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The detached dust and gas shells around the carbon star U Ant
Stockholm University, Faculty of Science, Department of Astronomy.
2010 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 511, no a37Article in journal (Other academic) Published
Abstract [en]

Context. Geometrically thin, detached shells of gas have been found around a handful of carbon stars. The current knowledge on these shells is mostly based on CO radio line data. However, imaging in scattered stellar light adds important new information as well as allows studies of the dust shells.

Aims. Previous observations of scattered stellar light in the circumstellar medium around the carbon star U Ant were taken through filters centred on the resonance lines of K and Na. These observations could not separate the scattering by dust and atoms. The aim of this paper is to remedy this situation.

Methods. We have obtained polarization data on stellar light scattered in the circumstellar medium around U Ant through filters which contain no strong lines, making it possible to differentiate between the two scattering agents. Kinematic, as well as spatial, information on the gas shells were obtained through high-resolution echelle spectrograph observations of the KI and NaD lines.

Results. We confirm the existence of two detached shells around U Ant. The inner shell (at a radius of 43´´ and a width of 2´´) consists mainly of gas, while the outer shell (at a radius of 50´´ and a width of 7´´) appears to consist exclusively of dust. Both shells appear to have an over-all spherical geometry. The gas shell mass is estimated to be 2  , while the mass of the dust shell is estimated to be 5  . The derived expansion velocity, from the KI and NaD lines, of the gas shell, 19.5 , agrees with that obtained from CO radio line data. The inferred shell age is 2700 years. There is structure, e.g. in the form of arcs, inside the gas shell, but it is not clear whether these are due to additional shells.

Conclusions. Our results support the hypothesis that the observed geometrically thin, detached shells around carbon stars are the results of brief periods of intense mass loss, probably associated with thermal pulses, and subsequent wind-wind interactions. The separation into a gas and a dust shell, with different widths, is most likely the effect of different dynamical evolutions of the two media after their ejection.

Place, publisher, year, edition, pages
EDP Sciences , 2010. Vol. 511, no a37
Keyword [en]
stars: abundances / stars: AGB and post-AGB / stars: evolution / stars: mass-loss
National Category
Astronomy, Astrophysics and Cosmology
URN: urn:nbn:se:su:diva-31423DOI: 10.1051/0004-6361/200913376ISI: 000275752400080OAI: diva2:276943
Available from: 2009-11-13 Created: 2009-11-13 Last updated: 2011-12-05Bibliographically approved
In thesis
1. Asymptotic Giant Branch stars viewed up-close and far-off: The physics, chemistry, and evolution of their circumstellar envelopes
Open this publication in new window or tab >>Asymptotic Giant Branch stars viewed up-close and far-off: The physics, chemistry, and evolution of their circumstellar envelopes
2009 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The asymptotic giant branch (AGB) is the last stage of stellar evolution for stars with masses between 0.8-8 M(sun). This phase is characterised by an intense mass loss, which builds up a circumstellar envelope of dust and gas(CSE). It is through this process that the AGB stars contribute to the chemical evolution of galaxies. In addition, a rich and varied chemistry is active within the CSEs.Observations of circumstellar H2O are of particular interest, as it is expected to be one of the most abundant molecules in the inner envelopes of M-type AGB stars (with C/O<1). The first part of this thesis concerns the modelling of water vapour emission lines from CSEs around M-type AGB stars. Using satellite observations and detailed radiative transfer models, H2O abundances in these stars are determined and compared with theoretical chemical models. The importance of resolved H2O line profiles and excitation through different vibrationally excited states are also demonstrated.The second part of the thesis has its focus on the detached shells of dust and gas observed around a handful of carbon AGB stars (with C/O>1), believed to be an effect of highly time-variable mass loss during a thermal pulse. The detached shells around three sources were observed in stellar light scattered by dust and gas in the shells using ground-based and space telescopes. The observations allow a separation of the scattering agents, and reveal information on the detached shells in unprecedented detail.

Place, publisher, year, edition, pages
Stockholm: Department of Astronomy, Stockhoom University, 2009. 65 p.
AGB stars, evolution, mass-loss, abundances
National Category
Astronomy, Astrophysics and Cosmology
Research subject
urn:nbn:se:su:diva-31425 (URN)978-91-7155-975-3 (ISBN)
Public defence
2010-01-18, Sal FB 52, AlbaNova universitetscentrum, Roslagstullsbacken 21, Stockholm, 10:00 (English)
At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 2: Submitted. Paper 4: In progress.Available from: 2009-12-17 Created: 2009-11-13 Last updated: 2009-12-22Bibliographically approved

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