Mass-loss properties of S-stars on the AGB

We have used a detailed non-LTE radiative transfer code to model new APEX CO(J = 3 -> 2) data, and existing CO radio line data, on a sample of 40 AGB S-stars. The derived mass-loss-rate distribution has a median value of 2 x 10(-7) M-circle dot yr(-1), and resembles values obtained for similar samples of M-stars and carbon stars. Possibly, there is a scarcity of high-mass-loss-rate (>= 10(-5) M-circle dot yr(-1)) S-stars. The distribution of envelope gas expansion velocities is similar to that of the M-stars, the median is 7.5 km s(-1), while the carbon stars, in general, have higher gas expansion velocities. The mass-loss rate correlates well with the gas expansion velocity, in accordance with results for M-stars and carbon stars.