MOLECULAR LINE OBSERVATIONS OF SOUTHERN S-STARS

We observed a sample of southern S stars with the SEST telescope, in t he SiO (v=O, J=3-2) transition at 130.3 GHz, and the HCN (J=1-0) trans ition at 88.6 GHz. SiO emission was detected in all seven stars observ ed, while HCN was detected in two. We employed a statistical equilibri um/radiative transfer model to estimate the SiO abundance for an assum ed molecular distribution. The inferred SiO abundances are consistent with formation of the molecule under thermodynamic equilibrium (TE) co nditions near the stellar photosphere, for reasonable physical conditi ons. We also model the HCN emission by a similar analysis, and find th at if HCN is produced near the stellar photosphere, our model abundanc es are much higher than predicted by TE chemistry, unless the gas temp erature is <1300 K and the gas density n(H-2) similar to 10(12) cm(-3) . Under such conditions, condensation of silicate grains may enhance p roduction of HCN for C/O approximate to 1. Alternatively, HCN may be f ormed by photochemical reactions in the outer circumstellar envelope, as has been proposed for O-rich giants.