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- Danilovich, T., et al.
(författare)
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Sulphur-bearing molecules in AGB stars : II. Abundances and distributions of CS and SiS
- 2018
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Ingår i: Astronomy and Astrophysics. - : EDP SCIENCES S A. - 0004-6361 .- 1432-0746. ; 617
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Tidskriftsartikel (refereegranskat)abstract
- Context: Sulphur has long been known to form different molecules depending on the chemical composition of its environment. More recently, the sulphur-bearing molecules SO and H2S have been shown to behave differently in oxygen-rich asymptotic giant branch (AGB) circumstellar envelopes of different densities.Aims: By surveying a diverse sample of AGB stars for CS and SiS emission, we aim to determine in which environments these sulphur bearing molecules most readily occur. We include sources with a range of mass-loss rates and carbon-rich, oxygen-rich, and mixed S-type chemistries. Where these molecules are detected, we aim to determine their CS and SiS abundances.Methods: We surveyed 20 AGB stars of different chemical types using the APEX telescope, and combined this with an IRAM 30 m and APEX survey of CS and SiS emission towards over 30 S-type stars. For those stars with detections, we performed radiative transfer modelling to determine abundances and abundance distributions.Results: We detect CS towards all the surveyed carbon stars, some S-type stars, and the highest mass-loss rate oxygen-rich stars, (M > 5 x 10(-6) M-circle dot yr(-1)). SiS is detected towards the highest mass-loss rate sources of all chemical types (M >= similar to 8 x 10(-7) M-circle dot yr(-1)). We find CS peak fractional abundances ranging from similar to 4 x 10(-7) to similar to 2 x 10(-5) for the carbon stars, from similar to 3 x 10(-8) to similar to 1 x 10(-7) for the oxygen-rich stars, and from similar to 1 x 10(-7) to similar to 8 x 10(-6) for the S-type stars. We find SiS peak fractional abundances ranging from similar to 9 x 10(-6) to similar to 2 x 10(-5) for the carbon stars, from similar to 5 x 10(-7) to similar to 2 x 10(-6) for the oxygen-rich stars, and from similar to 2 x 10(-7) to similar to 2 x 10(-6) for the S-type stars.Conclusions: Overall, we find that wind density plays an important role in determining the chemical composition of AGB circumstellar envelopes. It is seen that for oxygen-rich AGB stars both CS and SiS are detected only in the highest density circumstellar envelopes and their abundances are generally lower than for carbon-rich AGB stars by around an order of magnitude. For carbon-rich and S-type stars SiS was also only detected in the highest density circumstellar envelopes, while CS was detected consistently in all surveyed carbon stars and sporadically among the S-type stars.
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| 2. |
- Danilovich, T., et al.
(författare)
-
Sulphur-bearing molecules in AGB stars. Part 2. II. Abundances and distributions of CS and SiS
- 2018
-
Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 617
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Tidskriftsartikel (refereegranskat)abstract
- Context. Sulphur has long been known to form different molecules depending on the chemical composition of its environment. More recently, the sulphur-bearing molecules SO and H2S have been shown to behave differently in oxygen-rich asymptotic giant branch (AGB) circumstellar envelopes of different densities. Aims. By surveying a diverse sample of AGB stars for CS and SiS emission, we aim to determine in which environments these sulphur bearing molecules most readily occur. We include sources with a range of mass-loss rates and carbon-rich, oxygen-rich, and mixed S-type chemistries. Where these molecules are detected, we aim to determine their CS and SiS abundances. Methods. We surveyed 20 AGB stars of different chemical types using the APEX telescope, and combined this with an IRAM 30 m and APEX survey of CS and SiS emission towards over 30 S-type stars. For those stars with detections, we performed radiative transfer modelling to determine abundances and abundance distributions. Results. We detect CS towards all the surveyed carbon stars, some S-type stars, and the highest mass-loss rate oxygen-rich stars, (M > 5 x 10(-6) M-circle dot yr(-1)). SiS is detected towards the highest mass-loss rate sources of all chemical types (M >= similar to 8 x 10(-7) M-circle dot yr(-1)). We find CS peak fractional abundances ranging from similar to 4 x 10(-7) to similar to 2 x 10(-5) for the carbon stars, from similar to 3 x 10(-8) to similar to 1 x 10(-7) for the oxygen-rich stars, and from similar to 1 x 10(-7) to similar to 8 x 10(-6) for the S-type stars. We find SiS peak fractional abundances ranging from similar to 9 x 10(-6) to similar to 2 x 10(-5) for the carbon stars, from similar to 5 x 10(-7) to similar to 2 x 10(-6) for the oxygen-rich stars, and from similar to 2 x 10(-7) to similar to 2 x 10(-6) for the S-type stars. Conclusions. Overall, we find that wind density plays an important role in determining the chemical composition of AGB circumstellar envelopes. It is seen that for oxygen-rich AGB stars both CS and SiS are detected only in the highest density circumstellar envelopes and their abundances are generally lower than for carbon-rich AGB stars by around an order of magnitude. For carbon-rich and S-type stars SiS was also only detected in the highest density circumstellar envelopes, while CS was detected consistently in all surveyed carbon stars and sporadically among the S-type stars.
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| 3. |
- Nicolaes, D., et al.
(författare)
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PACS and SPIRE range spectroscopy of cool, evolved stars
- 2018
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 618
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Tidskriftsartikel (refereegranskat)abstract
- Context. At the end of their lives AGB stars are prolific producers of dust and gas. The details of this mass-loss process are still not understood very well. Herschel PACS and SPIRE spectra which cover the wavelength range from ∼55 to 670 μm almost continuously, offer a unique way of investigating properties of AGB stars in general and the mass-loss process in particular as this is the wavelength region where dust emission is prominent and molecules have many emission lines. Aims. We present the community with a catalogue of AGB stars and red supergiants (RSGs) with PACS and/or SPIRE spectra reduced according to the current state of the art. Methods. The Herschel interactive processing environment (HIPE) software with the latest calibration is used to process the available PACS and SPIRE spectra of 40 evolved stars. The SPIRE spectra of some objects close to the Galactic plane require special treatment because of the weaker fluxes in combination with the strong and complex background emission at those wavelengths. The spectra are convolved with the response curves of the PACS and SPIRE bolometers and compared to the fluxes measured in imaging data of these sources. Custom software is used to identify lines in the spectra, and to determine the central wavelengths and line intensities. Standard molecular line databases are used to associate the observed lines. Because of the limited spectral resolution of the PACS and SPIRE spectrometers (∼1500), several known lines are typically potential counterparts to any observed line. To help identifications in follow-up studies the relative contributions in line intensity of the potential counterpart lines are listed for three characteristic temperatures based on local thermodynamic equilibrium (LTE) calculations and assuming optically thin emission. Results. The following data products are released: the reduced spectra, the lines that are measured in the spectra with wavelength, intensity, potential identifications, and the continuum spectra, i.e. the full spectra with all identified lines removed. As simple examples of how this data can be used in future studies we have fitted the continuum spectra with three power laws (two wavelength regimes covering PACS, and one covering SPIRE) and find that the few OH/IR stars seem to have significantly steeper slopes than the other oxygen- and carbon-rich objects in the sample, possibly related to a recent increase in mass-loss rate. As another example we constructed rotational diagrams for CO (and HCN for the carbon stars) and fitted a two-component model to derive rotational temperatures.
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