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 Sun for emission from the J = 1-0 and 2-1 lines of CO. For those stars with 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 addition, 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 detected 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 Jura (1988) from IR flux densities and a dust model, but our values are systematically higher than Jura's by about a factor of 2. The derived gas 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 that S stars are less efficient at producing grains, per unit of ejected mass, than are carbon stars. Four of eight stars surveyed show definite detections of HCN in their circumstellar envelopes. Five of seven stars 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 SiO abundances and compare these with the results of thermodynamic equilibrium models for the formation of these species under photospheric conditions with C/O nearly equal to unity. The HCN and SiO abundances indicate that the formation of these species must occur at temperatures low enough for grain formation to be important, which may serve to enhance production of HCN. Our results show no evidence that S stars experience a cessation of mass loss, contrary to recent suggestions.