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Asymptotic Giant Branch stars viewed up-close and far-off: The physics, chemistry, and evolution of their circumstellar envelopes
Stockholm University, Faculty of Science, Department of Astronomy.
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 [en]
AGB stars, evolution, mass-loss, abundances
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy
Identifiers
URN: urn:nbn:se:su:diva-31425ISBN: 978-91-7155-975-3 (print)OAI: oai:DiVA.org:su-31425DiVA: diva2:276947
Public defence
2010-01-18, Sal FB 52, AlbaNova universitetscentrum, Roslagstullsbacken 21, Stockholm, 10:00 (English)
Opponent
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At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 2: Submitted.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
List of papers
1. Circumstellar water vapour in M-type AGB stars: constraints from H2O(1,10-1,01) lines obtained with odin
Open this publication in new window or tab >>Circumstellar water vapour in M-type AGB stars: constraints from H2O(1,10-1,01) lines obtained with odin
Show others...
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.

Keyword
stars: abundances, stars: AGB and post-AGB, stars: evolution, stars: mass-loss
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:su:diva-31234 (URN)10.1051/0004-6361:200810017 (DOI)000263040200020 ()
Available from: 2009-11-09 Created: 2009-11-09 Last updated: 2017-12-12Bibliographically approved
2. The detached dust and gas shells around the carbon star U Ant
Open this publication in new window or tab >>The detached dust and gas shells around the carbon star U Ant
2010 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 511, no a37Article in journal (Other academic) Published
Abstract [en]

Context. Geometrically thin, detached shells of gas have been found around a handful of carbon stars. The current knowledge on these shells is mostly based on CO radio line data. However, imaging in scattered stellar light adds important new information as well as allows studies of the dust shells.

Aims. Previous observations of scattered stellar light in the circumstellar medium around the carbon star U Ant were taken through filters centred on the resonance lines of K and Na. These observations could not separate the scattering by dust and atoms. The aim of this paper is to remedy this situation.

Methods. We have obtained polarization data on stellar light scattered in the circumstellar medium around U Ant through filters which contain no strong lines, making it possible to differentiate between the two scattering agents. Kinematic, as well as spatial, information on the gas shells were obtained through high-resolution echelle spectrograph observations of the KI and NaD lines.

Results. We confirm the existence of two detached shells around U Ant. The inner shell (at a radius of 43´´ and a width of 2´´) consists mainly of gas, while the outer shell (at a radius of 50´´ and a width of 7´´) appears to consist exclusively of dust. Both shells appear to have an over-all spherical geometry. The gas shell mass is estimated to be 2  , while the mass of the dust shell is estimated to be 5  . The derived expansion velocity, from the KI and NaD lines, of the gas shell, 19.5 , agrees with that obtained from CO radio line data. The inferred shell age is 2700 years. There is structure, e.g. in the form of arcs, inside the gas shell, but it is not clear whether these are due to additional shells.

Conclusions. Our results support the hypothesis that the observed geometrically thin, detached shells around carbon stars are the results of brief periods of intense mass loss, probably associated with thermal pulses, and subsequent wind-wind interactions. The separation into a gas and a dust shell, with different widths, is most likely the effect of different dynamical evolutions of the two media after their ejection.

Place, publisher, year, edition, pages
EDP Sciences, 2010
Keyword
stars: abundances / stars: AGB and post-AGB / stars: evolution / stars: mass-loss
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:su:diva-31423 (URN)10.1051/0004-6361/200913376 (DOI)000275752400080 ()
Available from: 2009-11-13 Created: 2009-11-13 Last updated: 2017-12-12Bibliographically approved
3. Circumstellar water vapour in M-type AGB stars: radiative transfer models, abundances, and predictions for HIFI
Open this publication in new window or tab >>Circumstellar water vapour in M-type AGB stars: radiative transfer models, abundances, and predictions for HIFI
Show others...
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
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:su:diva-17161 (URN)doi:10.1051/0004-6361:20078680 (DOI)
Available from: 2009-01-25 Created: 2009-01-25 Last updated: 2009-12-22Bibliographically approved
4. High-resolution HST/ACS images of detached shells around carbon stars  
Open this publication in new window or tab >>High-resolution HST/ACS images of detached shells around carbon stars  
Show others...
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.

Keyword
AGB and post-AGB, carbon, circumstellar matter, mass-loss
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:su:diva-31424 (URN)10.1051/0004-6361/200913929 (DOI)000278399000027 ()
Note
authorCount :5Available from: 2009-11-13 Created: 2009-11-13 Last updated: 2017-12-12Bibliographically approved

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