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- Maercker, Matthias, et al.
(författare)
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Circumstellar water vapour in M-type AGB stars: radiative transfer models, abundances, and predictions for HIFI
- 2008
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 479:1, s. 779-791
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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- Ramstedt, Sofia, et al.
(författare)
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Circumstellar molecular line emission from S-type AGB stars : Mass-loss rates and SiO abundances
- 2009
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 499:9, s. 515-527
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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- Ramstedt, Sofia, et al.
(författare)
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Mass-loss properties of S-stars on the AGB
- 2006
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Ingår i: Astronomy & Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 454:2, s. L103-L106
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 7. |
- Ramstedt, Sofia, 1974-
(författare)
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Molecules and Dust around AGB stars : Mass-loss rates and molecular abundances
- 2009
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- 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 10−4 MSun yr−1 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.
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- Ramstedt, Sofia, et al.
(författare)
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On the reliability of mass-loss-rate estimates for AGB stars
- 2008
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 487:2, s. 645-657
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 9. |
- Ramstedt, Sofia, et al.
(författare)
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The physics and chemistry of circumstellar envelopes of S-stars on the AGB
- 2008
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Ingår i: Proceedings of the International Astronomical Union. - 1743-9213 .- 1743-9221. ; 4:S251, s. 167-168, s. 167-168
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Tidskriftsartikel (refereegranskat)abstract
- Presented here are the preliminary results of a long-term study of S-stars on the AGB. S-stars are important as possible transition objects between oxygen-rich M-stars and carbon stars. The aim of the study is to compare results from our newly gathered observational database for the S-stars with those already obtained for the M- and carbon stars. We can thus follow the changes as the stars evolve along the AGB and more firmly establish the suggested M-MS-S-SC-C evolutionary sequence. It will also allow us to determine the relative importance of processes such as non-equilibrium chemistry, grain formation, and photodissociation in regulating the chemistry in circumstellar envelopes of AGB stars.
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