A MILLIMETER-WAVELENGTH SURVEY OF S STARS FOR MASS-LOSS AND CHEMISTRY

We report the results of a survey of 27 S stars within 1 kpc of the Su n for emission from the J = 1-0 and 2-1 lines of CO. For those stars w ith strong detections in CO, we also searched for emission from the J = 1-0 line of HCN and the J = 2-1, v = 0 emission lines of SiO. In add ition, we searched for SiO J = 2-1, v = 1 maser emission in a total of 14 stars in our sample. We derive mass-loss rates for the S stars det ected in CO and compare with other estimates. Our mass loss rates from the CO J = 1-0 line are well correlated with the rates derived by Jur a (1988) from IR flux densities and a dust model, but our values are s ystematically higher than Jura's by about a factor of 2. The derived g as mass-loss rates for our sample are very similar to those found for carbon stars. We find, however, that the average dust/gas ratio of our sample of S stars is lower than that for carbon stars, suggesting tha t S stars are less efficient at producing grains, per unit of ejected mass, than are carbon stars. Four of eight stars surveyed show definit e detections of HCN in their circumstellar envelopes. Five of seven st ars have thermal (v = 0) SiO emission, while eight of 14 surveyed show maser (v = 1) SiO emission. We use our results to estimate HCN and Si O abundances and compare these with the results of thermodynamic equil ibrium models for the formation of these species under photospheric co nditions with C/O nearly equal to unity. The HCN and SiO abundances in dicate that the formation of these species must occur at temperatures low enough for grain formation to be important, which may serve to enh ance production of HCN. Our results show no evidence that S stars expe rience a cessation of mass loss, contrary to recent suggestions.