An ALMA view of CS and SiS around oxygen-rich AGB stars

We aim to determine the distributions of molecular SiS and CS in the circumstellar envelopes of oxygen-rich asymptotic giant branch stars and how these distributions differ between stars that lose mass at different rates. In this study, we analyse ALMA observations of SiS and CS emission lines for three oxygen-rich galactic AGB stars: IK Tau, with a moderately high mass-loss rate of 5 × 10⁻⁶ M yr⁻¹, and W Hya and R Dor with low mass-loss rates of ∼1 × 10⁻⁷ M yr⁻¹. These molecules are usually more abundant in carbon stars but the high sensitivity of ALMA allows us to detect their faint emission in the low mass-loss rate AGB stars. The high spatial resolution of ALMA also allows us to precisely determine the spatial distribution of these molecules in the circumstellar envelopes. We run radiative transfer models to calculate the molecular abundances and abundance distributions for each star. We find a spread of peak SiS abundances with ∼10⁻⁸ for R Dor, ∼10⁻⁷ for W Hya, and ∼3 × 10⁻⁶ for IK Tau relative to H₂. We find lower peak CS abundances of ∼7 × 10⁻⁹ for R Dor, ∼7 × 10⁻⁸ for W Hya, and ∼4 × 10⁻⁷ for IK Tau, with some stratifications in the abundance distributions. For IK Tau, we also calculate abundances for the detected isotopologues: C³⁴S, ²⁹SiS, ³⁰SiS, Si³³S, Si³⁴S, ²⁹Si³⁴S, and ³⁰Si³⁴S. Overall, the isotopic ratios we derive for IK Tau suggest a lower metallicity than solar.