Molecular line observations of southern S stars

We observed a sample of southern S stars with the SEST telescope, in the SiO (v=0, J=3-2) transition at 130.3 GHz, and the HCN (J=1-0) transition at 88.6 GHz. SiO emission was detected in all seven stars observed, while HCN was detected in two. We employed a statistical equilibrium/radiative transfer model to estimate the SiO abundance for an assumed molecular distribution. The inferred SiO abundances are consistent with formation of the molecule under thermodynamic equilibrium (TE) conditions near the stellar photosphere, for reasonable physical conditions. We also model the HCN emission by a similar analysis, and find that if HCN is produced near the stellar photosphere, our model abundances are much higher than predicted by TE chemistry, unless the gas temperature is <1300 K and the gas density n(H₂) ∼ 10¹² cm^-3. Under such conditions, condensation of silicate grains may enhance production of HCN for C/O ≈ 1. Alternatively, HCN may be formed by photochemical reactions in the outer circumstellar envelope, as has been proposed for O-rich giants.