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High-resolution HST/ACS images of detached shells around carbon stars  
Stockholm University, Faculty of Science, Department of Astronomy.
Stockholm University, Faculty of Science, Department of Astronomy.
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2010 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 515, A27- p.Article in journal (Refereed) Published
Abstract [en]

Context. Overall spherically symmetric, geometrically thin gas and dust shells have been found around a handful of asymptotic giant branch (AGB) carbon stars. Their dynamical ages lie in the range of 103 to 104 years. A tentative explanation for their existence is that they have formed as a consequence of mass-loss-rate modulations during a He-shell flash.

Aims. The detached shells carry information on their formation process, as well as on the small-scale structure of the circumstellar medium around AGB stars due to the absence of significant line-of-sight confusion.

Methods. The youngest detached shells, those around the carbon stars R Scl and U Cam, are studied here in great detail in scattered stellar light with the Advanced Survey Camera on the Hubble Space Telescope. Quantitative results are derived assuming optically thin dust scattering.

Results. The detached dust shells around R Scl and U Cam are found to be consistent with an overall spherical symmetry. They have radii of 192 (corresponding to a linear size of 8 × 1016 cm) and 77 (5 × 1016 cm), widths of 12 (5 × 1015 cm) and 06 (4 × 1015 cm), and dust masses of 3 × 10-6 and 3 × 10-7 , respectively. The dynamical ages of the R Scl and U Cam shells are estimated to be 1700 and 700 yr, respectively, and the shell widths correspond to time scales of 100 and 50 yr, respectively. Small-scale structure in the form of less than arcsec-sized clumps is clearly seen in the images of the R Scl shell. Average clump dust masses are estimated to be about 2 × 10-9 . Comparisons with CO line interferometer data show that the dust and gas shells coincide spatially, within the errors (1´´ for U Cam and 2´´ for R Scl).

Conclusions. The results are consistent with the interpretation of geometrically thin gas and dust shells formed by a mass-loss eruption during a He-shell flash, and where interaction with a previous wind plays a role as well. The mass loss responsible for the shells must have been remarkably isotropic, and, if wind interaction plays a role, this also applies to the mass loss prior to the eruption. Clumpy structure is present in the R Scl shell, possibly as a consequence of the mass loss itself, but more likely as a consequence of instabilities in the expanding shell.

Place, publisher, year, edition, pages
2010. Vol. 515, A27- p.
Keyword [en]
AGB and post-AGB, carbon, circumstellar matter, mass-loss
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
URN: urn:nbn:se:su:diva-31424DOI: 10.1051/0004-6361/200913929ISI: 000278399000027OAI: oai:DiVA.org:su-31424DiVA: diva2:276945
Note
authorCount :5Available from: 2009-11-13 Created: 2009-11-13 Last updated: 2017-12-12Bibliographically 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.
Keyword
AGB stars, evolution, mass-loss, abundances
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy
Identifiers
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)
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Supervisors
Note
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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