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Circumstellar water vapour in M-type AGB stars: radiative transfer models, abundances, and predictions for HIFI
Stockholm University, Faculty of Science, Department of Astronomy.
Stockholm University, Faculty of Science, Department of Astronomy.
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2008 (English)In: Astronomy and Astrophysics, Vol. 479, no 1, 779-791 p.Article in journal (Refereed) Published
Abstract [en]

Context: Surprisingly high amounts of H2O have recently been reported in the circumstellar envelope around the M-type asymptotic giant branch star W Hya. This has lead to the speculation that evaporation of icy cometary or planetary bodies might be an effective ongoing mechanism in such systems. However, substantial uncertainties remain, as the required radiative transfer modelling is difficult due to high optical depths, sub-thermal excitation and the sensitivity to the combined radiation field from the central star and dust grains. Aims: By performing a detailed radiative transfer analysis, we determine fractional abundances of circumstellar H2O in the envelopes around six M-type asymptotic giant branch stars. The models are also used to predict H2O spectral line emission for the upcoming Herschel/HIFI mission. Methods: We use Infrared Space Observatory Long Wavelength Spectrometer spectra to constrain the circumstellar fractional abundance distribution of ortho-H2O, using a non-local thermal equilibrium, and non-local, radiative transfer code based on the accelerated lambda iteration formalism. The mass-loss rates and kinetic temperature structures for the sample stars are determined through radiative transfer modelling of CO line emission based on the Monte-Carlo method. The density and temperature profiles of the circumstellar dust grains are determined through spectral energy distribution modelling using the publicly available code Dusty. Results: The determined ortho-H2O abundances lie between 2×10-4 and 1.5×10-3 relative to H2, with the exception of WX Psc, which has a much lower estimated ortho-H2O abundance of only 2×10-6, possibly indicating H2O adsorption onto dust grains or recent mass-loss-rate modulations. The estimated abundances are uncertain by, at best, a factor of a few. Conclusions: The high water abundance found for the majority of the sources suggests that either the “normal” chemical processes are very effective in producing H2O, or else non-local thermal equilibrium atmospheric chemistry, grain surface reactions, or a release of H2O (e.g. from icy bodies like Kuiper belt objects) play a role. However, more detailed information on the physical structure and the velocity field of the region where the water vapour lines are formed is required to improve abundance estimates. We provide predictions for ortho-H2O lines in the spectral window of Herschel/HIFI. These spectrally resolved lines cover a wide range of excitation conditions and will provide valuable additional information on the physical and chemical properties of the inner stellar wind where H2O is abundant.

Place, publisher, year, edition, pages
EDP Sciences , 2008. Vol. 479, no 1, 779-791 p.
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
URN: urn:nbn:se:su:diva-17161DOI: doi:10.1051/0004-6361:20078680OAI: oai:DiVA.org:su-17161DiVA: diva2:183681
Available from: 2009-01-25 Created: 2009-01-25 Last updated: 2009-12-22Bibliographically 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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