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

Abstract

Aims. The main aim is to derive reliable mass-loss rates and circumstellar SiO abundances for a sample of 40 S-type AGB stars based on new multi-transitional CO and SiO radio line observations. In addition, the results are compared to previous results for M-type AGB stars 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 massloss 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 AGB stars and find that the derived mass-loss rates have a distribution that resembles those previously derived for similar samples of M-type AGB stars 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 AGB stars and carbon stars. In all, this indicates that the mass loss is driven by the same mechanism in all three chemical types of AGB stars. In addition, we have estimated the circumstellar fractional abundance of SiO relative to H 2 in 26 of the sample S-type AGB stars. 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 AGB stars 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 AGB stars with carbon stars and M-type AGB stars, we find no large differences in circumstellar physical properties or SiO abundances depending on the chemical type of the star.

Carbon isotopic abundance ratios in S-type stars

“…Four stars in our survey show signs of optically thick 12 CO emission: W Aql, R And, S Cas, and R Cyg. Of these, high opacity is confirmed in W Aql and S Cas (Ramstedt et al 2009;Nyman et al 1992). So we will apply the full optically thick case for them.…”

“…Such corrections are estimated assuming that all the emission at velocities well separated from the systemic one is well picked up by our single-point observations, since at these velocities the line emission comes from an angularly compact region along the central line of sight that can be assumed to be always unresolved. The peak intensity was then estimated by applying to our observations the relative increase between the central peak and the adjacent velocities found in the profiles of our targets obtained by Ramstedt et al (2009) andNyman et al (1992), who used a wider beam. Even so, the procedure is a bit risky for W Aql and S Cas owing to possible uncertainties in our respective intensity calibrations.…”

“…In Table 4 we show the estimated mass loss rates based on (1) sophisticated model fitting by Ramstedt et al (2009) and (2) from our simple estimates. For our estimates we have adopted the same stellar parameters as Ramstedt et al wherever possible.…”

Morphology and kinematics of the gas envelope of the Mira binary W Aquilae

“…panel). The latter corresponds to the APEX single dish observation of Ramstedt et al (2009) which is also displayed in the figure (right panel). Significant differences between the ALMA and ACA data are the effect of the important short spacing problem in the former.…”

“…Radio single dish observations in the millimetre/sub-millimetre range include several molecular lines (Nyman et al 1992, Knapp et al 1998, Ramstedt et al 2009, Wallerstein et al 2011and Ramstedt & Oloffson 2014. The associated line profiles cover typically a ±20 km s −1 Doppler velocity interval, giving evidence for significant winds.…”