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  • 1.
    Danilovich, T.
    et al.
    Chalmers, Onsala Space Observ, Gothenburg, Sweden..
    Bergman, P.
    Chalmers, Onsala Space Observ, Gothenburg, Sweden..
    Justtanont, K.
    Chalmers, Onsala Space Observ, Gothenburg, Sweden..
    Lombaert, R.
    Katholieke Univ Leuven, Inst Sterrenkunde, Louvain, Belgium..
    Maercker, M.
    Chalmers, Onsala Space Observ, Gothenburg, Sweden.;Univ Bonn, Argelander Inst Astron, Bonn, Germany..
    Olofsson, H.
    Chalmers, Onsala Space Observ, Gothenburg, Sweden..
    Ramstedt, Sofia
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Royer, P.
    Katholieke Univ Leuven, Inst Sterrenkunde, Louvain, Belgium..
    Detailed Modelling of the Circumstellar Envelope of the S-type AGB Star W Aquilae2015In: WHY GALAXIES CARE ABOUT AGB STARS III: A CLOSER LOOK IN SPACE AND TIME, ASTRONOMICAL SOC PACIFIC , 2015, Vol. 497, 219-220 p.Conference paper (Refereed)
    Abstract [en]

    We present new Herschel(1) HIFI (de Graauw et al. 2010) and PACS (Poglitsch et al. 2010) sub-millimeter and far-infrared line observations of several molecular species towards the S-type AGB star W Aql. We use these observations, which probe a wide range of gas temperatures, to constrain the circumstellar properties of W Aql, including mass-loss rate and molecular abundances.

  • 2. Danilovich, T.
    et al.
    Bergman, P.
    Justtanont, K.
    Lombaert, R.
    Maercker, M.
    Olofsson, H.
    Ramstedt, Sofia
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy.
    Royer, P.
    Detailed modelling of the circumstellar molecular line emission of the S-type AGB star W Aquilae2014In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 569, A76- p.Article in journal (Refereed)
    Abstract [en]

    Context. S-type AGB stars have a C/O ratio which suggests that they are transition objects between oxygen-rich M-type stars and carbon-rich C-type stars. As such, their circumstellar compositions of gas and dust are thought to be sensitive to their precise C/O ratio, and it is therefore of particular interest to examine their circumstellar properties. Aims. We present new Herschel HIFI and PACS sub-millimetre and far-infrared line observations of several molecular species towards the S-type AGB star W Aql. We use these observations, which probe a wide range of gas temperatures, to constrain the circumstellar properties of W Aql, including mass-loss rate and molecular abundances. Methods. We used radiative transfer codes to model the circumstellar dust and molecular line emission to determine circumstellar properties and molecular abundances. We assumed a spherically symmetric envelope formed by a constant mass-loss rate driven by an accelerating wind. Our model includes fully integrated H2O line cooling as part of the solution of the energy balance. Results. We detect circumstellar molecular lines from CO, H2O, SiO, HCN, and, for the first time in an S-type AGB star, NH3. The radiative transfer calculations result in an estimated mass-loss rate for W Aql of 4.0 x 10(-6) M-circle dot yr(-1) based on the (CO)-C-12 lines. The estimated (CO)-C-12/(CO)-C-13 ratio is 29, which is in line with ratios previously derived for S-type AGB stars. We find an H2O abundance of 1.5 x 10(-5), which is intermediate to the abundances expected for M and C stars, and an ortho/para ratio for H2O that is consistent with formation at warm temperatures. We find an HCN abundance of 3 x 10(-6), and, although no CN lines are detected using HIFI, we are able to put some constraints on the abundance, 6 x 10(-6), and distribution of CN in W Aql's circumstellar envelope using ground-based data. We find an SiO abundance of 3 x 10(-6), and an NH3 abundance of 1.7 x 10(-5), confined to a small envelope. If we include uncertainties in the adopted circumstellar model - in the adopted abundance distributions, etc. - the errors in the abundances are of the order of factors of a few. The data also suggest that, in terms of HCN, S-type and M-type AGB stars are similar, and in terms of H2O, S-type AGB stars are more like C-type than M-type AGB stars. We detect excess blue-shifted emission in several molecular lines, possibly due to an asymmetric outflow. Conclusions. The estimated abundances of circumstellar HCN, SiO and H2O place W Aql in between M-and C-type AGB stars, i.e., the abundances are consistent with an S-type classification.

  • 3.
    De Beck, E.
    et al.
    Chalmers, Onsala Space Observ, Dept Earth & Space Sci, S-43992 Onsala, Sweden..
    Decin, L.
    Inst Sterrenkunde, Dept Nat Kunde & Sterrenkunde, Celestijnenlaan 200D, B-3001 Heverlee, Belgium.;Univ Amsterdam, Sterrenkundig Inst Anton Pannekoek, Sci Pk 904, NL-1098 Amsterdam, Netherlands..
    Ramstedt, Sofia
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Observational Astronomy.
    Olofsson, H.
    Chalmers, Onsala Space Observ, Dept Earth & Space Sci, S-43992 Onsala, Sweden..
    Menten, K. M.
    Max Planck Inst Radioastron, Hugel 69, D-53121 Bonn, Germany..
    Patel, N. A.
    Harvard Smithsonian Ctr Astrophys, 60 Garden St,MS78, Cambridge, MA 02138 USA..
    Vlemmings, W. H. T.
    Chalmers, Onsala Space Observ, Dept Earth & Space Sci, S-43992 Onsala, Sweden..
    Search for aluminium monoxide in the winds of oxygen-rich AGB stars2017In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 598, A53Article in journal (Refereed)
    Abstract [en]

    Context. Aluminium monoxide (AlO) is likely efficiently depleted from the gas around oxygen-rich evolved stars to form alumina (Al2O3) clusters and dust seeds. The presence of AlO gas in the extended atmospheres of evolved stars has been derived from optical spectroscopy. More recently, AlO gas was also detected at long wavelengths around the supergiant VY CMa and the oxygenrich asymptotic giant branch star o Cet (Mira A). The central role aluminium might play in dust formation and wind driving, in combination with these first detections of AlO at long wavelengths, shows the need for a wider search for this molecule in the winds of evolved stars. Aims. The detection at long wavelengths of emission in rotational transitions of AlO towards asymptotic giant branch stars can help constrain the presence and location of AlO gas in the outflows and ultimately also the efficiency of the depletion process. Methods. In search of AlO, we mined data obtained with APEX, the IRAM 30m telescope, Herschel / HIFI, SMA, and ALMA, which were primarily aimed at studying other species around asymptotic giant branch stars. We report here on observations of AlO towards a sample of eight oxygen-rich asymptotic giant branch stars in different rotational transitions, up to seven for some stars. Results. We present definite detections of one rotational transition of AlO for o Cet and R Aqr, and tentative detections of one transition for R Dor and o Cet and two transitions for IK Tau and W Hya. The presented spectra of WX Psc, R Cas, and TX Cam show no signature of AlO. For o Cet, R Aqr, and IK Tau, we find that the AlO (N = 9 8) emission likely traces the inner parts of the wind, out to only a few tens of AU, where the gas has not yet been accelerated to its terminal velocity. This is in agreement with recently published results from a detailed study on o Cet. Conclusions. The conclusive detections of AlO emission in the case of o Cet and R Aqr confirm the presence of AlO in the gas phase in outflows of asymptotic giant branch stars. The tentative detections further support this. Since most of the observations presented in this study were obtained with stronger emission from other species than AlO in mind, observations with higher sensitivity in combination with high angular resolution will improve our understanding of the presence and behaviour of AlO. From the current data sets we cannot firmly conclude whether there is a direct correlation between the wind properties and the detection rate of AlO emission. We hope that this study can serve as a stimulus to perform sample studies in search of AlO in oxygen-rich outflows.

  • 4.
    De Nutte, R.
    et al.
    Katholieke Univ Leuven, Inst Astron, Louvain, Belgium..
    Decin, L.
    Katholieke Univ Leuven, Inst Astron, Louvain, Belgium.;Univ Amsterdam, Astron Inst Anton Pannekoek, NL-1012 WX Amsterdam, Netherlands..
    Olofsson, H.
    Chalmers, Onsala Space Observ, Dept Earth & Space Sci, S-43900 Onsala, Sweden..
    de Koter, A.
    Katholieke Univ Leuven, Inst Astron, Louvain, Belgium.;Univ Amsterdam, Astron Inst Anton Pannekoek, NL-1012 WX Amsterdam, Netherlands..
    Lombaert, R.
    Katholieke Univ Leuven, Inst Astron, Louvain, Belgium..
    Milam, S.
    NASA, Goddard Space Flight Ctr, Astrochem Lab, Greenbelt, MD USA..
    Ramstedt, Sofia
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Nucleosynthesis in AGB Stars Traced by Oxygen Isotopic Ratios2015In: WHY GALAXIES CARE ABOUT AGB STARS III: A CLOSER LOOK IN SPACE AND TIME, ASTRONOMICAL SOC PACIFIC , 2015, Vol. 497, 289-294 p.Conference paper (Refereed)
    Abstract [en]

    Isotopic ratios are by far the best diagnostic tracers of the stellar origin of elements, as they are very sensitive to the precise conditions in the nuclear burning regions. They allow us to give direct constraints on stellar evolution models and on the progenitor mass. However, up to now different isotopic ratios have been well constrained for only a handful of Asymptotic Giant Branch (AGB) stars. We present new data on isotopologue lines of a well-selected sample of AGB stars, covering the three spectral classes of C-, S- and M-type stars. We report on the first efforts made in determining accurate isotopologue fractions, focusing on oxygen isotopes which are a crucial tracer of the poorly constrained extra mixing processes in stellar atmospheres.

  • 5.
    De Nutte, R.
    et al.
    Katholieke Univ Leuven, Inst Astron, Celestijnenlaan 200D B2401, B-3001 Leuven, Belgium..
    Decin, L.
    Katholieke Univ Leuven, Inst Astron, Celestijnenlaan 200D B2401, B-3001 Leuven, Belgium.;Univ Amsterdam, Astron Inst Anton Pannekoek, POB 94249, NL-1090 GE Amsterdam, Netherlands..
    Olofsson, H.
    Chalmers, Onsala Space Observ, Dept Earth & Space Sci, S-43992 Onsala, Sweden..
    Lombaert, R.
    Katholieke Univ Leuven, Inst Astron, Celestijnenlaan 200D B2401, B-3001 Leuven, Belgium.;Chalmers, Onsala Space Observ, Dept Earth & Space Sci, S-43992 Onsala, Sweden..
    de Koter, A.
    Katholieke Univ Leuven, Inst Astron, Celestijnenlaan 200D B2401, B-3001 Leuven, Belgium.;Univ Amsterdam, Astron Inst Anton Pannekoek, POB 94249, NL-1090 GE Amsterdam, Netherlands..
    Karakas, A.
    Australian Natl Univ, Res Sch Astron & Astrophys, Canberra, ACT 2611, Australia.;Monash Univ, Monash Ctr Astrophys, Sch Phys & Astron, Clayton, Vic 3800, Australia..
    Milam, S.
    NASA, Goddard Space Flight Ctr, Astrochem Lab, Code 691-0, Greenbelt, MD 20771 USA..
    Ramstedt, Sofia
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Observational Astronomy.
    Stancliffe, R. J.
    Univ Bonn, Argelander Inst Astron, Hugel 71, D-53121 Bonn, Germany..
    Homan, W.
    Katholieke Univ Leuven, Inst Astron, Celestijnenlaan 200D B2401, B-3001 Leuven, Belgium..
    Van de Sande, M.
    Katholieke Univ Leuven, Inst Astron, Celestijnenlaan 200D B2401, B-3001 Leuven, Belgium..
    Nucleosynthesis in AGB stars traced by oxygen isotopic ratios I. Determining the stellar initial mass by means of the O-17/O-18 ratio2017In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 600, A71Article in journal (Refereed)
    Abstract [en]

    Aims. We seek to investigate the O-17/O-18 ratio for a sample of AGB stars containing M-, S-, and C-type stars. These ratios are evaluated in relation to fundamental stellar evolution parameters: the stellar initial mass and pulsation period.

    Methods. Circumstellar (CO)-C-13-O-16, (CO)-C-12-O-17, and (CO)-C-12-O-18 line observations were obtained for a sample of nine stars with various single-dish long-wavelength facilities. Line intensity ratios are shown to relate directly to the surface O-17/O-18 abundance ratio.

    Results. Stellar evolution models predict the O-17/O-18 ratio to be a sensitive function of initial mass and to remain constant throughout the entire TP-AGB phase for stars initially less massive than 5 M-circle dot. This makes the measured ratio a probe of the initial stellar mass.

    Conclusions. Observed O-17/O-18 ratios are found to be well in the range predicted by stellar evolution models that do not consider convective overshooting. From this, accurate initial mass estimates are calculated for seven sources. For the remaining two sources, there are two mass solutions, although there is a larger probability that the low-mass solution is correct. Finally, we present hints at a possible separation between M/S- and C-type stars when comparing the O-17/O-18 ratio to the stellar pulsation period.

  • 6.
    Doan, Lam
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Ramstedt, Sofia
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Vlemmings, W. H. T.
    Department of Earth and Space Sciences, Chalmers University of Technology.
    Höfner, Susanne
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    De Beck, E.
    Department of Earth and Space Sciences, Chalmers University of Technology.
    Kerschbaum, F.
    Department of Astrophysics, University of Vienna.
    Lindqvist, M.
    Department of Earth and Space Sciences, Chalmers University of Technology.
    Maercker, M.
    Department of Earth and Space Sciences, Chalmers University of Technology.
    Mohamed, S.
    South African Astronomical Observatory; Astronomy Department, University of Cape Town; National Institute for Theoretical Physics, Private Bag X1, 7602 Matieland.
    Paladini, C.
    Institut d’Astronomie et d’Astrophysique, Université Libre de Bruxelles.
    Wittkowski, M.
    European Southern Observatory, Germany.
    The extended molecular envelope of the asymptotic giant branchstar π1 Gruis as seen by ALMA: I. Large-scale kinematic structure and CO excitation properties2017In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 605, A28Article in journal (Refereed)
    Abstract [en]

    Context. The S-type asymptotic giant branch (AGB) star pi(1) Gru has a known companion at a separation of 2 ''.7 (approximate to 400 AU). Previous observations of the circumstellar envelope (CSE) show strong deviations from spherical symmetry. The envelope structure, including an equatorial torus and a fast bipolar outflow, is rarely seen in the AGB phase and is particularly unexpected in such a wide binary system. Therefore a second, closer companion has been suggested, but the evidence is not conclusive.

    Aims. The aim is to make a 3D model of the CSE and to constrain the density and temperature distribution using new spatially resolved observations of the CO rotational lines.

    Methods. We have observed the J = 3-2 line emission from (CO)-C-12 and (CO)-C-13 using the compact arrays of the Atacama Large Millimeter/submillimeter Array (ALMA). The new ALMA data, together with previously published (CO)-C-12 J = 2-1 data from the Submillimeter Array (SMA), and the (CO)-C-12 J = 5-4 and J = 9-8 lines observed with Herschel/Heterodyne Instrument for the Far-Infrared (HIFI), is modeled with the 3D non-LTE radiative transfer code SHAPEMOL.

    Results. The data analysis clearly confirms the torus-bipolar structure. The 3D model of the CSE that satisfactorily reproduces the data consists of three kinematic components: a radially expanding torus with velocity slowly increasing from 8 to 13 km s(-1) along the equator plane; a radially expanding component at the center with a constant velocity of 14 km s(-1); and a fast, bipolar outflow with velocity proportionally increasing from 14 km s(-1) at the base up to 100 km s(-1) at the tip, following a linear radial dependence. The results are used to estimate an average mass-loss rate during the creation of the torus of 7.7 x 10(-7) M-circle dot yr(-1). The total mass and linear momentum of the fast outflow are estimated at 7.3 x 10(-4) M-circle dot and 9.6 x 10(37) g cm s(-1), respectively. The momentum of the outflow is in excess (by a factor of about 20) of what could be generated by radiation pressure alone, in agreement with recent findings for more evolved sources. The best-fit model also suggests a (CO)-C-12/(CO)-C-13 abundance ratio of 50. Possible shaping scenarios for the gas envelope are discussed.

  • 7.
    Kerschbaum, F.
    et al.
    Univ Vienna, Dept Astrophys.
    Maercker, M.
    Chalmers Univ Technol, Dept Earth & Space Sci, Onsala Space Observ.
    Brunner, M.
    Univ Vienna, Dept Astrophys.
    Lindqvist, M.
    Chalmers Univ Technol, Dept Earth & Space Sci, Onsala Space Observ.
    Olofsson, H.
    Chalmers Univ Technol, Dept Earth & Space Sci, Onsala Space Observ.
    Mecina, M.
    Univ Vienna, Dept Astrophys.
    De Beck, E.
    Chalmers Univ Technol, Dept Earth & Space Sci, Onsala Space Observ.
    Groenewegen, M. A. T.
    Koninklijke Sterrenwacht van Belgie.
    Lagadec, E.
    Univ Cote Azur, Observ Cote Azur, Lab Lagrange, CNRS,Bd Observ.
    Mohamed, S.
    South African Astron Observ.; Univ Cape Town, Dept Astron.; Natl Inst Theoret Phys, Private Bag X1.
    Paladini, C.
    Univ Libre Bruxelles, Inst Astron & Astrophys, Campus Plaine CP 226.
    Ramstedt, Sofia
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Vlemmings, W. H. T.
    Chalmers Univ Technol, Dept Earth & Space Sci, Onsala Space Observ.
    Wittkowski, M.
    European Southern Observ, Karl Schwarzschild Str 2.
    Rings and filaments: The remarkable detached CO shell of U Antliae2017In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 605, A116Article in journal (Refereed)
    Abstract [en]

    Aims. Our goal is to characterize the intermediate age, detached shell carbon star U Antliae morphologically and physically in order to study the mass-loss evolution after a possible thermal pulse.

    Methods. High spatial resolution ALMA observations of unprecedented quality in thermal CO lines allow us to derive first critical spatial and temporal scales and constrain modeling efforts to estimate mass-loss rates for both the present day as well as the ejection period of the detached shell.

    Results. The detached shell is remarkably thin, overall spherically symmetric, and shows a barely resolved filamentary substructure possibly caused by instabilities in the interaction zone of winds with different outflow velocities. The expansion age of the detached shell is of the order of 2700 yr and its overall width indicates a high expansion-velocity and high mass-loss period of only a few hundred years at an average mass-loss rate of approximate to 10(-5) M-circle dot yr(-1). The post-high-mass-loss-rate-epoch evolution of U Ant shows a significant decline to a substantially lower gas expansion velocity and a mass-loss rate amounting to 4 x 10(-8) M-circle dot yr(-1), at present being consistent with evolutionary changes as predicted for the period between thermal pulses.

  • 8.
    Khouri, T.
    et al.
    Chalmers, Onsala Space Observ, Dept Earth & Space Sci, SE-43992 Onsala, Sweden..
    Vlemmings, W. H. T.
    Chalmers, Onsala Space Observ, Dept Earth & Space Sci, SE-43992 Onsala, Sweden..
    Ramstedt, Sofia
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Lombaert, R.
    Chalmers, Onsala Space Observ, Dept Earth & Space Sci, SE-43992 Onsala, Sweden..
    Maercker, M.
    Chalmers, Onsala Space Observ, Dept Earth & Space Sci, SE-43992 Onsala, Sweden..
    De Beck, E.
    Chalmers, Onsala Space Observ, Dept Earth & Space Sci, SE-43992 Onsala, Sweden..
    ALMA observations of the vibrationally excited rotational CO transition v=1, J=3-2 towards five AGB stars2016In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 463, no 1, L74-L78 p.Article in journal (Refereed)
    Abstract [en]

    We report the serendipitous detection with the Atacama Large Millimeter/submillimeter Array (ALMA) of the vibrationally excited pure-rotational CO transition, J = 3 - 2 towards five asymptotic giant branch (AGB) stars, Cet, R Aqr, R Scl, W Aql, and pi(1) Gru. The observed lines are formed in the poorly understood region located between the stellar surface and the region where the wind starts, the so-called warm molecular layer. We successfully reproduce the observed lines profiles using a simple model. We constrain the extents, densities, and kinematics of the region where the lines are produced. R Aqr and R Scl show inverse P-Cygni line profiles which indicate infall of material on to the stars. The line profiles of Cet and R Scl show variability. The serendipitous detection towards these five sources shows that vibrationally excited rotational lines can be observed towards a large number of nearby AGB stars using ALMA. This opens a new possibility for the study of the innermost regions of AGB circumstellar envelopes.

  • 9.
    Lykou, F.
    et al.
    Univ Vienna, Inst Astron, A-1010 Vienna, Austria..
    Hron, J.
    Univ Vienna, Inst Astron, A-1010 Vienna, Austria..
    Zijlstra, A. A.
    Jodrell Bank Ctr Astrophys, Macclesfield, Cheshire, England..
    Tuthill, P. C.
    Sydney Inst Astron, Sydney, NSW, Australia..
    Norris, B. R. M.
    Sydney Inst Astron, Sydney, NSW, Australia..
    Kluska, J.
    Univ Exeter, Exeter EX4 4QJ, Devon, England..
    Paladini, C.
    Univ Libre Bruxelles, Inst Astron & Astrophys, Brussels, Belgium..
    Lagadec, E.
    Observ Cote Azur, F-06003 Nice, France..
    Wittkowski, M.
    European So Observ, Dresden, Germany..
    Ramstedt, Sofia
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy.
    Maercker, M.
    Onsala Space Observ, Uppsala, Sweden..
    Unraveling Disks In Agb Stars2015In: Physics Of Evolved Stars: A Conference Dedicated To The Memory Of Olivier Chesneau / [ed] Lagadec, E; Millour, F, EDP Sciences, 2015, 217-222 p.Conference paper (Refereed)
    Abstract [en]

    It is commonly accepted that asymmetries found in the post-AGB stars and planetary nebulae should originate as early as during the AGB phase. We present results from our high-angular resolution observing programs, with an aperture masking technique on the VLT, of a sample of evolved stars that were known to present asymmetries at larger spatial scales (e.g. jets, torii and/or bipolar nebulae). Disk-like structures have been found in the vicinity of at least two of these stars.

  • 10.
    Lykou, F.
    et al.
    Univ Vienna, Inst Astrophys, A-1180 Vienna, Austria.
    Klotz, D.
    Univ Vienna, Inst Astrophys, A-1180 Vienna, Austria.
    Paladini, C.
    Univ Vienna, Inst Astrophys, A-1180 Vienna, Austria; Univ Libre Bruxelles, Inst Astron & Astrophys, B-1050 Brussels, Belgium.
    Hron, J.
    Univ Vienna, Inst Astrophys, A-1180 Vienna, Austria.
    Zijlstra, A. A.
    Univ Manchester, Jodrell Bank, Ctr Astrophys, Manchester M13 9PL, Lancs, England.
    Kluska, J.
    UJF Grenoble 1, CNRS INSU, IPAG, UMR Grenoble 5274, Grenoble, France.
    Norris, B. R. M.
    Univ Sydney, Sydney Inst Astron, Sydney, NSW 2016, Australia.
    Tuthill, P. G.
    Univ Sydney, Sydney Inst Astron, Sydney, NSW 2016, Australia.
    Ramstedt, Sofia
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Lagadec, E.
    Cornell Univ, Dept Astron, Ithaca, NY 14853 USA; Univ Nice Sophia Antipolis, CNRS, Observ Cote Azur, Lab Lagrange,UMR 7293, F-06304 Nice 4, France.
    Wittkowski, M.
    European So Observ, D-85748 Garching, Germany.
    Maercker, M.
    Chalmers, Onsala Space Observ, Dept Earth & Space Sci, S-43992 Onsala, Sweden.
    Mayer, A.
    Univ Vienna, Inst Astrophys, A-1180 Vienna, Austria.
    Dissecting the AGB star L-2 Puppis: a torus in the making2015In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 576, UNSP A46Article in journal (Refereed)
    Abstract [en]

    Aims. The circumstellar environment of L-2 Pup, an oxygen-rich semiregular variable, was observed to understand the evolution of mass loss and the shaping of ejecta in the late stages of stellar evolution. Methods. High-angular resolution observations from a single 8 m telescope were obtained using aperture masking in the near-infrared (1.64, 2.30 and 3.74 mu m) on the NACO/VLT, both in imaging and polarimetric modes. Results. The aperture-masking images of L-2 Pup at 2.30 mu m show a resolved structure that resembles a toroidal structure with a major axis of similar to 140 milliarcseconds (mas) and an east-west orientation. Two clumps can be seen on either side of the star, similar to 65 mas from the star, beyond the edge of the circumstellar envelope (estimated diameter is similar to 27 mas), while a faint, hook-like structure appear toward the northeast. The patterns are visible both in the imaging and polarimetric mode, although the latter was only used to measure the total intensity (Stokes I). The overall shape of the structure is similar at the 3.74 mu m pseudo-continuum (dust emission), where the clumps appear to be embedded within a dark, dusty lane. The faint, hook-like patterns are also seen at this wavelength, extending northeast and southwest with the central, dark lane being an apparent axis of symmetry. We interpret the structure as a circumstellar torus with inner radius of 4.2 au. With a rotation velocity of 10 kms(-1) as suggested by the SiO maser profile, we estimate a stellar mass of 0.7 M-circle dot.

  • 11. Maercker, M.
    et al.
    Ramstedt, Sofia
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy.
    Leal-Ferreira, M. L.
    Olofsson, G.
    Floren, H. G.
    The detached dust shells around the carbon AGB stars R Sculptoris and V644 Scorpii2014In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 570, A101- p.Article in journal (Refereed)
    Abstract [en]

    Context. The morphology of the circumstellar envelopes (CSE) around asymptotic giant branch (AGB) stars gives information on the mass-loss process from the star, its evolution and wind, and on the effect of binary interaction. However, determining the distribution of dust in the circumstellar envelopes is difficult. Observations of polarised, dust-scattered stellar light in the optical have produced images with high-spatial resolution of the envelopes around evolved stars. For sources with detached shells in particular, this method has proven extremely successful. Detached shells are believed to be created during a thermal pulse, and studying them can constrain the time scales and physical properties of one of the main drivers of late stellar evolution. Aims. We aim at determining the morphology of the detached shells around the carbon AGB stars R Scl and V644 Sco. In particular, we attempt to constrain the radii and widths of the detached dust shells around the stars and compare them to observations of the detached gas shells. Methods. We observed the polarised, dust-scattered stellar light around the carbon AGB stars R Scl and V644 Sco using the PolCor instrument mounted on the ESO 3.6 m telescope. Observations were done with a coronographic mask to block out the direct stellar light. The polarised images clearly show the detached shells around R Scl and V644 Sco. Using a dust radiative transfer code to model the dust-scattered polarised light, we constrained the radii and widths of the shells. Results. We determine radii of 19 ''.5 and 9 ''.4 for the detached dust shells around R Scl and V644 Sco, respectively. Both shells have an overall spherical symmetry and widths of approximate to 2 ''. For R Scl, we can compare the observed dust emission directly with high spatialresolution maps of CO(3-2) emission from the shell observed with ALMA. We find that the dust and gas coincide almost exactly, indicating a common evolution. The data presented here for R Scl are the most detailed observations of the entire dusty detached shell to date. For V644 Sco, these are the first direct measurements of the detached shell. Also here we find that the dust most likely coincides with the gas shell. Conclusions. The observations are consistent with a scenario where the detached shells are created during a thermal pulse. The determined radii and widths will constrain hydrodynamical models describing the pre-pulse mass loss, the thermal pulse, and postpulse evolution of the star.

  • 12.
    Maercker, M.
    et al.
    Chalmers, Onsala Space Observ, Dept Earth & Space Sci, S-43992 Onsala, Sweden..
    Vlemmings, W. H. T.
    Chalmers, Onsala Space Observ, Dept Earth & Space Sci, S-43992 Onsala, Sweden..
    Brunner, M.
    Univ Vienna, Dept Astrophys, Turkenschanzstr 17, A-1180 Vienna, Austria..
    De Beck, E.
    Chalmers, Onsala Space Observ, Dept Earth & Space Sci, S-43992 Onsala, Sweden..
    Humphreys, E. M.
    European So Observ, Karl Schwarzschild Str 2, D-85748 Garching, Germany..
    Kerschbaum, F.
    Univ Vienna, Dept Astrophys, Turkenschanzstr 17, A-1180 Vienna, Austria..
    Lindqvist, M.
    Chalmers, Onsala Space Observ, Dept Earth & Space Sci, S-43992 Onsala, Sweden..
    Olofsson, H.
    Chalmers, Onsala Space Observ, Dept Earth & Space Sci, S-43992 Onsala, Sweden..
    Ramstedt, Sofia
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Observational Astronomy.
    A detailed view of the gas shell around R Sculptoris with ALMA2016In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 586, A5Article in journal (Refereed)
    Abstract [en]

    Context. During the asymptotic giant branch (AGB) phase, stars undergo thermal pulses - short-lived phases of explosive helium burning in a shell around the stellar core. Thermal pulses lead to the formation and mixing-up of new elements to the stellar surface. They are hence fundamental to the chemical evolution of the star and its circumstellar envelope. A further consequence of thermal pulses is the formation of detached shells of gas and dust around the star, several of which have been observed around carbon-rich AGB stars. Aims. We aim to determine the physical properties of the detached gas shell around R Sculptoris, in particular the shell mass and temperature, and to constrain the evolution of the mass-loss rate during and after a thermal pulse. Methods. We analyse (CO)-C-12(1-0), (CO)-C-12(2-1), and (CO)-C-12(3-2) emission, observed with the Atacama Large Millimeter/submillimeter Array (ALMA) during Cycle 0 and complemented by single-dish observations. The spatial resolution of the ALMA data allows us to separate the detached shell emission from the extended emission inside the shell. We perform radiative transfer modelling of both components to determine the shell properties and the post-pulse mass-loss properties. Results. The ALMA data show a gas shell with a radius of 19.'' 5 expanding at 14.3 km s(-1). The different scales probed by the ALMA Cycle 0 array show that the shell must be entirely filled with gas, contrary to the idea of a detached shell. The comparison to single-dish spectra and radiative transfer modelling confirms this. We derive a shell mass of 4.5 x 10(-3) M-circle dot with a temperature of 50 K. Typical timescales for thermal pulses imply a pulse mass-loss rate of 2.3 x 10(-5) M-circle dot yr(-1). For the post-pulse mass-loss rate, we find evidence for a gradual decline of the mass-loss rate, with an average value of 1.6 x 10(-5) M-circle dot yr(-1). The total amount of mass lost since the last thermal pulse is 0.03 M-circle dot, a factor four higher compared to classical models, with a sharp decline in mass-loss rate immediately after the pulse. Conclusions. We find that the mass-loss rate after a thermal pulse has to decline more slowly than generally expected from models of thermal pulses. This may cause the star to lose significantly more mass during a thermal pulse cycle, which affects the lifetime on the AGB and the chemical evolution of the star, its circumstellar envelope, and the interstellar medium.

  • 13.
    McDonald, I.
    et al.
    Univ Manchester, Sch Phys & Astron, Jodrell Bank Ctr Astrophys, Manchester M13 9PL, Lancs, England..
    Zijlstra, A. A.
    Univ Manchester, Sch Phys & Astron, Jodrell Bank Ctr Astrophys, Manchester M13 9PL, Lancs, England..
    Lagadec, E.
    Cornell Univ, Cornell Ctr Astrophys & Planetary Sci, Ithaca, NY 14853 USA.;Observ Cote dAzur, F-06304 Nice 4, France..
    Sloan, G. C.
    Cornell Univ, Cornell Ctr Astrophys & Planetary Sci, Ithaca, NY 14853 USA..
    Boyer, M. L.
    NASA, Goddard Flight Ctr, Observat Cosmol Lab, Greenbelt, MD 20771 USA..
    Matsuura, M.
    UCL, Dept Phys & Astron, London WC1E 6BT, England.;Cardiff Univ, Sch Phys & Astron, Cardiff CF24 3AA, S Glam, Wales..
    Smith, R. J.
    Univ Manchester, Sch Phys & Astron, Jodrell Bank Ctr Astrophys, Manchester M13 9PL, Lancs, England..
    Smith, C. L.
    Univ Manchester, Sch Phys & Astron, Jodrell Bank Ctr Astrophys, Manchester M13 9PL, Lancs, England..
    Yates, J. A.
    UCL, Dept Phys & Astron, London WC1E 6BT, England..
    van Loon, J. Th.
    Keele Univ, Astrophys Grp, Lennard Jones Labs, Keele ST5 5BG, Staffs, England..
    Jones, O. C.
    Univ Manchester, Sch Phys & Astron, Jodrell Bank Ctr Astrophys, Manchester M13 9PL, Lancs, England.;STScI, Baltimore, MD 21218 USA..
    Ramstedt, Sofia
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy.
    Avison, A.
    Univ Manchester, Sch Phys & Astron, Jodrell Bank Ctr Astrophys, Manchester M13 9PL, Lancs, England..
    Justtanont, K.
    Chalmers, Onsala Space Observ, Dept Earth & Space Sci, SE-43992 Onsala, Sweden..
    Olofsson, H.
    Chalmers, Onsala Space Observ, Dept Earth & Space Sci, SE-43992 Onsala, Sweden..
    Blommaert, J. A. D. L.
    Vrije Univ Brussel, Dept Phys & Astrophys, Astron & Astrophys Res Grp, B-1050 Brussels, Belgium..
    Goldman, S. R.
    Keele Univ, Astrophys Grp, Lennard Jones Labs, Keele ST5 5BG, Staffs, England..
    Groenewegen, M. A. T.
    Koninklijke Sterrenwacht Belgie, B-1180 Brussels, Belgium..
    ALMA reveals sunburn: CO dissociation around AGB stars in the globular cluster 47 Tucanae2015In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 453, no 4, 4324-4336 p.Article in journal (Refereed)
    Abstract [en]

    Atacama Large Millimetre Array observations show a non-detection of carbon monoxide around the four most luminous asymptotic giant branch (AGB) stars in the globular cluster 47 Tucanae. Stellar evolution models and star counts show that the mass-loss rates from these stars should be similar to 1.2-3.5x10(-7) M-circle dot yr(-1). We would naively expect such stars to be detectable at this distance (4.5 kpc). By modelling the ultraviolet radiation field from post-AGB stars and white dwarfs in 47 Tuc, we conclude that CO should be dissociated abnormally close to the stars. We estimate that the CO envelopes will be truncated at a few hundred stellar radii from their host stars and that the line intensities are about two orders of magnitude below our current detection limits. The truncation of CO envelopes should be important for AGB stars in dense clusters. Observing the CO (3-2) and higher transitions and targeting stars far from the centres of clusters should result in the detections needed to measure the outflow velocities from these stars.

  • 14.
    Mohamed, S.
    et al.
    S African Astron Observ, POB 9, ZA-7935 Cape Town, South Africa..
    Booth, R.
    Univ Oxford, Dept Astrophys, Oxford OX1 3RH, England..
    Podsiadlowski, Ph.
    Univ Oxford, Dept Astrophys, Oxford OX1 3RH, England..
    Ramstedt, Sofia
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy.
    Vlemmings, W.
    Chalmers, Dept Earth & Space Sci, Onsala Space Observ, S-43992 Onsala, Sweden..
    Maercker, M.
    Chalmers, Dept Earth & Space Sci, Onsala Space Observ, S-43992 Onsala, Sweden..
    3D Models Of Symbiotic Binaries2015In: Physics Of Evolved Stars: A Conference Dedicated To The Memory Of Olivier Chesneau / [ed] Lagadec, E; Millour, F, EDP Sciences, 2015, 81-86 p.Conference paper (Refereed)
    Abstract [en]

    Symbiotic binaries consist of a cool, mass-losing giant and an accreting, compact companion. We present 3D Smoothed Particle Hydrodynamics (SPH) models of two such interacting binaries, RS Oph and Mira AB. RS Oph is also a recurrent nova system, thus we model multiple quiescent mass transfer-nova outburst cycles. The resulting circumstellar structures of both systems are highly complex with the formation of spirals, arcs, shells, equatorial and bipolar outflows. We compare the models to recent observations and discuss the implications of our results for related systems, e.g., bipolar nebulae and jets, chemically peculiar stars, and the progenitors of Type Ia supernovae.

  • 15.
    Montez, Rodolfo, Jr.
    et al.
    Smithsonian Astrophys Observ, Cambridge, MA 02138 USA..
    Ramstedt, Sofia
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Observational Astronomy.
    Kastner, Joel H.
    Rochester Inst Technol, Chester F Carlson Ctr Imaging Sci, Sch Phys & Astron, 54 Lomb Mem Dr, Rochester, NY 14623 USA.;Rochester Inst Technol, Lab Multiwavelength Astrophys, 54 Lomb Mem Dr, Rochester, NY 14623 USA..
    Vlemmings, Wouter
    Chalmers, Onsala Space Observ, Dept Space Earth & Environm, SE-43992 Onsala, Sweden..
    Sanchez, Enmanuel
    Univ Cincinnati, Dept Phys, 400 Geol Phys Bldg, Cincinnati, OH 45221 USA..
    A Catalog of GALEX Ultraviolet Emission from Asymptotic Giant Branch Stars2017In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 841, no 1, 33Article in journal (Refereed)
    Abstract [en]

    We have performed a comprehensive study of the UV emission detected from asymptotic giant branch (AGB) stars by the Galaxy Evolution Explorer (GALEX). Of the 468 AGB stars in our sample, 316 were observed by GALEX. In the near-UV (NUV) bandpass (lambda(eff) similar to 2310 angstrom), 179 AGB stars were detected and 137 were not detected. Only 38 AGB stars were detected in the far-UV (FUV) bandpass (lambda(eff) similar to 1528 angstrom). We find that NUV emission is correlated with optical to near-infrared emission, leading to higher detection fractions among the brightest, and hence closest, AGB stars. Comparing the AGB time-variable visible phased light curves to corresponding GALEX NUV phased light curves, we find evidence that for some AGB stars the NUV emission varies in phase with the visible light curves. We also find evidence that the NUV emission and possibly the FUV emission are anticorrelated with the circumstellar envelope density. These results suggest that the origin of the GALEX-detected UV emission is an inherent characteristic of the AGB stars that can most likely be traced to a combination of photospheric and chromospheric emission. In most cases, UV detections of AGB stars are not likely to be indicative of the presence of binary companions.

  • 16.
    Olofsson, H.
    et al.
    Chalmers, Onsala Space Observ, Dept Earth & Space Sci, S-43992 Onsala, Sweden..
    Vlemmings, W. H. T.
    Chalmers, Onsala Space Observ, Dept Earth & Space Sci, S-43992 Onsala, Sweden..
    Maercker, M.
    Chalmers, Onsala Space Observ, Dept Earth & Space Sci, S-43992 Onsala, Sweden..
    Humphreys, E. M. L.
    ESO, D-85748 Garching, Germany..
    Lindqvist, M.
    Chalmers, Onsala Space Observ, Dept Earth & Space Sci, S-43992 Onsala, Sweden..
    Nyman, L.
    Joint ALMA Observ, Vitacura, Santiago De Chi, Chile.;ESO, Santiago, Chile..
    Ramstedt, Sofia
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    ALMA view of the circumstellar environment of the post-common-envelope-evolution binary system HD 1015842015In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 576, L15Article in journal (Refereed)
    Abstract [en]

    Aims. We study the circumstellar evolution of the binary HD 101584, consisting of a post-AGB star and a low-mass companion, which is most likely a post-common-envelope-evolution system. Methods. We used ALMA observations of the (CO)-C-12, (CO)-C-13, and (CO)-O-18 J = 2-1 lines and the 1.3 mm continuum to determine the morphology, kinematics, masses, and energetics of the circumstellar environment. Results. The circumstellar medium has a bipolar hour-glass structure, seen almost pole-on, formed by an energetic jet, approximate to 150 km s(-1). We conjecture that the circumstellar morphology is related to an event that took place approximate to 500 yr ago, possibly a capture event where the companion spiraled in towards the AGB star. However, the kinetic energy of the accelerated gas exceeds the released orbital energy, and, taking into account the expected energy transfer efficiency of the process, the observed phenomenon does not match current common-envelope scenarios. This suggests that another process must augment, or even dominate, the ejection process. A significant amount of material resides in an unresolved region, presumably in the equatorial plane of the binary system.

  • 17.
    Olofsson, H.
    et al.
    Chalmers, Dept Earth & Space Sci, S-41296 Gothenburg, Sweden..
    Vlemmings, W.
    Chalmers, Dept Earth & Space Sci, S-41296 Gothenburg, Sweden..
    Maercker, M.
    Chalmers, Dept Earth & Space Sci, S-41296 Gothenburg, Sweden..
    Humphreys, E.
    European So Observ, Garching, Germany..
    Lindqvist, M.
    Chalmers, Dept Earth & Space Sci, S-41296 Gothenburg, Sweden..
    Nyman, L.
    Joint ALMA Observ, Santiago, Chile..
    Ramstedt, Sofia
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    An ALMA View of the Complex Circumstellar Environment of the Post-AGB Object HD 1015842015In: Revolution in Astronomy with ALMA: The Third Year, 2015, Vol. 499, 319-322 p.Conference paper (Refereed)
    Abstract [en]

    We use (CO)-C-12, (CO)-C-13, and (CO)-O-18 J = 2-1 lines and 1.3 mm continuum ALMA observations to study the circumstellar evolution of the binary HD 101584, a post-AGB star and a low-mass companion, which is-most likely a post-common envelope-evolution system. It is inferred that the circumstellar medium has a bipolar hour-glass structure, seen almost pole-on, formed by an energetic, approximate to 150 km s(-1), jet. Significant amount of material resides in an unresolved central region. It is proposed that the circumstellar morphology is related to an event which took place approximate to 500 yr ago, possibly a capture event where the companion spiraled in towards the AGB star. However, the kinetic energy of the accelerated gas exceeds the released orbital energy. Hence, the observed phenomenon does not match current common-envelope scenarios, and another process must augment, or even dominate, the ejection process.

  • 18.
    Olofsson, H.
    et al.
    Chalmers, Dept Earth & Space Sci, Gothenburg, Sweden..
    Vlemmings, W.
    Chalmers, Dept Earth & Space Sci, Gothenburg, Sweden..
    Maercker, M.
    Chalmers, Dept Earth & Space Sci, Gothenburg, Sweden..
    Humphreys, E.
    European Southern Observ, Garching, Germany..
    Lindqvist, M.
    Chalmers, Dept Earth & Space Sci, Gothenburg, Sweden..
    Nyman, L.
    Joint ALMA Observ, Santiago, Chile..
    Ramstedt, Sofia
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    An ALMA view of the post-AGB object HD1015842016In: 11th Pacific Rim Conference On Stellar Astrophysics: Physics And Chemistry Of The Late Stages Of Stellar Evolution, Pts 1-6 / [ed] Kwok, S Leung, KC, IOP PUBLISHING LTD , 2016, UNSP 042005Conference paper (Refereed)
    Abstract [en]

    ALMA cycles 1 and 3 observations of CO isotopologues and 1.3mm continuum are used in a study of the circumstellar environment of the binary HD 101584, a post-AGB star and a low-mass companion that is most likely a post-common-envelope-evolution system. These data are supplemented with new information from OH maser emission. It is inferred that the large- scale circumstellar medium has a bipolar hour-glass structure, seen almost pole-on, formed by an energetic, >= 150 km s(-1), jet. Significant amount of material still resides in the central region. It is proposed that the circumstellar morphology is related to an event which took place <= 500 yr ago, possibly a capture event where the companion spiralled in towards the AGB star. Several observed features remain to be explained, and may hint to a more complicated scenario.

  • 19.
    Olofsson, Hans
    et al.
    Chalmers, Dept Earth & Space Sci, Stockholm, Sweden..
    Ramstedt, Sofia
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    The Circumstellar (CO)-C-12/(CO)-C-13 Ratio in AGB Stars of Different Chemical Type2015In: WHY GALAXIES CARE ABOUT AGB STARS III: A CLOSER LOOK IN SPACE AND TIME, ASTRONOMICAL SOC PACIFIC , 2015, Vol. 497, 295-300 p.Conference paper (Refereed)
    Abstract [en]

    We have investigated the evolution of the C-12/C-13 ratio along the AGB through the circumstellar isotopologue (CO)-C-12/(CO)-C-13 abundance ratio for a significant number of M-, S-, and C-type AGB stars. The isotopologue ratios are estimated through a detailed radiative transfer analysis of single-dish radio-line observations. The circumstellar (CO)-C-12/(CO)-C-13 ratio differs between the three spectral types; the median values are 13, 26, and 34 for the M-, S-type, and carbon AGB stars, respectively, consistent with what is expected from stellar evolutionary models assuming that the spectral types constitute an evolutionary sequence. The spread of the abundance ratio among carbon stars is much larger than for the other chemical types. We find no correlation between the isotopologue ratio and the mass-loss rate.

  • 20.
    Ramstedt, Sofia
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Mohamed, S.
    South African Astron Observ.; Univ Cape Town, Dept Astron.; South Africa Natl Inst Theoret Phys.
    Vlemmings, W. H. T.
    Chalmers Univ Technol, Dept Earth & Space Sci, Onsala Space Observ.
    Danilovich, T.
    Katholieke Univ Leuven, Inst Sterrenkunde.
    Brunner, M.
    Univ Vienna, Dept Astrophys.
    De Beck, E.
    Chalmers Univ Technol, Dept Earth & Space Sci, Onsala Space Observ.
    Humphreys, E. M. L.
    ESO, D-85748 Garching.
    Lindqvist, M.
    Chalmers Univ Technol, Dept Earth & Space Sci, Onsala Space Observ.
    Maercker, M.
    Chalmers Univ Technol, Dept Earth & Space Sci, Onsala Space Observ.
    Olofsson, H.
    Chalmers Univ Technol, Dept Earth & Space Sci, Onsala Space Observ.
    Kerschbaum, F.
    Univ Vienna, Dept Astrophys.
    Quintana-Lacaci, G.
    CSIC, Inst Ciencia Mat Madrid.
    The circumstellar envelope around the S-type AGB star W Aql Effects of an eccentric binary orbit2017In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 605, A126Article in journal (Refereed)
    Abstract [en]

    Context. Recent observations at subarcsecond resolution, now possible also at submillimeter wavelengths, have shown intricate circumstellar structures around asymptotic giant branch (AGB) stars, mostly attributed to binary interaction. The results presented here are part of a larger project aimed at investigating the effects of a binary companion on the morphology of circumstellar envelopes (CSEs) of AGB stars.

    Aims. AGB stars are characterized by intense stellar winds that build CSEs around the stars. Here, the CO(J = 3 -> 2) emission from the CSE of the binary S-type AGB star W Aql has been observed at subarcsecond resolution using ALMA. The aim of this paper is to investigate the wind properties of the AGB star and to analyse how the known companion has shaped the CSE.

    Methods. The average mass-loss rate during the creation of the detected CSE is estimated through modelling, using the ALMA brightness distribution and previously published single-dish measurements as observational constraints. The ALMA observations are presented and compared to the results from a 3D smoothed particle hydrodynamics (SPH) binary interaction model with the same properties as the W Aql system and with two different orbital eccentricities. Three-dimensional radiative transfer modelling is performed and the response of the interferometer is modelled and discussed.

    Results. The estimated average mass-loss rate of W Aql is (M) over dot = 3.0 x 10(-6) M-circle dot yr(-1) and agrees with previous results based on single-dish CO line emission observations. The size of the emitting region is consistent with photodissociation models. The inner 10 0 0 of the CSE is asymmetric with arc-like structures at separations of 23" scattered across the denser sections. Further out, weaker spiral structures at greater separations are found, but this is at the limit of the sensitivity and field of view of the ALMA observations.

    Conclusions. The CO(J = 3 -> 2) emission is dominated by a smooth component overlayed with two weak arc patterns with different separations. The larger pattern is predicted by the binary interaction model with separations of similar to 10" and therefore likely due to the known companion. It is consistent with a binary orbit with low eccentricity. The smaller separation pattern is asymmetric and coincides with the dust distribution, but the separation timescale (200 yr) is not consistent with any known process of the system. The separation of the known companions of the system is large enough to not have a very strong effect on the circumstellar morphology. The density contrast across the envelope of a binary with an even larger separation will not be easily detectable, even with ALMA, unless the orbit is strongly asymmetric or the AGB star has a much larger mass-loss rate.

  • 21.
    Ramstedt, Sofia
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy.
    Mohamed, S.
    Vlemmings, W. H. T.
    Maercker, M.
    Montez, R.
    Baudry, A.
    De Beck, E.
    Lindqvist, M.
    Olofsson, H.
    Humphreys, E. M. L.
    Jorissen, A.
    Kerschbaum, F.
    Mayer, A.
    Wittkowski, M.
    Cox, N. L. J.
    Lagadec, E.
    Leal-Ferreira, M. L.
    Paladini, C.
    Perez-Sanchez, A.
    Sacuto, S.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy.
    The wonderful complexity of the Mira AB system2014In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 570, L14- p.Article in journal (Refereed)
    Abstract [en]

    We have mapped the (CO)-C-12(3-2) line emission around the Mira AB system at 0 ''.5 resolution using the Atacama Large Millimeter/submillimeter Array (ALMA). The CO map shows amazing complexity. The circumstellar gas has been shaped by different dynamical actors during the evolution of the system, and several morphological components can be identified. The companion is marginally resolved in continuum emission and is currently at 0 ''.487 +/- 0 ''.006 separation. In the main line component, centered on the stellar velocity, spiral arcs around Mira A are found. The spiral appears to be relatively flat and oriented in the orbital plane. An accretion wake behind the companion is clearly visible, and the projected arc separation is about 5 ''. In the blue wing of the line emission, offset from the main line, several large (similar to 5-10 '') opposing arcs are found. We tentatively suggest that this structure is created by the wind of Mira B blowing a bubble in the expanding envelope of Mira A.

  • 22.
    Ramstedt, Sofia
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy.
    Olofsson, H.
    The (CO)-C-12/(CO)-C-13 ratio in AGB stars of different chemical type Connection to the C-12/C-13 ratio and the evolution along the AGB2014In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 566, A145- p.Article in journal (Refereed)
    Abstract [en]

    Aims. The aim of this paper is to investigate the evolution of the C-12/C-13 ratio along the AGB through the circumstellar (CO)-C-12/(CO)-C-13 abundance ratio. This is the first time a sample including a significant number of M-and S-type stars is analysed together with a carbon-star sample of equal size, making it possible to investigate trends among the different types and establish evolutionary effects. Methods. The circumstellar (CO)-C-12/(CO)-C-13 abundance ratios are estimated through a detailed radiative transfer analysis of single-dish radio line emission observations. Several different transitions have been observed for each source to ensure that a large extent of the circumstellar envelope is probed and the radiative transfer model is well constrained. The radiative transfer model is based on the Monte Carlo method and has been benchmarked against a set of similar codes. It assumes that the radiation field is non-local and solves the statistical equilibrium equations in full non-local thermodynamic equilibrium. The energy balance equation, determining the gas temperature distribution, is solved self-consistently, and the effects of thermal dust radiation (as estimated from the spectral energy distribution) are taken into account. First, the (CO)-C-12 radiative transfer is solved, assuming an abundance (dependent on the chemical type of the star), to give the physical parameters of the gas, i.e. mass-loss rate. M. gas expansion velocity, v(e), and gas temperature distribution. Then, the (CO)-C-13 radiative transfer is solved using the results of the (CO)-C-12 model giving the (CO)-C-13 abundance. Finally, the (CO)-C-12/(CO)-C-13 abundance ratio is calculated. Results. The circumstellar (CO)-C-12/(CO)-C-13 abundance ratio differs between the three spectral types. This is consistent with what is expected from stellar evolutionary models assuming that the spectral types constitute an evolutionary sequence; however, this is the first time this has been shown observationally for a relatively large sample covering all three spectral types. The median value of the (CO)-C-13 abundance in the inner circumstellar envelope is 1.6 x 10(-5), 2.3 x 10(-5), and 3.0 x 10(-5) for the M-type. S-type, and carbon stars of the sample, respectively, corresponding to (CO)-C-12/(CO)-C-13 abundance ratios of 13, 26, and 34, respectively. The spread in the (CO)-C-13 abundance, quantified by the ratio between the 90th and 10th percentile, is 4, 3, and 15 for the M-type. S-type, and carbon stars, respectively. Interestingly, the abundance ratio spread of the carbon stars is much larger than for the M-and S-type stars, even when excluding J-type carbon stars, in line with what could be expected from evolution on the AGB. We find no correlation between the isotopologue ratio and the mass-loss rate, as would be expected if both increase as the star evolves.

  • 23.
    Ramstedt, Sofia
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Vlemmings, W. H. T.
    Chalmers, Dept Earth & Space Sci, Onsala, Sweden..
    Mohamed, S.
    South African Astron Observ, Cape Town, South Africa..
    The Outflows of Binary AGB Stars2015In: Revolution in Astronomy with ALMA: The Third Year, 2015, Vol. 499, 331-334 p.Conference paper (Refereed)
    Abstract [en]

    The required conditions for stars to evolve into planetary nebulae (PNs) continues to puzzle. Since PNs are found in a wide variety of shapes, processes that could sculpt circumstellar envelopes (CSEs) are being investigated. A binary companion will have a strong gravitational effect, but known binary AGB stars are rare. Using ALMA in Cycle 1 and 2, we have observed a small sample of well-studied, binary AGB stars, covering a decisive range in separation, in order to determine the influence of a companion on the circumstellar morphology of the AGB primary. The first steps toward interpreting and analyzing the data have been taken, and the results will be compared to 3D Smoothed Particle Hydrodynamics (SPH) models of the gravitational interaction.

  • 24.
    Ramstedt, Sofia
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Vlemmings, Wouter H. T.
    Chalmers, Onsala Space Observ, Dept Earth & Space Sci, S-43900 Onsala, Sweden..
    Mohamed, Shazrene
    S African Astron Observ, Observatory, South Africa..
    Shaping the Outflows of Binary AGB Stars2015In: WHY GALAXIES CARE ABOUT AGB STARS III: A CLOSER LOOK IN SPACE AND TIME, ASTRONOMICAL SOC PACIFIC , 2015, Vol. 497, 187-192 p.Conference paper (Refereed)
    Abstract [en]

    The required conditions for stars to evolve into planetary nebulae (PNe) continue to puzzle. Since PNe are found in a wide variety of shapes, processes that could sculpt circumstellar envelopes (CSEs) are being investigated. A binary companion will have a strong gravitational effect, but known binary AGB stars are rare. We have observed the CO emission from a small sample of known binary AGB stars (R Aqr, Mira, W Aql, and pi(1). Gru) with ALMA. The stars cover a decisive range in binary separation, necessary to provide essential constraints for 3-D models of the binary interaction. They have previously been observed with Herschel/PACS and VLTI/MIDI allowing the circumstellar morphology to be studied from the very inner CSE out to the very largest scales. The circumstellar gas distribution will strongly depend on how the mass is initially lost from the primary. These observations will therefore not only help us understand the important processes for the binary interaction, but will also provide crucial information needed to understand the mass-loss mechanisms of the primary. The first set of observations has been delivered and preliminary results are presented.

  • 25.
    Sacuto, Stéphane
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Observational Astronomy.
    Ramstedt, Sofia
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Observational Astronomy.
    Höfner, Susanne
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Olofsson, H.
    Bladh, Sara
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Eriksson, Kjell
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Aringer, B.
    Klotz, D.
    Maercker, M.
    The wind of the M-type AGB star RT Virginis probed by VLTI/MIDI2013In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 551, A72- p.Article in journal (Refereed)
    Abstract [en]

    Aims. We study the circumstellar environment of the M-type AGB star RT Vir using mid-infrared high spatial resolution observations from the ESO-VLTI focal instrument MIDI. The aim of this study is to provide observational constraints on theoretical prediction that the winds of M-type AGB objects can be driven by photon scattering on iron-free silicate grains located in the close environment (about 2 to 3 stellar radii) of the star. Methods. We interpreted spectro-interferometric data, first using wavelength-dependent geometric models. We then used a self-consistent dynamic model atmosphere containing a time-dependent description of grain growth for pure forsterite dust particles to reproduce the photometric, spectrometric, and interferometric measurements of RT Vir. Since the hydrodynamic computation needs stellar parameters as input, a considerable effort was first made to determine these parameters. Results. MIDI differential phases reveal the presence of an asymmetry in the stellar vicinity. Results from the geometrical modeling give us clues to the presence of aluminum and silicate dust in the close circumstellar environment (<5 stellar radii). Comparison between spectro-interferometric data and a self-consistent dust-driven wind model reveals that silicate dust has to be present in the region between 2 to 3 stellar radii to reproduce the 59 and 63 m baseline visibility measurements around 9.8 mu m. This gives additional observational evidence in favor of winds driven by photon scattering on iron-free silicate grains located in the close vicinity of an M-type star. However, other sources of opacity are clearly missing to reproduce the 10-13 mu m visibility measurements for all baselines. Conclusions. This study is a first attempt to understand the wind mechanism of M-type AGB stars by comparing photometric, spectrometric, and interferometric measurements with state-of-the-art, self-consistent dust-driven wind models. The agreement of the dynamic model atmosphere with interferometric measurements in the 8-10 mu m spectral region gives additional observational evidence that the winds of M-type stars can be driven by photon scattering on iron-free silicate grains. Finally, a larger statistical study and progress in advanced self-consistent 3D modeling are still required to solve the remaining problems.

  • 26. Sanchez, Enmanuel
    et al.
    Montez, Rodolfo, Jr.
    Ramstedt, Sofia
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Stassun, Keivan G.
    First Detection of Ultraviolet Emission from a Detached Dust Shell: Galaxy Evolution Explorer Observations of the Carbon Asymptotic Giant Branch Star U Hya2015In: Astrophysical Journal Letters, ISSN 2041-8205, E-ISSN 2041-8213, Vol. 798, no 2, L39- p.Article in journal (Refereed)
    Abstract [en]

    We present the discovery of an extended ring of ultraviolet (UV) emission surrounding the asymptotic giant branch (AGB) star U Hya in archival observations performed by the Galaxy Evolution Explorer. This is the third discovery of extended UV emission from a carbon AGB star and the first from an AGB star with a detached shell. From imaging and photometric analysis of the FUV and NUV images, we determined that the UV ring has a radius of similar to 110 '', thus indicating that the emitting material is likely associated with the detached shell seen in the infrared. We find that scattering of the central point source of NUV and FUV emission by the dust shell is negligible. Moreover, we find that scattering of the interstellar radiation field by the dust shell can contribute at most similar to 10% of the FUV flux. Morphological and photometric evidence suggests that shocks caused by the star's motion through space and, possibly, shock-excited H-2 molecules are the most likely origins of the UV flux. In contrast to previous examples of extended UV emission from AGB stars, the extended UV emission from U Hya does not show a bow-shock-like structure, which is consistent with a lower space velocity and lower interstellar medium density. This suggests the detached dust shell is the source of the UV-emitting material and can be used to better understand the formation of detached shells.

  • 27. Ueta, T.
    et al.
    Ladjal, D.
    Exter, K. M.
    Otsuka, M.
    Szczerba, R.
    Siodmiak, N.
    Aleman, I.
    van Hoof, P. A. M.
    Kastner, J. H.
    Montez, R., Jr.
    McDonald, I.
    Wittkowski, M.
    Sandin, C.
    Ramstedt, Sofia
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    De Marco, O.
    Villaver, E.
    Chu, Y-H
    Vlemmings, W.
    Izumiura, H.
    Sahai, R.
    Lopez, J. A.
    Balick, B.
    Zijlstra, A.
    Tielens, A. G. G. M.
    Rattray, R. E.
    Behar, E.
    Blackman, E. G.
    Hebden, K.
    Hora, J. L.
    Murakawa, K.
    Nordhaus, J.
    Nordon, R.
    Yamamura, I.
    The Herschel Planetary Nebula Survey (HerPlaNS) I. Data overview and analysis demonstration with NGC 67812014In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 565, A36- p.Article in journal (Refereed)
    Abstract [en]

    Context. This is the first of a series of investigations into far-IR characteristics of 11 planetary nebulae (PNe) under the Herschel Space Observatory open time 1 program, Herschel Planetary Nebula Survey (HerPlaNS). Aims. Using the HerPlaNS data set, we look into the PN energetics and variations of the physical conditions within the target nebulae. In the present work, we provide an overview of the survey, data acquisition and processing, and resulting data products. Methods. We performed (1) PACS/SPIRE broadband imaging to determine the spatial distribution of the cold dust component in the target PNe and (2) PACS/SPIRE spectral-energy-distribution and line spectroscopy to determine the spatial distribution of the gas component in the target PNe. Results. For the case of NGC 6781, the broadband maps confirm the nearly pole-on barrel structure of the amorphous carbon-rich dust shell and the surrounding halo having temperatures of 26-40 K. The PACS/SPIRE multiposition spectra show spatial variations of far-.IR lines that reflect the physical stratification of the nebula. We demonstrate that spatially resolved far-IR line diagnostics yield the (T-e, n(e)) profiles, from which distributions of ionized, atomic, and molecular gases can be determined. Direct comparison of the dust and gas column mass maps constrained by the HerPlaNS data allows to construct an empirical gas-to-dust mass ratio map, which shows a range of ratios with the median of 195 +/- 110. The present analysis yields estimates of the total mass of the shell to be 0.86 M-circle dot, consisting of 0.54 M-circle dot of ionized gas, 0.12 M-circle dot of atomic gas, 0.2 M-circle dot of molecular gas, and 4 x 10(-3) M-circle dot of dust grains. These estimates' also suggest that the central star of about 1.5 M-circle dot initial mass is terminating its PN evolution onto the white dwarf cooling track. Conclusions. The HerPlaNS data provide various diagnostics for both the dust and gas components in a spatially resolved manner. In the forthcoming papers of the HerPlaNS series we will explore the HerPlaNS data set fully for the entire sample of 11 PNe.

  • 28. Vlemmings, W. H. T.
    et al.
    Maercker, M.
    Lindqvist, M.
    Mohamed, S.
    Olofsson, H.
    Ramstedt, Sofia
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Brunner, M.
    Groenewegen, M. A. T.
    Kerschbaum, F.
    Wittkowski, M.
    ALMA observations of the variable (CO)-C-12/(CO)-C-13 ratio around the asymptotic giant branch star R Sculptoris2013In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 556, L1- p.Article in journal (Refereed)
    Abstract [en]

    The (CO)-C-12/(CO)-C-13 ratio is often used as a measure of the C-12/C-13 ratio in the circumstellar environment, carrying important information about the stellar nucleosynthesis. External processes can change the (CO)-C-12 and (CO)-C-13 abundances, and spatially resolved studies of the (CO)-C-12/(CO)-C-13 ratio are needed to quantify the effect of these processes on the globally determined values. Additionally, such studies provide important information on the conditions in the circumstellar environment. The detached-shell source R Scl, displaying CO emission from recent mass loss, in a binary-induced spiral structure as well as in a clumpy shell produced during a thermal pulse, provides a unique laboratory for studying the differences in CO isotope abundances throughout its recent evolution. We observed both the (CO)-C-12(J = 3 -> 2) and the (CO)-C-13(J = 3 -> 2) line using ALMA. We find significant variations in the (CO)-C-12/(CO)-C-13 intensity ratios and consequently in the abundance ratios. The average CO isotope abundance ratio is at least a factor three lower in the shell (similar to 19) than that in the present-day (less than or similar to 300 years) mass loss (>60). Additionally, variations in the ratio of more than an order of magnitude are found in the shell itself. We attribute these variations to the competition between selective dissociation and isotope fractionation in the shell, of which large parts cannot be warmer than similar to 35 K. However, we also find that the (CO)-C-12/(CO)-C-13 ratio in the present-day mass loss is significantly higher than the C-12/C-13 ratio determined in the stellar photosphere from molecular tracers (similar to 19). The origin of this discrepancy is still unclear, but we speculate that it is due to an embedded source of UV-radiation that is primarily photo-dissociating (CO)-C-13. This radiation source could be the hitherto hidden companion. Alternatively, the UV-radiation could originate from an active chromosphere of R Scl itself. Our results indicate that caution should be taken when directly relating the (CO)-C-12/(CO)-C-13 intensity and C-12/C-13 abundance ratios for specific asymptotic giant branch stars, in particular binaries or stars that display signs of chromospheric stellar activity.

  • 29.
    Vlemmings, W. H. T.
    et al.
    Chalmers, Onsala Space Observ, Dept Earth & Space Sci, S-43992 Onsala, Sweden..
    Ramstedt, Sofia
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    O'Gorman, E.
    Chalmers, Onsala Space Observ, Dept Earth & Space Sci, S-43992 Onsala, Sweden..
    Humphreys, E. M. L.
    ESO, D-85748 Garching, Germany..
    Wittkowski, M.
    ESO, D-85748 Garching, Germany..
    Baudry, A.
    Univ Bordeaux 1, LAB, UMR 5804, F-33270 Floirac, France.;CNRS, LAB, UMR 5804, F-33270 Floirac, France..
    Karovska, M.
    Smithsonian Astrophys Observ, Cambridge, MA 02138 USA..
    Resolving the stellar activity of the Mira AB binary with ALMA2015In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 577, L4Article in journal (Refereed)
    Abstract [en]

    Aims. We present the size, shape, and flux densities at millimeter continuum wavelengths, based on ALMA science verification observations in Band 3 (similar to 94.6 GHz) and Band 6 (similar to 228.7 GHz), from the binary Mira A (o Ceti) and Mira B. Methods. The Mira AB system was observed with ALMA at a spatial resolution down to similar to 25 mas. The extended atmosphere of Mira A and the wind around Mira B sources were resolved, and we derived the sizes of Mira A and of the ionized region around Mira B. The spectral indices within Band 3 (between 89-100 GHz) and between Bands 3 and 6 were also derived. Results. The spectral index of Mira A is found to change from 1.71 +/- 0.05 within Band 3 to 1.54 +/- 0.04 between Bands 3 and 6. The spectral index of Mira B is 1.3 +/- 0.2 in Band 3, in good agreement with measurements at longer wavelengths; however, it rises to 1.72 +/- 0.11 between the bands. For the first time, the extended atmosphere of a star is resolved at these frequencies, and for Mira A the diameter is similar to 3.8 x 3.2 AU in Band 3 (with brightness temperature T-b similar to 5300 K) and similar to 4.0 x 3.6 AU in Band 6 (T-b similar to 2500 K). Additionally, a bright hotspot similar to 0.4 AU, with T-b similar to 10 000 K, is found on the stellar disk of Mira A. The size of the ionized region around the accretion disk of Mira B is found to be similar to 2.4 AU. Conclusions. The emission around Mira B is consistent with emission from a partially ionized wind of gravitationally bound material from Mira A close to the accretion disk of Mira B. The Mira A atmosphere does not fully match predictions with brightness temperatures in Band 3 significantly higher than expected, potentially owing to shock heating. The hotspot is very likely due to magnetic activity and could be related to the previously observed X-ray flare of Mira A.

  • 30.
    Wittkowski, M.
    et al.
    European Southern Observ, Karl Schwarzschild Str 2, D-85748 Garching, Germany..
    Hofmann, K. -H
    Max Planck Inst Radioastron, Hugel 69, D-53121 Bonn, Germany.
    Höfner, S.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy.
    Le Bouquin, J. B.
    Univ Grenoble Alpes, CNRS, IPAG, F-38000 Grenoble, France..
    Nowotny, W.
    Univ Vienna, Dept Astrophys, Turkenschanzstr 17, A-1180 Vienna, Austria..
    Paladini, C.
    Univ Libre Bruxelles, Inst Astron & Astrophys, CP 226,Blvd Triomphe, B-1050 Brussels, Belgium..
    Young, J.
    Cavendish Lab, Astrophys Grp, JJ Thomson Ave, Cambridge CB3 0HE, England..
    Berger, J. -P
    Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France .
    Brunner, M.
    Univ Vienna, Dept Astrophys, Turkenschanzstr 17, A-1180 Vienna, Austria..
    de Gregorio-Monsalvo, I.
    Joint ALMA Off, Alonso de Cordova 3107,Casilla 19001, Santiago 19, Chile.;European Southern Observ, Alonso de Cordova 3107, Santiago, Chile..
    Eriksson, K.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy.
    Hron, J.
    Univ Vienna, Dept Astrophys, Turkenschanzstr 17, A-1180 Vienna, Austria..
    Humphreys, E. M. L.
    European Southern Observ, Karl Schwarzschild Str 2, D-85748 Garching, Germany..
    Lindqvist, M.
    Chalmers, Onsala Space Observ, Dept Earth & Space Sci, S-43992 Onsala, Sweden..
    Maercker, M.
    Chalmers, Onsala Space Observ, Dept Earth & Space Sci, S-43992 Onsala, Sweden..
    Mohamed, S.
    South African Astron Observ, POB 9, ZA-7935 Observatory, South Africa.;Univ Cape Town, Astron Dept, ZA-7701 Rondebosch, South Africa.;Natl Inst Theoret Phys, Private Bag X1, ZA-7602 Matieland, South Africa..
    Olofsson, H.
    Chalmers, Onsala Space Observ, Dept Earth & Space Sci, S-43992 Onsala, Sweden..
    Ramstedt, S.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy.
    Weigelt, G.
    Max Planck Inst Radioastron, Hugel 69, D-53121 Bonn, Germany..
    Aperture synthesis imaging of the carbon AGB star R Sculptoris Detection of a complex structure and a dominating spot on the stellar disk2017In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 601, A3Article in journal (Refereed)
    Abstract [en]

    Aims. We present near-infrared interferometry of the carbon-rich asymptotic giant branch (AGB) star R Sculptoris (R Scl). Methods. We employ medium spectral resolution K-band interferometry obtained with the instrument AMBER at the Very Large Telescope Interferometer (VLTI) and H-band low spectral resolution interferometric imaging observations obtained with the VLTI instrument PIONIER. We compare our data to a recent grid of dynamic atmosphere and wind models. We compare derived fundamental parameters to stellar evolution models. Results. The visibility data indicate a broadly circular resolved stellar disk with a complex substructure. The observed AMBER squared visibility values show drops at the positions of CO and CN bands, indicating that these lines form in extended layers above the photosphere. The AMBER visibility values are best fit by a model without a wind. The PIONIER data are consistent with the same model. We obtain a Rosseland angular diameter of 8.9 +/- 0.3 mas, corresponding to a Rosseland radius of 355 +/- 55 R-Theta, an effective temperature of 2640 +/- 80 K, and a luminosity of log L/L-Theta = 3.74 +/- 0.18. These parameters match evolutionary tracks of initial mass 1.5 +/- 0.5 M-Theta and current mass 1.3 +/- 0.7 M-Theta. The reconstructed PIONIER images exhibit a complex structure within the stellar disk including a dominant bright spot located at the western part of the stellar disk. The spot has an H- band peak intensity of 40% to 60% above the average intensity of the limb-darkening-corrected stellar disk. The contrast between the minimum and maximum intensity on the stellar disk is about 1:2.5. Conclusions. Our observations are broadly consistent with predictions by dynamic atmosphere and wind models, although models with wind appear to have a circumstellar envelope that is too extended compared to our observations. The detected complex structure within the stellar disk is most likely caused by giant convection cells, resulting in large-scale shock fronts, and their effects on clumpy molecule and dust formation seen against the photosphere at distances of 2-3 stellar radii.

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