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Circumstellar molecular line emission from S-type AGB stars: Mass-loss rates and SiO abundances
Stockholm University, Faculty of Science, Department of Astronomy.
2009 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 499, no 9, 515-527 p.Article in journal (Refereed) Published
Abstract [en]

Aims. The main aim is to derive reliable mass-loss rates and circumstellar SiO abundances for a sample of 40 S-type AGB stars based on new multi-transitional CO and SiO radio line observations. In addition, the results are compared to previous results for M-type AGB stars and carbon stars to look for trends with chemical type.Methods. The circumstellar envelopes are assumed to be spherically symmetric and formed by a constant mass-loss rate. The mass-loss rates are estimated from fitting the CO observations using a non-local, non-LTE radiative transfer code based on the Monte Carlo method. In the excitation analysis, the energy balance equation is solved self-consistently simultaneously as the radiative transfer and the temperature structure of the gas is derived. Effects of dust grains are also included in the molecular excitation analysis. Once the physical properties of the circumstellar envelopes are determined, the same radiative transfer code is used to model the observed SiO lines in order to derive circumstellar abundances and the sizes of the SiO line-emitting regions.Results. We have estimated mass-loss rates of 40 S-type AGB stars and find that the derived mass-loss rates have a distribution that resembles those previously derived for similar samples of M-type AGB stars and carbon stars. The estimated mass-loss rates also correlate well with the corresponding expansion velocity of the envelope, in accordance with results for M-type AGB stars and carbon stars. In all, this indicates that the mass loss is driven by the same mechanism in all three chemical types of AGB stars. In addition, we have estimated the circumstellar fractional abundance of SiO relative to H2 in 26 of the sample S-type AGB stars. The derived SiO abundances are, on average, about an order of magnitude higher than predicted by stellar atmosphere thermal equilibrium chemistry, indicating that non-equilibrium chemical processes determines the abundance of SiO in the circumstellar envelope. Moreover, a comparison with the results for M-type AGB stars and carbon stars show that for a certain mass-loss rate, the circumstellar SiO abundance seems independent (although with a large scatter) of the C/O-ratio.Conclusions. In our comparison of S-type AGB stars with carbon stars and M-type AGB stars, we find no large differences in circumstellar physical properties or SiO abundances depending on the chemical type of the star.

Place, publisher, year, edition, pages
2009. Vol. 499, no 9, 515-527 p.
Keyword [en]
Stars: AGB and post-AGB, Stars: abundances, Stars: carbon, Stars: late-type, Stars: mass-loss
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy
Identifiers
URN: urn:nbn:se:su:diva-26778DOI: 10.1051/0004-6361/200911730ISI: 000266730500016OAI: oai:DiVA.org:su-26778DiVA: diva2:211335
Available from: 2009-04-14 Created: 2009-04-14 Last updated: 2017-12-13Bibliographically approved
In thesis
1. Molecules and Dust around AGB stars: Mass-loss rates and molecular abundances
Open this publication in new window or tab >>Molecules and Dust around AGB stars: Mass-loss rates and molecular abundances
2009 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

All stars with a stellar mass of about 0.8–8 MSun will end their lives as asymptotic giant branch (AGB) stars. Through their extensive mass loss the AGB stars constitutes an important source of nuclear processed material. They also provide us with fascinating systems where an interchange of different physical and chemical processes occur, making them excellent astrophysical laboratories.

Being the most important process for the evolution of an AGB star, the mass loss is well established, but its details are less well known. On the AGB, the mass-loss rate can span several orders of magnitude, reaching 104 MSun yr1 toward the end of the AGB. It is challenging to find reliable methods to estimate the mass-loss rates of individual objects. Nevertheless, it is important, since the mass-loss rate affects the derived abundances of other molecules in the circumstellar envelope, and therefore the estimates of the amount and composition of the recycled material. In the first part of the thesis the reliability of mass-loss rate estimates is evaluated using two main methods; the observations and radiative transfer analysis of CO radio line emission, and dust radiative transfer combined with a dynamical model.

The second part of the thesis focuses on a particular chemical type of AGB stars; the S-type. The S-stars are believed to have approximately the same amount of carbon as oxygen in the photosphere, and to be an intermediate evolutionary stage as the star evolves from an oxygen-rich M-star into a carbon star. As possible transition objects the S-stars might give important clues to the mass-loss mechanisms and to the chemical evolution along the AGB. Using observations of circumstellar radio line emission in combination with detailed radiative transfer analysis, we have estimated mass-loss rates and abundances of chemically important molecules for a sample of 40 S-stars. The results are compared to previous results for M- and carbon stars.

Place, publisher, year, edition, pages
Stockholm: Department of Astronomy, Stockholm University, 2009. 74 p.
Keyword
AGB stars, mass-loss, circumstellar abundances
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy
Identifiers
urn:nbn:se:su:diva-27405 (URN)978-91-7155-871-8 (ISBN)
Public defence
2009-06-04, FB42, AlbaNova universitetscentrum, Roslagstullsbacken 21, Stockholm, 10:00 (English)
Opponent
Supervisors
Available from: 2009-05-14 Created: 2009-05-01 Last updated: 2012-11-27Bibliographically approved

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