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We report the results of a survey of 27 S stars within 1 kpc of the sun for emission from the J=1-0 and 2-1 lines of CO. For those stars detected strongly in CO, we also searched for emission from the J=1-0 line of HCN and the J=2-1, v=0 and v=1 emission lines of SiO. We derive mass loss rates for the S stars detected in CO, and compare with other estimates. Our mass loss rates from the CO J=1-0 line (and using the model of Kastner, 1992 ApJ 401, 337) are well-correlated with the rates derived by Jura (1988 ApJSuppl, 66, 33) from IR flux densities and a dust model, but our values are systematically higher than Jura's by about a factor of 2.
Eight stars detected in CO were surveyed for emission from the HCN J=1-0 line. Four show definite detections of HCN in their circumstellar envelopes. Seven stars were searched for thermal (v=0, J=2-1) SiO emission, of which 5 were detected. Eight of 14 stars surveyed showed maser (v=1, J=2-1) SiO emission. We use our results to estimate HCN and SiO abundances.
We have computed thermodynamic equilibrium models for the chemical abundances of species expected to form in and near the photospheres of AGB stars. We compare our model abundances for HCN and SiO formed under conditions with C/O nearly equal to unity (appropriate for S stars), with the abundances estimated from our observations of HCN and SiO thermal emission from our survey stars. We discuss the implications of our results for the formation of molecules, and for the status of S stars in post-main sequence evolution.
This work was supported in part by NSF grant AST-9115137.
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