Sulphur-bearing molecules in AGB stars II: Abundances and distributions of CS and SiS

Abstract

We surveyed 20 AGB stars of different chemical types using the APEX telescope, and combined this with an IRAM 30 m and APEX survey of CS and SiS emission towards over 30 S-type stars. For those stars with detections, we performed radiative transfer modelling to determine abundances and abundance distributions. We detect CS towards all the surveyed carbon stars, some S-type stars, and the highest mass-loss rate oxygen-rich stars ($>5\times 10^{-6}$ Msol yr${}^{-1}$). SiS is detected towards the highest mass-loss rate sources of all chemical types ($>8\times 10^{-7}$ Msol yr${}^{-1}$). We find CS peak fractional abundances ranging from ~ $4\times 10^{-7}$ to ~ $2\times 10^{-5}$ for the carbon stars, from ~ $3\times 10^{-8}$ to ~ $1\times 10^{-7}$ for the oxygen-rich stars and from ~ $1\times 10^{-7}$ to ~ $8\times 10^{-6}$ for the S-type stars. We find SiS peak fractional abundances ranging from ~ $9\times 10^{-6}$ to ~ $2\times 10^{-5}$ for the carbon stars, from ~ $5\times 10^{-7}$ to ~ $2\times 10^{-6}$ for the oxygen-rich stars, and from ~ $2\times 10^{-7}$ to ~ $2\times 10^{-6}$ for the S-type stars. We derived Si${}^{32}$S/Si${}^{34}$S = 11.4 for AI Vol, the only star for which we had a reliable isotopologue detection. Overall, we find that wind density plays an important role in determining the chemical composition of AGB CSEs. It is seen that for oxygen-rich AGB stars both CS and SiS are detected only in the highest density circumstellar envelopes and their abundances are generally lower than for carbon-rich AGB stars by around an order of magnitude. For carbon-rich and S-type stars SiS was also only detected in the highest density circumstellar envelopes, while CS was detected consistently in all surveyed carbon stars and sporadically among the S-type stars.