employ a new, state-of-the-art theoretical mass-loss prescription for dust-driven winds, which takes effects of the carbon excess into account and implement a set of composition-dependent molecular opacities, which incorporates the changes due to the evolution of the carbon excess. This work should be regarded as an exploratory and comparative study. The stellar evolution models were computed using the MESA code-package (http://mesa.sourceforge.net), which includes a new fast and efficient code for stellar evolution, but still needs further calibration before quantitative comparisons with observations can be made. We find that the development of the carbon excess is controlling much of the carbon-star evolution. A very pronounced superwind is forming and the termination of the AGB happens soon after the star becomes carbon rich, in fact, after only 4-5 thermal pulses, which is much less than in existing models.

    National Category
    Astronomy, Astrophysics and Cosmology
    Research subject
    Astronomy; Physics
    Identifiers
    urn:nbn:se:uu:diva-99623 (URN)
    Available from: 2009-03-17 Created: 2009-03-17 Last updated: 2010-01-14
    6. The origin of carbon: Low-mass stars and an evolving, initially top-heavy IMF?
    Open this publication in new window or tab >>The origin of carbon: Low-mass stars and an evolving, initially top-heavy IMF?
    2010 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 515, p. A68-Article in journal (Refereed) Published
    Abstract [en]

    Multi-zone chemical evolution models (CEMs), differing in the nucleosynthesis prescriptions (yields) and prescriptions of star formation, have been computed for the Milky Way. All models fit the observed O/H and Fe/H gradients well and reproduce the main characteristics of the gas distribution, but they are also designed to do so. For the C/H gradient the results are inconclusive with regards to yields and star formation. The C/Fe and O/Fe vs. Fe/H, as well as C/O vs. O/H trends predicted by the models for the solar neighbourhood zone were compared with stellar abundances from the literature. For O/Fe vs. Fe/H all models fit the data, but for C/O vs. O/H, only models with increased carbon yields for zero-metallicity stars or an evolving initial mass function provide good fits. Furthermore, a steep star formation threshold in the disc can be ruled out since it predicts a steep fall-off in all abundance gradients beyond a certain galactocentric distance (similar to 13 kpc) and cannot explain the possible flattening of the C/H and Fe/H gradients in the outer disc seen in observations. Since in the best-fit models the enrichment scenario is such that carbon is primarily produced in low-mass stars, it is suggested that in every environment where the peak of star formation happened a few Gyr back in time, winds of carbon-stars are responsible for most of the carbon enrichment. However, a significant contribution by zero-metallicity stars, especially at very early stages, and by winds of high-mass stars, which are increasing in strength with metallicity, cannot be ruled out by the CEMs presented here. In the solar neighbourhood, as much as 80%, or as little as 40% of the carbon may have been injected to the interstellar medium by low- and intermediate-mass stars. The stellar origin of carbon remains an open question, although production in low-and intermediate-mass stars appears to be the simplest explanation of observed carbon abundance trends.

    Keyword
    Galaxy: abundances; Galaxy: evolution; Galaxy: formation; Galaxy: stellar content; Stars: carbon; Stars: mass-loss
    National Category
    Physical Sciences
    Research subject
    Astronomy; Physics
    Identifiers
    urn:nbn:se:uu:diva-99581 (URN)10.1051/0004-6361/200913315 (DOI)000280505000037 ()
    Note
    Uppdaterad från Manuskript till Artikel 20101208Available from: 2009-03-16 Created: 2009-03-16 Last updated: 2017-12-13Bibliographically approved
  • 29.
    Mattsson, Lars
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Astronomy and Space Physics.
    The origin of carbon revisited: winds of carbon-stars2008In: Physica Scripta, ISSN 0031-8949, E-ISSN 1402-4896, Vol. T133, p. 014027-Article in journal (Refereed)
    Abstract [en]

    Chemical evolution models, differing in the nucleosynthesis prescriptions (yields) for carbon, nitrogen and oxygen, have been computed for the Milky Way and Andromeda (NGC 224). All models fit the observed O/H gradients well and reproduce the main characteristics of the gas distributions, but they are also designed to do so. The N/O gradient for NGC 224 cannot be reproduced without ad hoc modifications to the yields and a similar result is obtained for the Milky Way N/O gradient, although in the latter case the slopes of the gradients obtained with unmodified yields are consistent with the observed gradient. For the C/O gradients (obtained from B stars) the results are inconclusive. The C/Fe, N/Fe, O/Fe versus Fe/H, as well as C/O versus O/H trends predicted by the models for the solar neighbourhood were compared with stellar abundances from the literature. For O/Fe versus Fe/H, all models fit the data, but for C/Fe, N/Fe versus Fe/H and C/O versus O/H, only modified sets of yields provide good fits. Since in the best-fit model, the yields were modified such that carbon should be primarily produced in low-mass stars, it is quite possible that in every environment where the peak of star formation happened a few Gyr back in time, the winds of carbon stars are responsible for most of the carbon enrichment, although models with a significant contribution from high-mass stars cannot be ruled out. In the solar neighbourhood, almost two-thirds of the carbon in the interstellar medium may come from carbon stars. Finally, the challenges met by stellar evolution and nucleosynthesis modelling due to this 'carbon star hypothesis' for the origin of carbon are discussed. It is suggested that a mass-loss prescription where the mass-loss rate depends on the carbon excess may act as a self-regulating mechanism for how much carbon a carbon star can deliver to the interstellar medium.

  • 30.
    Mattsson, Lars
    et al.
    Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Physics, Department of Physics and Astronomy. Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Physics, Department of Physics and Astronomy, Division of Astronomy and Space Physics.
    Herwig, Falk
    Dept. of Physics and Astronomy, University of Victoria, Canada.
    Höfner, Susanne
    Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Physics, Department of Physics and Astronomy. Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Physics, Department of Physics and Astronomy, Division of Astronomy and Space Physics.
    Wahlin, Rurik
    Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Physics, Department of Physics and Astronomy. Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Physics, Department of Physics and Astronomy, Division of Astronomy and Space Physics.
    Lederer, Michael
    Dept. of Astronomy, University of Vienna, Austria.
    Paxton, Bill
    Kavli Institute for Theoretical Physics, University of California, Santa Barbara, USA..
    Effects of Carbon-excess dependent Mass Loss and Molecular Opacities on Models of C-star EvolutionManuscript (Other academic)
    Abstract [en]

    We present models of stellar evolution for a 2Msun-star of initial metallicity Z = 0.01 (scaled solar), with focus on the carbon-star phase, the effects of increasing carbon excess on mass loss and molecular opacities. We employ a new, state-of-the-art theoretical mass-loss prescription for dust-driven winds, which takes effects of the carbon excess into account and implement a set of composition-dependent molecular opacities, which incorporates the changes due to the evolution of the carbon excess. This work should be regarded as an exploratory and comparative study. The stellar evolution models were computed using the MESA code-package (http://mesa.sourceforge.net), which includes a new fast and efficient code for stellar evolution, but still needs further calibration before quantitative comparisons with observations can be made. We find that the development of the carbon excess is controlling much of the carbon-star evolution. A very pronounced superwind is forming and the termination of the AGB happens soon after the star becomes carbon rich, in fact, after only 4-5 thermal pulses, which is much less than in existing models.

  • 31.
    Mattsson, Lars
    et al.
    Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Physics, Department of Physics and Astronomy. Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Physics, Department of Physics and Astronomy, Division of Astronomy and Space Physics.
    Höfner, Susanne
    Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Physics, Department of Physics and Astronomy. Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Physics, Department of Physics and Astronomy, Division of Astronomy and Space Physics.
    Dust Driven Mass Loss from Carbon Stars as Function of Stellar Parameters II: Effects of Relaxing the Small Particle ApproximationManuscript (Other academic)
    Abstract [en]

    Context. It is well-established that the winds of carbon-rich AGB stars (carbon stars) can be driven by radiation pressure on grains of amorphous carbon and collisional transfer of momentum to the gas. This has been demonstrated by different numerical wind models including time-dependent dust formation, where it has been assumed that the dust grains that form never grow to sizes comparable to wavelengths around the stellar flux maximum (or beyond), which simplifies the treatment of grain opacities considerably. It is not clear, however, if this small particle approximation (SPA) is always valid.

    Aims. In this paper we explore the effects of relaxing the SPA by considering a few less severe approximations for the radiation pressure efficiency,  which include the effects of grain sizes. The purpose of the study is mainly to establish when the SPA can be applied and to quantify the possible errors that may occur when it does not hold.

    Methods. We have computed wind models with time-dependent dust formation and grain-size dependent opacities, where (1) the radiation pressure efficiency is approximated using grain sizes based on various means of the grain size distribution, and (2) where the problem is simplified by assuming a single dust-grain size.

    Results. It is shown that in critical cases, the effect of grain sizes can be significant. Mass-loss rates may increase by a factor of two, or more, and wind speeds may be an order of magnitude higher. Furthermore, all models with grain-size dependent opacities that have resultant winds appear to have much lower degrees of dust condensation, compared to corresponding SPA models. Consequently, the "dust-loss rates" are much lower in the new models. However, for well-developed dust-driven winds, where the dust formation has saturated, the effect of grain sizes on the mass-loss rate and wind speed is almost negligible.

    Conclusions. We conclude that the SPA is, under many circumstances, a reasonable simplification in models of carbon star mass loss. However, critical cases do exist, where especially the effects of momentum transfer due to scattering become significant. It is therefore uncertain whether previous results for winds in the transition regions (between no dust-driven mass loss and well-developed winds) are quantitatively correct. However, we argue that the SPA is a reasonable simplification for strong dust-driven winds. Furthermore, we note that there are other effects (not yet included in the model) than those of grain-size dependent opacities, which may become important in the critical wind regime.

  • 32.
    Mattsson, Lars
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Astronomy and Space Physics.
    Wahlin, Rurik
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Astronomy and Space Physics.
    Höfner, Susanne
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Astronomy and Space Physics.
    Dust driven mass loss from carbon stars as a function of stellar parameters I: A grid of solar-metallicity wind models2010In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 509, no 1, p. 13-Article in journal (Refereed)
    Abstract [en]

    Context. Knowing how the mass loss of carbon-rich AGB stars depends on stellar parameters is crucial for stellar evolution modelling, as well as for the understanding of when and how circumstellar structures emerge around these stars, e.g., dust shells and so-called detached shells of expelled gas.

    Aims. The purpose of this paper is to explore the stellar parameter space using a numerical radiation hydrodynamic (RHD) model of carbon-star atmospheres, including a detailed description of dust formation and frequency-dependent radiative transfer, in order to determine how the mass loss of carbon stars changes with stellar parameters.

    Methods. We have computed a grid of 900 numeric dynamic model atmospheres (DMAs) using a well-tested computer code. This grid of models covers most of the expected combinations of stellar parameters, which are the stellar temperature, the stellar luminosity, the stellar mass, the abundance of condensible carbon, and the velocity amplitude of the pulsation.

    Results. The resultant mass-loss rates and wind speeds are clearly affected by the choice of stellar temperature, mass, luminosity and the abundance of available carbon. In certain parts of the parameter space there is also an inevitable mass-loss threshold, below which a dust-driven wind is not possible. Contrary to some previous studies, we find a strong dependence on the abundance of free carbon, which turns out to be a critical parameter. Furthermore, we have found that the dust grains that form in the atmosphere may grow too large for the commonly-used small-particle approximation of the dust opacity to be strictly valid. This may have some bearing on the wind properties, although further study of this problem is needed before quantitative conclusions can be drawn.

    Conclusions. The wind properties show relatively simple dependences on stellar parameters above the mass-loss threshold, while the threshold itself is of a more complicated nature. Hence, we chose not to derive any simplistic mass-loss formula, but rather provide a mass-loss prescription in the form of aeasy-to-use FORTRAN routine. Since this mass-loss routine is based on data coming from an essentially self-consistent model of mass loss, it may therefore serve as a better mass-loss prescription for stellar evolution calculations than empirical formulae. Furthermore, we conclude that there are still some issues that need to be investigated, such as the role of grain-sizes.

  • 33.
    Mattsson, Lars
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Division of Astronomy and Space Physics.
    Wahlin, Rurik
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Division of Astronomy and Space Physics.
    Höfner, Susanne
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Division of Astronomy and Space Physics.
    Eriksson, Kjell
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Division of Astronomy and Space Physics.
    Intense mass loss from C-rich AGB stars at low metallicity?2008In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 484, no 1, p. L5-L8Article in journal (Refereed)
    Abstract [en]

    We argue that the energy injection of pulsations may be of greater importance to the mass-loss rate of AGB stars than metallicity, and that the mass-loss trend with metallicity is not as simple as sometimes assumed. Using our detailed radiation hydrodynamical models that include dust formation, we illustrate the effects of pulsation energy on wind properties. We find that the mass-loss rate scales with the kinetic energy input by pulsations as long as a dust-saturated wind does not occur, and all other stellar parameters are kept constant. This includes the absolute abundance of condensible carbon (not bound in CO), which is more relevant than keeping the C/O-ratio constant when comparing stars of different metallicity. The pressure and temperature gradients in the atmospheres of stars, become steeper and flatter, respectively, when the metallicity is reduced, while the radius where the atmosphere becomes opaque is typically associated with a higher gas pressure. This effect can be compensated for by adjusting the velocity amplitude of the variable inner boundary (piston), which is used to simulate the effects of pulsation, to obtain models with comparable kinetic-energy input. Hence, it is more relevant to compare models with similar energy-injections than of similar velocity amplitude. Since there is no evidence for weaker pulsations in low-metallicity AGB stars, we conclude that it is unlikely that low-metallicity C-stars have lower mass-loss rates, than their more metal-rich counterparts with similar stellar parameters, as long as they have a comparable amount of condensible carbon.

  • 34.
    Norin, Lars
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Astronomy and Space Physics.
    Secondary Electromagnetic Radiation Generated by HF Pumping of the Ionosphere2008Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Electromagnetic waves can be used to transmit information over long distances and are therefore often employed for communication purposes. The electromagnetic waves are reflected off material objects on their paths and interact with the medium through which they propagate. For instance, the plasma in the ionosphere can refract and even reflect radio waves propagating through it.

    By increasing the power of radio waves injected into the ionosphere, the waves start to modify the plasma, resulting in the generation of a wide range of nonlinear processes, including turbulence, in particular near the reflection region. By systematically varying the injected radio waves in terms of frequency, power, polarisation, duty cycle, inclination, etc. the ionosphere can be used as an outdoor laboratory for investigating fundamental properties of the near-Earth space environment as well as of plasma turbulence. In such ionospheric modification experiments, it has been discovered that the irradiation of the ionosphere by powerful radio waves leads to the formation of plasma density structures and to the emission of secondary electromagnetic radiation, a phenomenon known as stimulated electromagnetic emission. These processes are highly repeatable and have enabled systematic investigations of the nonlinear properties of the ionospheric plasma.

    In this thesis we investigate features of the plasma density structures and the secondary electromagnetic radiation. In a theoretical study we analyse a certain aspect of the formation of the plasma structures. The transient dynamics of the secondary radiation is investigated experimentally in a series of papers, focussing on the initial stage as well as on the decay. In one of the papers we use the transient dynamics of the secondary radiation to reveal the intimate relation between certain features of the radiation and structures of certain scales. Further, we present measurements of unprecedentedly strong secondary radiation, attributed to stimulated Brillouin scattering, and report measurements of the secondary radiation using a novel technique imposed on the transmitted radio waves.

    List of papers
    1. On the linear stage of thermal parametric instabilities in the ionosphere excited by HF pumping near electron gyroharmonics
    Open this publication in new window or tab >>On the linear stage of thermal parametric instabilities in the ionosphere excited by HF pumping near electron gyroharmonics
    2006 (English)In: Advances in Space Research, ISSN 0273-1177, E-ISSN 1879-1948, Vol. 38, no 11, p. 2527-2532Article in journal (Refereed) Published
    Abstract [en]

    Thresholds and growth rates of the thermal parametric instability in the ionosphere were studied taking into account the regular inhomogeneity (altitude dependence) of the plasma density and of the magnetic field strength. A noticeable decrease of the pump wave threshold power and an increase of the growth rate occurs when f0 approaches the double resonance frequency fd = nfce(zd) = fUH(zd), zd being the altitude for double resonance. Particularly, the threshold power decreases relative to the non-resonant case by a factor 3-7 at |f0 - fd(zd)| ≲ 15 kHz for n = 3 and up to a factor 3 for n = 4. The instability growth rate can increase with a factor of 103-104 relative to the non-resonant case. The decrease of the threshold and increase of the growth rate occur for quite small scales of the striations, l ∼ 1 m for n = 3 and l ∼ 0.5 m for n = 4.

    Keyword
    Electron gyroharmonics, Inhomogeneous inosphere, Thermal parametric instability
    National Category
    Physical Sciences
    Identifiers
    urn:nbn:se:uu:diva-97793 (URN)10.1016/j.asr.2004.12.014 (DOI)000202988800036 ()
    Available from: 2008-11-21 Created: 2008-11-21 Last updated: 2017-12-14Bibliographically approved
    2. Recovery of the ponderomotive parametric instability after long pumping of the ionosphere
    Open this publication in new window or tab >>Recovery of the ponderomotive parametric instability after long pumping of the ionosphere
    Show others...
    2007 (English)In: Advances in Space Research, ISSN 0273-1177, E-ISSN 1879-1948, Vol. 40, no 3, p. 377-383Article in journal (Refereed) Published
    Abstract [en]

    We show that during high frequency (HF) pumping of the overhead ionosphere with a pump wave frequency slightly above the local fifth electron gyroharmonic, when one switches the pump wave from continuous pumping to low duty cycle pulse pumping, the stimulated electromagnetic emission (SEE) features related to the ponderomotive parametric instability (PPI) near the pump wave reflection point, such as the ponderomotive narrow continuum and the narrow continuum maximum, recover and become dominating in the SEE spectra after a few seconds after the switch. Simultaneously, the ionospherically reflected pump wave signal exhibits a recovery of ponderomotive self-action and the damped quasi-periodical oscillations (spikes) of the reflected pump wave, also related to the PPI development. The intensity of the down-shifted maximum (DM) feature in the SEE spectra increases for about 1–2 s after the switch to the pulse mode, but later, together with PPI related phenomena recovery, the DM intensity decreases noticeably. The phenomena described are attributed to the relaxation of small-scale irregularities (striations). The latter provide strong scattering of the pump wave into upper hybrid waves, strong shielding of the pump wave reflection point, and generation of the upper-hybrid related DM feature.

    Keyword
    HF pumping of the ionosphere, Stimulated electromagnetic emissions, Ponderomotive and thermal parametric instabilities
    National Category
    Physical Sciences
    Identifiers
    urn:nbn:se:uu:diva-97794 (URN)10.1016/j.asr.2007.04.067 (DOI)000253589400013 ()
    Available from: 2008-11-21 Created: 2008-11-21 Last updated: 2017-12-14Bibliographically approved
    3. Transient dynamics of secondary radiation from an HF pumped magnetized space plasma
    Open this publication in new window or tab >>Transient dynamics of secondary radiation from an HF pumped magnetized space plasma
    Show others...
    2007 (English)In: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 112, no A9, p. A09303-Article in journal (Refereed) Published
    Abstract [en]

    In order to systematically analyze the transient wave and radiation processes that are excited when a high-frequency (HF) radio wave is injected into a magnetized space plasma, we have measured the secondary radiation, or stimulated electromagnetic emission (SEE), from the ionosphere, preconditioned such that geomagnetic field-aligned plasma irregularities are already present. The transient dynamics experiments were made using a duty cycle of the HF radio wave of 200 ms (180 ms on and 20 ms off) and 100 ms (80 ms on and 20 ms off) for various frequencies near the fifth harmonic of the local ionospheric electron cyclotron frequency. Within the first 10 ms after the radio pulse turn-on, frequency downshifted structures of the SEE exhibit an overshoot with a maximum at 3 ms < t < 8 ms, whereas the upshifted spectral components do not exhibit this feature. The relative magnitude of the overshoot is strongly dependent on the frequency offset of the pump from the harmonic of the electron cyclotron frequency. A transient blue-shifted frequency component is identified. This component is upshifted from the pump by 14 kHz < Δ f< 55 kHz and exists only within the first 10 ms after the radio pulse turn-on. On a longer time scale we analyze the amplitude modulation, or "ringing," of the reflected radio wave, (also known as "quasi-periodic oscillations" or "spikes"). The ringing has a frequency of the order 15-20 Hz and we show that this phenomenon is also present in the SEE sidebands and is synchronized with the ringing of the reflected HF wave itself.

    Keyword
    Quasi periodic variation, Amplitude modulation, Frequency shift, Blueshift, exhibits, harmonics, Electron cyclotron frequency, cycles, experimental studies, geomagnetic field, ionosphere, electromagnetic emission, high frequency, Radio wave, Plasma, radiation, dynamics, transient phenomena
    National Category
    Physical Sciences
    Identifiers
    urn:nbn:se:uu:diva-97795 (URN)10.1029/2007JA012454 (DOI)000249373600012 ()
    Available from: 2008-11-21 Created: 2008-11-21 Last updated: 2017-12-14Bibliographically approved
    4. Ionospheric plasma density irregularities measured by stimulated electromagnetic emission
    Open this publication in new window or tab >>Ionospheric plasma density irregularities measured by stimulated electromagnetic emission
    Show others...
    2008 (English)In: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 113, no A9, p. A09314-Article in journal (Refereed) Published
    Abstract [en]

    It is well known that ionospheric plasma turbulence can be conveniently generated by controlled injection of powerful high-frequency radio beams from the ground. Irradiation of the ionosphere with such radio waves leads to the formation of plasma density structures, striations, and the generation of secondary electromagnetic radiation, a phenomenon known as stimulated electromagnetic emission (SEE). In this paper we present experimental results of the dependence of SEE on decreasing excitation levels of the striations. In the experiments the frequency of the injected radio beam was varied near the fifth harmonic of the local ionospheric electron gyro frequency. We use the SEE measurements to obtain transverse length scales of the striations involved in the generation of the SEE. Our results show that different spectral features of the SEE display different temporal dynamics, suggesting that they are related to striations with different transverse length scales (1 less than or similar to L-perpendicular to less than or similar to 25 m).

    National Category
    Astronomy, Astrophysics and Cosmology
    Identifiers
    urn:nbn:se:uu:diva-97796 (URN)10.1029/2008JA013338 (DOI)000259398700002 ()
    Available from: 2008-11-21 Created: 2008-11-21 Last updated: 2017-12-14Bibliographically approved
    5. Unprecedentedly strong and narrow electromagnetic emissions stimulated by high-frequency radio waves in the ionosphere
    Open this publication in new window or tab >>Unprecedentedly strong and narrow electromagnetic emissions stimulated by high-frequency radio waves in the ionosphere
    Show others...
    2009 (English)In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 102, no 6, p. 065003-Article in journal (Refereed) Published
    Abstract [en]

    Experimental results of secondary electromagnetic radiation, stimulated by high-frequency radio waves irradiating the ionosphere, are reported. We have observed emission peaks, shifted in frequency up to a few tens of Hertz from radio waves transmitted at several megahertz. These emission peaks are by far the strongest spectral features of secondary radiation that have been reported. The emissions are attributed to stimulated Brillouin scattering, long predicted but hitherto never unambiguously identified in high-frequency ionospheric interaction experiments. The experiments were performed at the High-Frequency Active Auroral Research Program (HAARP), Alaska, USA.

    National Category
    Physical Sciences
    Identifiers
    urn:nbn:se:uu:diva-97797 (URN)10.1103/PhysRevLett.102.065003 (DOI)000263389500029 ()
    Available from: 2008-11-21 Created: 2008-11-21 Last updated: 2017-12-14Bibliographically approved
    6. Radio pumping of the ionosphere with orbital angular momentum
    Open this publication in new window or tab >>Radio pumping of the ionosphere with orbital angular momentum
    Show others...
    In: Physical Review Letters, ISSN 0031-9007Article in journal (Refereed) Submitted
    Identifiers
    urn:nbn:se:uu:diva-97798 (URN)
    Available from: 2008-11-21 Created: 2008-11-21Bibliographically approved
  • 35.
    Norin, Lars
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Astronomy and Space Physics. Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Grach, S. M.
    Leyser, Thomas
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Thidé, Bo
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Sergeev, E. N.
    Berlin, M.
    Ionospheric plasma density irregularities measured by stimulated electromagnetic emission2008In: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 113, no A9, p. A09314-Article in journal (Refereed)
    Abstract [en]

    It is well known that ionospheric plasma turbulence can be conveniently generated by controlled injection of powerful high-frequency radio beams from the ground. Irradiation of the ionosphere with such radio waves leads to the formation of plasma density structures, striations, and the generation of secondary electromagnetic radiation, a phenomenon known as stimulated electromagnetic emission (SEE). In this paper we present experimental results of the dependence of SEE on decreasing excitation levels of the striations. In the experiments the frequency of the injected radio beam was varied near the fifth harmonic of the local ionospheric electron gyro frequency. We use the SEE measurements to obtain transverse length scales of the striations involved in the generation of the SEE. Our results show that different spectral features of the SEE display different temporal dynamics, suggesting that they are related to striations with different transverse length scales (1 less than or similar to L-perpendicular to less than or similar to 25 m).

  • 36.
    Norin, Lars
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Astronomy and Space Physics. Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Grach, S. M.
    Thidé, Bo
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Sergeev, E. N.
    Leyser, T. B.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
    Transient dynamics of secondary radiation from an HF pumped magnetized space plasma2007In: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 112, no A9, p. A09303-Article in journal (Refereed)
    Abstract [en]

    In order to systematically analyze the transient wave and radiation processes that are excited when a high-frequency (HF) radio wave is injected into a magnetized space plasma, we have measured the secondary radiation, or stimulated electromagnetic emission (SEE), from the ionosphere, preconditioned such that geomagnetic field-aligned plasma irregularities are already present. The transient dynamics experiments were made using a duty cycle of the HF radio wave of 200 ms (180 ms on and 20 ms off) and 100 ms (80 ms on and 20 ms off) for various frequencies near the fifth harmonic of the local ionospheric electron cyclotron frequency. Within the first 10 ms after the radio pulse turn-on, frequency downshifted structures of the SEE exhibit an overshoot with a maximum at 3 ms < t < 8 ms, whereas the upshifted spectral components do not exhibit this feature. The relative magnitude of the overshoot is strongly dependent on the frequency offset of the pump from the harmonic of the electron cyclotron frequency. A transient blue-shifted frequency component is identified. This component is upshifted from the pump by 14 kHz < Δ f< 55 kHz and exists only within the first 10 ms after the radio pulse turn-on. On a longer time scale we analyze the amplitude modulation, or "ringing," of the reflected radio wave, (also known as "quasi-periodic oscillations" or "spikes"). The ringing has a frequency of the order 15-20 Hz and we show that this phenomenon is also present in the SEE sidebands and is synchronized with the ringing of the reflected HF wave itself.

  • 37. Obbrugger, M.
    et al.
    Lueftinger, T.
    Nesvacil, N.
    Kochukhov, Oleg
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Astronomy and Space Physics.
    Weiss, W. W.
    First results on the multi-element Doppler imaging of the CP star HD 39802008In: Contributions of the Astronomical Observatory Skalnate Pleso, ISSN 1335-1842, Vol. 38, no 2, p. 347-352Article in journal (Refereed)
    Abstract [en]

    We present the first results of an ongoing analysis of the inhomogeneous elemental surface distribution in the lithium-rich magnetic Ap star HD 3980. To derive surface maps for Fe, Pr, Li, Gd and Cr the Doppler imaging inversion code INVERS12 was applied. For this work high resolution time-series observations obtained with the VLT Ultraviolet and Visual Echelle Spectrograph (UVES) were used.

  • 38. Oestlin, Goeran
    et al.
    Zackrisson, Erik
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Astronomy and Space Physics.
    Sollerman, Jesper
    Mattila, Seppo
    Hayes, Matthew
    Constraining the mass of the GRB 030329 progenitor2008In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 387, no 3, p. 1227-1236Article in journal (Refereed)
    Abstract [en]

    The long-duration gamma-ray burst (GRB) GRB 030329, associated with the supernova (SN) SN 2003dh, occurred inside a star-forming dwarf galaxy at redshift z = 0.1685. The low redshift, and a rich set of archival Hubble Space Telescope (HST ) images, makes this GRB well suited for a detailed study of the stellar population in the immediate vicinity of the explosion. Since the lifetime of a star is directly tied to its mass, the age of the stellar population can be used to put constraints on the GRB and SN progenitor mass. From the HST images, we extract the colours of the precise site from which the GRB originated, and find that the colours are significantly different from those of the overall host galaxy and the surrounding starburst environment. We have used spectral evolutionary models, including nebular emission, to carefully constrain the age of the stellar population, and hence the progenitor, at the very explosion site. For instantaneous burst models, we find that a population age of similar to 5 Myr best matches the data, suggesting a very massive (M(ZAMS) > 50 M(circle dot)) star as the progenitor, with an upper limit of 8 Myr (M(ZAMS) > 25 M(circle dot)). For more extended star formation scenarios, the inferred progenitor age is in most cases still very young (< 7 Myr, implying M(ZAMS) > 25 M(circle dot)), with an upper limit of 20 Myr (M(ZAMS) > 12 M(circle dot)). These age estimates are an order of magnitude lower than the ages inferred from the overall host galaxy colours, indicating that progenitor mass estimates based on data for spatially unresolved GRB host galaxies will in general be of limited use. Our results are consistent with the collapsar scenario.

  • 39.
    Onehag, Anna
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Astronomy and Space Physics.
    The calibration of Stromgren uvby-H beta photometry for late-type stars: a model atmosphere approach2008In: Physica Scripta, ISSN 0031-8949, E-ISSN 1402-4896, Vol. T133, p. 014021-Article in journal (Refereed)
    Abstract [en]

    The use of model atmospheres for deriving fundamental parameters of stars, such as T-eff, log g and [Fe/H], will increase as we find and explore extreme stellar populations where empirical calibrations are not yet available. Moreover, calibrations for upcoming large satellite missions of new spectrophotometric indices, similar to the uvby-H beta system, will be needed. I aim to test the power of theoretical calibrations based on a new generation of MARCS models by comparisons with observational photometric data. I calculate synthetic uvby-H beta colour indices from synthetic spectra. A sample of 389 stars is used for a direct comparison of the synthetic indices versus empirical data and for scrutinizing the possibilities of theoretical calibrations for temperature, metallicity and gravity. I show that the temperature sensitivity of the synthetic (b-y) colour is very close to its empirical counterpart, whereas the temperature scale based on H beta shows a slight offset. The theoretical metallicity sensitivity of the m(1) index (and for G-type stars its combination with c(1)) is somewhat higher than the empirical one, based on spectroscopic determinations. The gravity sensitivity of the synthetic c(1) index shows a satisfactory behaviour when compared with observations of F stars. For stars cooler than the sun, a deviation is significant in the c(1)-(b-y) diagram. The theoretical calibrations of (b-y), (v-y) and c(1) seem to work well for Pop II stars and leads to effective temperatures for globular cluster stars supporting recent claims by Korn et al (2007) that atomic diffusion occurs in stars near the turn-off point of NGC 6397. Synthetic colours of stellar atmospheres can indeed be used, in many cases, to derive reliable fundamental stellar parameters. The deviations seen as compared with observational data could be due to incomplete line lists but are possibly also due to effects of assuming plane-parallel or spherical geometry and LTE. For a full length article, see Onehag et al (2008, in preparation).

  • 40. Perez, A. E. Garcia
    et al.
    Christlieb, N.
    Ryan, S. G.
    Beers, T. C.
    Aoki, W.
    Asplund, M.
    Barklem, Paul S.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Astronomy and Space Physics.
    Bessell, M. S.
    Eriksson, Kjell
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Astronomy and Space Physics.
    Frebel, A.
    Gustafsson, Bengt
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Astronomy and Space Physics.
    Korn, Andreas J.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Astronomy and Space Physics.
    Nordstrom, B.
    Norris, J. E.
    A new sample of extremely/ultra metal-poor stars2008In: Physica Scripta, ISSN 0031-8949, E-ISSN 1402-4896, Vol. T133, p. 014036-Article in journal (Refereed)
    Abstract [en]

    A sample of 30 very metal-poor stars from the Hamburg-European Southern Observatory (ESO) objective-prism survey have been observed at high spectral resolution at the Very Large Telescope (VLT) using the Ultraviolet and Visual Echelle Spectrograph (UVES). Two of the observed stars are very interesting not only because of their very low iron content, approximately four orders of magnitude lower than the solar value, but also because we detected the neutral lithium resonance line at 670.8 nm. Hydrogen lines suggest that the two observed stars have effective temperatures around 6000-6250K and according to isochrones, they are either on the main-sequence or on the subgiant branch, in which case they would probably be the most metal-poor dwarfs or warm subgiants with lithium detections known. These detections would allow to determine more accurately the slope of the trend of the lithium abundance with [Fe/H] than was possible with samples of unevolved stars restricted to higher metallicities.

  • 41.
    Piskunov, Nikolai
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Astronomy and Space Physics.
    Doppler imaging2008In: Physica Scripta, ISSN 0031-8949, E-ISSN 1402-4896, Vol. T133, p. 014017-Article in journal (Refereed)
    Abstract [en]

    In this paper, I present a short review of the history and modern status of Doppler imaging techniques, highlighting their dependence on the knowledge of the fundamental stellar parameters, the quality of stellar atmospheric models and the accuracy of spectral synthesis.

  • 42.
    Plez, Bertrand
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Astronomy and Space Physics.
    MARCS model atmospheres2008In: Physica Scripta, ISSN 0031-8949, E-ISSN 1402-4896, Vol. T133, p. 014003-Article in journal (Refereed)
    Abstract [en]

    In this review presented at the Symposium A Stellar Journey in Uppsala, June 2008, I give an account of the historical development of the MARCS code, and its premises from the first version published in 1975 to the 2008 grid. The primary driver for the development team who constantly strive to include the best possible physical data, is the science that can be done with the models. A few preliminary comparisons of M star model spectra to spectrophotometric observations are presented. Particular results related to opacity effects are discussed. The size of errors in spectral energy distribution (SED) and model thermal stratification is estimated for different densities of wavelength sampling. The number of points used in the MARCS 2008 grid (108 000) is large enough to ensure errors of only a few K in all models of the grid, except the optically very thin layers of metal-poor stars. Errors in SEDs may reach about 10% locally in the UV. The published sampled SEDs are thus adequate to compute synthetic broadband photometry, but higher resolution spectra will be computed in the near future and published as well on the MARCS site (marcs.astro.uu.se). Test model calculations with TiO line opacity accounted for in scattering show significant cooling of the upper atmospheric layers of red giants. Rough estimates of radiative and collisional time scales for electronic transitions of TiO indicate that scattering may well be the dominant mechanism in these lines. However, models constructed with this hypothesis are incompatible with optical observations of TiO (Arcturus) or IR observations of OH (Betelgeuse), although they may succeed in explaining H2O line observations. More work is needed in that direction.

  • 43.
    Ryde, Nils
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Astronomy and Space Physics.
    Stellar spectroscopy in the infrared at high spectral resolution2009In: Physica Scripta, ISSN 0031-8949, E-ISSN 1402-4896, Vol. T134, p. 014001-Article in journal (Refereed)
    Abstract [en]

    I present aspects of infrared (IR) stellar spectroscopy at high spectral resolution, where consideration is mostly given to near-IR (1-5 mu m) spectroscopy of cool stars. Discussion is presented on applying IR spectroscopy to study the origin of sulfur, the origin of the Galactic Bulge, and the 12 mu m Mg I emission feature. I briefly discuss the atomic and molecular data requirements of stellar astrophysicists.

  • 44. Sachkov, M.
    et al.
    Kochukhov, Oleg
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Astronomy and Space Physics.
    Ryabchikova, T.
    Leone, F.
    Bagnulo, S.
    Weiss, W. W.
    Spectroscopic study of pulsations in the atmosphere of roAp star 10 Aql2008In: Contributions of the Astronomical Observatory Skalnate Pleso, ISSN 1335-1842, Vol. 38, no 2, p. 323-328Article in journal (Refereed)
    Abstract [en]

    We present the analysis of spectroscopic time-series observations of the roAp star 10 Aql. Observations were carried out in July 2006 with the UVES and SARG spectrographs and simultaneously with the MOST mini-satellite photometry. All these data were analysed for radial velocity (RV) variations. About 150 lines out of the 1000 measured reveal a clear pulsation signal. Frequency analysis of the spectroscopic data gives three frequencies that coincide with the photometric ones. Phase-amplitude diagrams created for the lines of different elements/ions show that atmospheric pulsations may be represented by a superposition of standing and running wave components, similar to other roAp stars. The highest RV amplitudes, 300-400 ms(-1), were measured for Ce II, Dy III, Tb III, and two unidentified lines at lambda 5471, 5556 angstrom. We discovered a approximate to 0.4 period phase jump in the RV measurements across the Nd III line profiles, indicating the presence of the pulsation node in stellar atmospheres. The phase jump occurs at nearly the same atmospheric layer for the two main frequencies. There is no rotational modulation in the average spectra for the 6 different nights we analysed.

  • 45. Shulyak, D.
    et al.
    Ryabchikova, T.
    Kildiyarova, R.
    Kochukhov, Oleg
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Astronomy and Space Physics.
    Realistic model atmosphere and revised abundances of the coolest Ap star HD 1010652010In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 520, p. A88-Article in journal (Refereed)
    Abstract [en]

    Aims. Among the known Ap stars, HD 101065 is probably one of the most interesting objects, demonstrating very rich spectra of rare-earth elements (REE). Strongly peculiar photometric parameters of this star can not be fully reproduced by any modern theoretical calculations, even those accounting for realistic chemistry of its atmosphere. In this study we investigate a role of missing REE line opacity and construct a self-consistent atmospheric model based on accurate abundance and chemical stratification analysis. Methods. We employed the LLMODELS stellar model atmosphere code together with DDAFIT and SYNTHMAG software packages to derive homogeneous and stratified abundances for 52 chemical elements and to construct a self-consistent model of HD 101065 atmosphere. The opacity in REE lines is accounted for in details, by using up-to-date extensive theoretical calculations. Results. We show that REE elements play a key role in the radiative energy balance in the atmosphere of HD 101065, leading to the strong suppression of the Balmer jump and energy redistribution very different from that of normal stars. Introducing new line lists of REEs allowed us to reproduce, for the first time, spectral energy distribution of HD 101065 and achieve a better agreement between the unusually small observed Stromgren c(1) index and the model predictions. Using combined photometric and spectroscopic approaches and based on the iterative procedure of abundance and stratification analysis we find effective temperature of HD 101065 to be T-eff = 6400 K.

  • 46. Sprague, A. L.
    et al.
    Hanna, Donaldson
    Kozlowski, H
    Helbert, J
    Maturilli, A
    Warell, J. B.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Astronomy and Space Physics.
    Hora, L
    Spectral emissivity measurements of Mercury's surface indicate Mg- and Ca-rich mineralogy, K-spar, Na-rich plagioclase, rutile, with possible perovskite, and garnet2009In: Planetary and Space Science, ISSN 0032-0633, E-ISSN 1873-5088, Vol. 57, no 3, p. 364-383Article in journal (Refereed)
    Abstract [en]

    Mid-infrared 2-D spectroscopic measurements from 8.0 to 12.7 gm of Mercury were taken using Boston University's Mid-Infrared Spectrometer and Imager (MIRSI) mounted on the NASA Infrared Telescope Facility (IRTF) on Mauna Kea, Hawaii, 7-11 April 2006. Measurements reported here cover radar bright region C, a dark plains region west of Caloris Basin, and the interior of Caloris Basin. By use of spectral deconvolution with a large spectral library composed of many mineral compositions and grain size separates, we fitted, or "unmixed", the Mercury spectra. We find mineral suites composed of magnesium-rich orthopyroxene and olivine, Ca-, Mg-, Na-rich clinopyroxene, potassium feldspar, and Na-bearing plagioclase feldspar. Both Ca- and Mg-rich garnet (pyrope and grossular, respectively) are apparently present in small amounts. Opaque minerals are required for spectral matching, with rutile (TiO2) repeatedly providing the "best fit". However, in the case of the radar bright region C, perovskite also contributed to a very good fit. Caloris Basin infill is rich in both potassium feldspar and Na-rich plagioclase. There is little or no olivine in the Caloris interior smooth plains. Together with the high alkali content, this indicates that resurfacing magmas were low to intermediate in SiO2. Data suggest the dark plains exterior to Caloris are highly differentiated low-iron basaltic magmas resulting in material that might be classified as oligorclase basalts. (C) 2009 Elsevier Ltd. All rights reserved

  • 47.
    Susanne, Höfner
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Astronomy and Space Physics.
    Bladh, S.
    Univ Padua, Dipartimento Fis & Astron Galileo Galilei, Vicolo Osservatorio 3, I-35122 Padua, Italy..
    Aringer, B.
    Univ Padua, Dipartimento Fis & Astron Galileo Galilei, Vicolo Osservatorio 3, I-35122 Padua, Italy.;Osserv Astron Padova, INAF, Vicolo Osservatorio 5, I-35122 Padua, Italy..
    Ahuja, Rajeev
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Dynamic atmospheres and winds of cool luminous giants I. Al2O3 and silicate dust in the close vicinity of M-type AGB stars2016In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 594, article id A108Article in journal (Refereed)
    Abstract [en]

    Context. In recent years, high spatial resolution techniques have given valuable insights into the complex atmospheres of AGB stars and their wind-forming regions. They make it possible to trace the dynamics of molecular layers and shock waves, to estimate dust condensation distances, and to obtain information on the chemical composition and size of dust grains close to the star. These are essential constraints for understanding the mass loss mechanism, which presumably involves a combination of atmospheric levitation by pulsation-induced shock waves and radiation pressure on dust, forming in the cool upper layers of the atmospheres. Aims. Spectro-interferometric observations indicate that Al2O3 condenses at distances of about 2 stellar radii or less, prior to the formation of silicates. Al2O3 grains are therefore prime candidates for producing the scattered light observed in the close vicinity of several M-type AGB stars, and they may be seed particles for the condensation of silicates at lower temperatures. The purpose of this paper is to study the necessary conditions for the formation of Al2O3 and the potential effects on mass loss, using detailed atmosphere and wind models. Methods. We have constructed a new generation of Dynamic Atmosphere and Radiation-driven Wind models based on Implicit Numerics (DARWIN), including a time-dependent treatment of grain growth and evaporation for both Al2O3 and Fe-free silicates (Mg2SiO4). The equations describing these dust species are solved in the framework of a frequency-dependent radiation hydrodynamical model for the atmosphere and wind structure, taking pulsation-induced shock waves and periodic luminosity variations into account. Results. Condensation of Al2O3 at the close distances and in the high concentrations implied by observations requires high transparency of the grains in the visual and near-IR region to avoid destruction by radiative heating. We derive an upper limit for the imaginary part of the refractive index k around 10(-3) at these wavelengths. For solar abundances, radiation pressure due to Al2O3 is too low to drive a wind. Nevertheless, this dust species may have indirect effects on mass loss. The formation of composite grains with an Al2O3 core and a silicate mantle can give grain growth a head start, increasing both mass loss rates and wind velocities. Furthermore, our experimental core-mantle grain models lead to variations of visual and near-IR colors during a pulsation cycle which are in excellent agreement with observations. Conclusions. Al2O3 grains are promising candidates for explaining the presence of gravitationally bound dust shells close to M-type AGB stars, as implied by both scattered light observations and mid-IR spectro-interferometry. The required level of transparency at near-IR wavelengths is compatible with impurities due to a few percent of transition metals (e.g., Cr), consistent with cosmic abundances. Grains consisting of an Al2O3 core and an Fe-free silicate mantle with total grain radii of about 0.1-1 micron may be more efficient at driving winds by the scattering of stellar photons than pure Fe-free silicate grains.

  • 48.
    Wachter, A.
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Astronomy and Space Physics.
    Winters, J. M.
    Schroeder, K. -P.
    Sedlmayr, E.
    Dust-driven winds and mass loss of C-rich AGB stars with subsolar metallicities2008In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 486, no 2, p. 497-504Article in journal (Refereed)
    Abstract [en]

    Aims. We investigate the mass loss of highly evolved, low- and intermediate mass stars and stellar samples with subsolar metallicity. We give a qualitative as well as quantitative description which can be applied to LMC/SMC-type stellar populations. Methods. For that purpose we apply the same approach as we did for solar metallicity stars and calculate hydrodynamical wind models including dust formation with LMC and SMC abundances under consideration of an adapted model assumption. In particular, we improved the treatment of the radiative transfer problem in order to accommodate larger non-local contributions occurring with smaller opacities. For each wind model we determine an averaged mass-loss rate. The resulting, approximate mass-loss formulae are then applied to well-tested and calibrated stellar evolution calculations in order to quantify the stellar mass loss. Results. The dynamical models for LMC and SMC metallicity result in mass-loss rates of the same order of magnitude as the solar metallicity models which is in this basic approach in agreement with observations. The hydrodynamical properties like e. g. the outflow velocity differ (for fixed C/O abundance ratio) noticeably, though. While critical luminosities of LMC and solar metallicity models fairly coincide, the SMC models need higher luminosities to develop dust-driven winds.

  • 49.
    Zackrisson, Erik
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Astronomy and Space Physics.
    Bergvall, Nils
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Astronomy and Space Physics.
    Leitet, Elisabet
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Astronomy and Space Physics.
    The impact of nebular emission on the broadband fluxes of high-redshift galaxies2008In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 676, no 1, p. L9-L12Article in journal (Refereed)
    Abstract [en]

    A substantial fraction of the light emitted from young or star-forming galaxies at ultraviolet to near-infrared wavelengths comes from the ionized interstellar medium in the form of emission lines and a nebular continuum. At high redshifts, star formation rates are on average higher and stellar populations younger than in the local universe. Both of these effects act to boost the impact of nebular emission on the overall spectrum of galaxies. Even so, the broadband fluxes and colors of high-redshift galaxies are routinely analyzed under the assumption that the light observed originates directly from stars. Here we assess the impact of nebular emission on broadband fluxes in Johnson/Cousins BVR1JHK, Sloan Digital Sky Survey griz and Spitzer IRAC/MIPS filters as a function of observed redshift (up to z = 15) for galaxies with different star formation histories. We find that nebular z p 15 emission may account for a nonnegligible fraction of the light received from high-redshift galaxies. The ages and masses inferred for such objects through the use of spectral evolutionary models that omit the nebular contribution are therefore likely to contain systematic errors. We argue that a careful treatment of the nebular component will be essential for the interpretation of the rest-frame ultraviolet-to-infrared properties of the first galaxies formed, like the ones expected to be detected with the James Webb Space Telescope.

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