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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.
    Brunetti, D.
    et al.
    CNR, IFP, Via R Cozzi 53, I-20125 Milan, Italy.
    Graves, J. P.
    Ecole Polytech Fed Lausanne, SPC, CH-1015 Lausanne, Switzerland.
    Lazzaro, E.
    CNR, IFP, Via R Cozzi 53, I-20125 Milan, Italy.
    Mariani, A.
    CNR, IFP, Via R Cozzi 53, I-20125 Milan, Italy.
    Nowak, S.
    CNR, IFP, Via R Cozzi 53, I-20125 Milan, Italy.
    Cooper, W. A.
    Ecole Polytech Fed Lausanne, SPC, CH-1015 Lausanne, Switzerland.
    Wahlberg, Christer
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Space Plasma Physics. Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Astronomy and Space Physics. EURATOM VR Fus Assoc, POB 515, SE-75120 Uppsala, Sweden.
    Helical equilibrium magnetohydrodynamic flow effects on the stability properties of low-n ideal external-infernal modes in weak shear tokamak configurations2019In: Plasma Physics and Controlled Fusion, ISSN 0741-3335, E-ISSN 1361-6587, Vol. 61, no 6, article id 064003Article in journal (Refereed)
    Abstract [en]

    The impact of equilibrium helical flows on the stability properties of low shear tokamak plasmas is assessed. The corrections due to such helical flow to the equilibrium profiles (mass density, pressure, Shafranov shift, magnetic fluxes) are computed by minimising order by order the generalised Grad-Shafranov equation. By applying the same minimisation procedure, a set of three coupled equations, suitable for the study of magnetohydrodynamic perturbations localised within core or edge transport barriers is derived in circular tokamak geometry. We apply these equations to modelling the impact of strong poloidal flow shear in the edge region caused by a radial electric field on the stability of edge infernal modes retaining vacuum effects. Due to the poloidal flow shearing, the effect of plasma rotation is not simply a Doppler shift of the eigenfrequency. Stabilisation is found even for weak flow amplitude.

  • 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.
    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.

  • 5.
    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)
  • 6.
    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.

    Keywords
    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
    Keywords
    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
  • 7.
    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.

  • 8. 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.

  • 9.
    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.

  • 10.
    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.

  • 11. 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.

  • 12. 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.

  • 13.
    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.

  • 14. 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.

  • 15.
    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.

  • 16.
    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)
  • 17.
    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.

  • 18.
    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.

  • 19. 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.

  • 20.
    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.

  • 21.
    Höfner, Susanne
    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.

  • 22. 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.

  • 23. 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.

  • 24.
    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.

  • 25.
    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.

  • 26.
    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.

  • 27.
    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.

  • 28. 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)
  • 29. 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.

  • 30.
    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.

    Keywords
    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: 2018-06-04Bibliographically 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.

    Keywords
    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: 2018-06-04Bibliographically 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