000002222 001__ 2222
000002222 005__ 20160706143335.0
000002222 0247_ $$2DOI$$a10.1051/0004-6361/201118242
000002222 037__ $$aASTROimport-379
000002222 100__ $$aSmolders, K.
000002222 245__ $$aThe Spitzer spectroscopic survey of S-type stars
000002222 260__ $$c2012
000002222 520__ $$aContext. 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 µm and 20 µm 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. Appendix A is only available in electronic form at http://www.aanda.org
000002222 700__ $$aNeyskens, P.
000002222 700__ $$aBlommaert, J. A. D. L.
000002222 700__ $$aHony, S.
000002222 700__ $$avan Winckel, H.
000002222 700__ $$aDecin, L.
000002222 700__ $$avan Eck, S.
000002222 700__ $$aSloan, G. C.
000002222 700__ $$aCami, J.
000002222 700__ $$aUttenthaler, S.
000002222 700__ $$aDegroote, P.
000002222 700__ $$aBarry, D.
000002222 700__ $$aFeast, M.
000002222 700__ $$aGroenewegen, M. A. T.
000002222 700__ $$aMatsuura, M.
000002222 700__ $$aMenzies, J.
000002222 700__ $$aSahai, R.
000002222 700__ $$avan Loon, J. T.
000002222 700__ $$aZijlstra, A. A.
000002222 700__ $$aAcke, B.
000002222 700__ $$aBloemen, S.
000002222 700__ $$aCox, N.
000002222 700__ $$aDe Cat, P.
000002222 700__ $$aDesmet, M.
000002222 700__ $$aExter, K.
000002222 700__ $$aLadjal, D.
000002222 700__ $$aØstensen, R.
000002222 700__ $$aSaesen, S.
000002222 700__ $$avan Wyk, F.
000002222 700__ $$aVerhoelst, T.
000002222 700__ $$aZima, W.
000002222 773__ $$cA72$$pAstronomy and Astrophysics$$v540$$y2012
000002222 85642 $$ahttp://esoads.eso.org/abs/2012A%26A...540A..72S
000002222 905__ $$apublished in
000002222 980__ $$aREFERD