Circumstellar molecular line emission from S-type AGB?stars: mass-loss rates and SiO abundances

摘要

Aims. The main aim is to derive reliable mass-loss rates and circumstellar SiO abundances for a sample of 40S-type AGBstars based on new multi-transitional CO and SiO radio line observations. In addition, the results are compared to previous results for M-type AGBstars and carbon stars to look for trends with chemical type. Methods. The circumstellar envelopes are assumed to be spherically symmetric and formed by a constant mass-loss rate. The mass-loss rates are estimated from fitting the CO observations using a non-local, non-LTE radiative transfer code based on the Monte Carlo method. In the excitation analysis, the energy balance equation is solved self-consistently simultaneously as the radiative transfer and the temperature structure of the gas is derived. Effects of dust grains are also included in the molecular excitation analysis. Once the physical properties of the circumstellar envelopes are determined, the same radiative transfer code is used to model the observed SiO lines in order to derive circumstellar abundances and the sizes of the SiO line-emitting regions. Results. We have estimated mass-loss rates of 40 S-type AGBstars and find that the derived mass-loss rates have a distribution that resembles those previously derived for similar samples of M-type AGBstars and carbon stars. The estimated mass-loss rates also correlate well with the corresponding expansion velocity of the envelope, in accordance with results for M-type AGBstars and carbon stars. In all, this indicates that the mass loss is driven by the same mechanism in all three chemical types of AGBstars. In addition, we have estimated the circumstellar fractional abundance of SiOrelative to H2 in 26 of the sample S-type AGBstars. The derived SiO abundances are, on average, about an order of magnitude higher than predicted by stellar atmosphere thermal equilibrium chemistry, indicating that non-equilibrium chemical processes determines the abundance of SiO in the circumstellar envelope. Moreover, a comparison with the results for M-type AGBstars and carbon stars show that for a certain mass-loss rate, the circumstellar SiO abundance seems independent (although with a large scatter) of the C/O-ratio. Conclusions. In our comparison of S-type AGBstars with carbon stars and M-type AGBstars, we find no large differences in circumstellar physical properties or SiO abundances depending on the chemical type of the star. Key words: stars: AGB and post-AGB - stars: abundances - stars: carbon - stars: late-type - stars: mass-loss