Journal
ASTRONOMY & ASTROPHYSICS
Volume 540, Issue -, Pages -Publisher
EDP SCIENCES S A
DOI: 10.1051/0004-6361/201118242
Keywords
stars: AGB and post-AGB; circumstellar matter; stars: mass-loss; infrared: stars
Categories
Funding
- Fund for Scientific Research of Flanders [G.0470.07]
- Austrian Science Fund (FWF) [P 22911-N16]
- National Research Foundation (NRF) of South Africa
- Fund for Scientific Research of Flanders (FWO), Belgium
- Research Council of K.U. Leuven, Belgium
- Fonds de la Recherche Scientifique (FNRS), Belgium
- Royal Observatory of Belgium
- Observatoire de Geneve, Switzerland
- Thuringer Landessternwarte Tautenburg, Germany
- Action de recherche concerte (ARC) from Direction generale de l'Enseignement non obligatoire et de la Recherche scientifique - Direction de la recherche scientifique - Communaute francaise de Belgique
- European Research Council under the European Community [227224]
- Research Council of K.U. Leuven [GOA/2008/04]
- Austrian Science Fund (FWF) [P 22911] Funding Source: researchfish
- Science and Technology Facilities Council [ST/J000035/1, ST/I001425/1, ST/G002827/1, PP/D000955/1, ST/J001384/1, PP/F000057/1, ST/G002355/1] Funding Source: researchfish
- STFC [ST/J000035/1, ST/G002827/1, ST/G002355/1, ST/J001384/1, ST/I001425/1, PP/D000955/1, PP/F000057/1] Funding Source: UKRI
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Context. S-type AGB stars are thought to be in the transitional phase between M-type and C-type AGB stars. Because the composition of the circumstellar environment reflects the photospheric abundances, one may expect a strong influence of the stellar C/O ratio on the molecular chemistry and the mineralogy of the circumstellar dust. Aims. In this paper, we present a large sample of 87 intrinsic galactic S-type AGB stars, observed at infrared wavelengths with the Spitzer Space Telescope, and supplemented with ground-based optical data. Methods. On the one hand, we derive the stellar parameters from the optical spectroscopy and photometry, using a grid of model atmospheres. On the other, we decompose the infrared spectra to quantify the flux-contributions from the different dust species. Finally, we compare the independently determined stellar parameters and dust properties. Results. For the stars without significant dust emission features, we detect a strict relation between the presence of SiS absorption in the Spitzer spectra and the C/O ratio of the stellar atmosphere. These absorption bands can thus be used as an additional diagnostic for the C/O ratio. For stars with significant dust emission, we define three distinct groups, based on the relative contribution of certain dust species to the infrared flux. We find a strong link between group-membership and C/O ratio. Furthermore, we show that these groups can be explained by assuming that the dust-condensation can be cut short before silicates are produced, while the remaining free atoms and molecules can then be used to form the observed magnesium sulfides or the carriers of the unidentified 13 mu m and 20 mu m features. Finally, we present the detection of emission features attributed to molecules and dust characteristic to C-type stars, such as molecular SiS, hydrocarbons and magnesium sulfide grains. We show that we often detect magnesium sulfides together with molecular SiS and we propose that it is formed by a reaction of SiS molecules with Mg.
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