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Circumstellar water vapour in M-type AGB stars: constraints from H2O(1,10-1,01) lines obtained with odin
Stockholm University, Faculty of Science, Department of Astronomy.
Stockholm University, Faculty of Science, Department of Astronomy. Onsala Space Observatory, Sweden.
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2009 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 494, 243-252 p.Article in journal (Refereed) Published
Abstract [en]

Context. A detailed radiative transfer code has been previously used to model circumstellar ortho-H2O line emission towards six M-type asymptotic giant branch stars using Infrared Space Observatory Long Wavelength Spectrometer data. Collisional and radiative excitation, including the v(2) = 1 state, was considered. Aims. Spectrally resolved circumstellar H2O(1(10)-1(01)) lines have been obtained towards three M-type AGB stars using the Odin satellite. This provides additional strong constraints on the properties of circumstellar H2O, in particular on the chemistry in the stellar atmosphere, and the photodissociation in the outer envelope. Methods. Infrared Space Observatory and Odin satellite H2O line data are used as constraints for radiative transfer models. Special consideration is taken to the spectrally resolved Odin line profiles, and the effect of excitation to the first excited vibrational states of the stretching modes (v(1) = 1 and v(3) = 1) on the derived abundances is estimated. A non-local, radiative transfer code based on the accelerated lambda iteration formalism is used. A statistical analysis is performed to determine the best-fit models. Results. The H2O abundance estimates are in agreement with previous estimates. The inclusion of the Odin data sets stronger constraints on the size of the H2O envelope. The H2O(1(10)-1(01)) line profiles require a significant reduction in expansion velocity compared to the terminal gas expansion velocity determined in models of CO radio line emission, indicating that the H2O emission lines probe a region where the wind is still being accelerated. Including the v(3) = 1 state significantly lowers the estimated abundances for the low-mass-loss-rate objects. This shows the importance of detailed modelling, in particular the details of the infrared spectrum in the range 3 to 6 mu m, to estimate accurate circumstellar H2O abundances. Conclusions. Spectrally resolved circumstellar H2O emission lines are important probes of the physics and chemistry in the inner regions of circumstellar envelopes around asymptotic giant branch stars. Predictions for H2O emission lines in the spectral range of the upcoming Herschel/HIFI mission indicate that these observations will be very important in this context.

Place, publisher, year, edition, pages
2009. Vol. 494, 243-252 p.
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-31234DOI: 10.1051/0004-6361:200810017ISI: 000263040200020OAI: diva2:275852
Available from: 2009-11-09 Created: 2009-11-09 Last updated: 2015-11-16Bibliographically 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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