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Molecules and Dust around AGB stars: Mass-loss rates and molecular abundances
Stockholm University, Faculty of Science, Department of Astronomy.
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 [en]
AGB stars, mass-loss, circumstellar abundances
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy
Identifiers
URN: urn:nbn:se:su:diva-27405ISBN: 978-91-7155-871-8 (print)OAI: oai:DiVA.org:su-27405DiVA: diva2:214054
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
List of papers
1. The abundance of HCN in the circumstellar envelopes of AGB stars different chemical type
Open this publication in new window or tab >>The abundance of HCN in the circumstellar envelopes of AGB stars different chemical type
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2013 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 550, A78Article in journal (Refereed) Published
Abstract [en]

Aims. A multi-transition survey of HCN (sub-) millimeter line emission from a large sample of asymptotic giant branch (AGB) stars of different chemical type is presented. The data are analysed and circumstellar HCN abundances are estimated. The sample stars span a large range of properties such as mass-loss rate and photospheric C/O-ratio. The analysis of the new data allows for more accurate estimates of the circumstellar HCN abundances and puts new constraints on chemical models. Methods. In order to constrain the circumstellar HCN abundance distribution a detailed non-local thermodynamic equilibrium (LTE) excitation analysis, based on the Monte Carlo method, is performed. Effects of line overlaps and radiative excitation from dust grains are included. Results. The median values for the derived abundances of HCN (with respect to H-2) are 3 x 10(-5), 7 x 10(-7) and 10(-7) for carbon stars (25 stars), S-type AGB stars (19 stars) and M-type AGB stars (25 stars), respectively. The estimated sizes of the HCN envelopes are similar to those obtained in the case of SiO for the same sample of sources and agree well with previous results from interferometric observations, when these are available. Conclusions. We find that there is a clear dependence of the derived circumstellar HCN abundance on the C/O-ratio of the star, in that carbon stars have about two orders of magnitude higher abundances than M-type AGB stars, on average. The derived HCN abundances of the S-type AGB stars have a larger spread and typically fall in between those of the two other types, however, slightly closer to the values for the M-type AGB stars. For the M-type stars, the estimated abundances are much higher than what would be expected if HCN is formed in thermal equilibrium. However, the results are also in contrast to predictions from recent non-LTE chemical models, where very little difference is expected in the HCN abundances between the various types of AGB stars.

National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy
Identifiers
urn:nbn:se:su:diva-26780 (URN)10.1051/0004-6361/201220400 (DOI)000314879700078 ()
Funder
Swedish Research CouncilEU, FP7, Seventh Framework Programme, 227290
Available from: 2009-04-14 Created: 2009-04-14 Last updated: 2017-12-13Bibliographically approved
2. Mass-loss properties of S-stars on the AGB
Open this publication in new window or tab >>Mass-loss properties of S-stars on the AGB
2006 (English)In: Astronomy & Astrophysics, ISSN 0004-6361, Vol. 454, no 2, L103-L106 p.Article in journal (Refereed) Published
Abstract [en]

We have used a detailed non-LTE radiative transfer code to model new APEX CO(J = 3 → 2) data, and existing CO radio line data, on a sample of 40 AGB S-stars. The derived mass-loss-rate distribution has a median value of 2 × 10-7~Mȯ yr-1, and resembles values obtained for similar samples of M-stars and carbon stars. Possibly, there is a scarcity of high-mass-loss-rate (≥10-5~Mȯ yr-1) S-stars. The distribution of envelope gas expansion velocities is similar to that of the M-stars, the median is 7.5 km s-1, while the carbon stars, in general, have higher gas expansion velocities. The mass-loss rate correlates well with the gas expansion velocity, in accordance with results for M-stars and carbon stars.

Keyword
stars: AGB and post-AGB, stars: circumstellar matter, stars: late-type, stars: mass-loss
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:su:diva-20159 (URN)doi:10.1051/0004-6361:20065285 (DOI)
Available from: 2007-02-27 Created: 2007-02-27 Last updated: 2011-01-11Bibliographically approved
3. On the reliability of mass-loss-rate estimates for AGB stars
Open this publication in new window or tab >>On the reliability of mass-loss-rate estimates for AGB stars
2008 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 487, no 2, 645-657 p.Article in journal (Refereed) Published
Abstract [en]

Context. In the recent literature there has been some doubt as to the reliability of CO multi-transitional line observations as a mass-loss-rate estimator for AGB stars. Aims. Using new well-calibrated CO radio line observations, the main aim of the work presented here is to carefully evaluate the reliability of CO mass-loss-rate estimates for intermediate-to high-mass-loss-rate AGB stars with different photospheric chemistries. Methods. Mass-loss rates for 10 intermediate-to high-mass-loss-rate AGB stars are derived using a detailed non-LTE, non-local radiative transfer code based on the Monte-Carlo method to model the CO radio line intensities. The circumstellar envelopes are assumed to be spherically symmetric and formed by constant mass-loss rates. The energy balance is solved self-consistently and the effects of dust on the radiation field and thermal balance included. An independent estimate of the mass-loss rate is also obtained from the combination of dust radiative transfer modelling with a dynamical model of the gas and dust particles. Results. We find that the CO radio line intensities and shapes are successfully reproduced for the majority of our objects when assuming a constant mass-loss rate. Moreover, the CO line intensities are only weakly dependent on the adopted micro-turbulent velocity, in contrast to recent claims in the literature. The two methods used in the present work to derive mass-loss rates are consistent within a factor of similar to 3 for intermediate- to high-mass-loss-rate objects, indicating that this is a lower limit to the uncertainty in present mass-loss-rate estimates. We find a tentative trend with chemistry. Mass-loss rates from the dust/dynamical model are systematically higher than those from the CO model for the carbon stars and vice versa for the M-type stars. This could be ascribed to a discrepancy in the adopted CO/H(2)-abundance ratio, but we caution that the sample is small and systematic errors cannot be excluded.

Keyword
stars: AGB and post-AGB, stars: carbon, stars: late-type, stars: mass-loss
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:su:diva-18324 (URN)10.1051/0004-6361:20078876 (DOI)000258327900035 ()
Available from: 2009-01-25 Created: 2009-01-25 Last updated: 2017-12-13Bibliographically approved
4. Circumstellar molecular line emission from S-type AGB stars: Mass-loss rates and SiO abundances
Open this publication in new window or tab >>Circumstellar molecular line emission from S-type AGB stars: Mass-loss rates and SiO abundances
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.

Keyword
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:nbn:se:su:diva-26778 (URN)10.1051/0004-6361/200911730 (DOI)000266730500016 ()
Available from: 2009-04-14 Created: 2009-04-14 Last updated: 2017-12-13Bibliographically approved

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