Charting circumstellar chemistry of carbon-rich asymptotic giant branch stars: III. SiO and SiS abundances

Journal article, 2026

Context. The circumstellar envelopes of AGB stars are sites of rich molecular chemistry. The present understanding of C-rich AGB chemistry largely relies on observations of the archetypal carbon star IRC+10 216. Current molecular abundance estimates for carbon stars are based either on single-dish spectra sampling a range of excitation conditions, or on interferometric mapping of a few lines.
Aims. We aim to estimate the circumstellar abundances of SiO, SiS, and their most abundant isotopologues ((SiO)-Si-29, (SiO)-Si-30, (SiS)-Si-29, (SiS)-Si-30, and (SiS)-S-34) for a sample of five carbon stars. This study compares molecular abundances across the sources, tests chemical modelling predictions, and examines whether IRC+10 216 is representative of the broader carbon star population.
Methods. We derived molecular abundances using detailed 1D non-local thermodynamic equilibrium (non-LTE) radiative transfer (RT) modelling, constrained by both morphological and excitation information obtained from spatially resolved ALMA maps and single-dish observations. We further compared the derived abundances to chemical modelling results.
Results. We obtain good fits to the SiO and SiS line profiles, and derived well-constrained abundance profiles and reliable isotopic ratios for all sources except AFGL 3068. While the SiS peak abundances are very similar across the sample (2.0 x 10(-6) - 4.7 x 10(-6)), we find that the SiO peak abundances of the rest of the stars are a factor of similar to 5 larger than that of IRC +10 216. The e-folding radii (R-e) are in the range 1.3 x 10(16) - 7.0 x 10(16)cm for SiO and 6.0 x 10(15) - 1.0 x 10(17) cm for SiS. The R-e increases with gas density for both SiO and SiS. Our RT models cannot simultaneously fit the low- and high-J SiO lines of IRC+10216. Chemical models reproduce the derived SiO abundance profiles well, while over-predicting the SiS R-e values.
Conclusions. Our models highlight the necessity of having spatially resolved observations across a broad range of excitation conditions to robustly constrain molecular abundance profiles, while also making evident the limitations inherent in 1D RT modelling using simplified (circum)stellar models. We find that the currently assumed SiS photodissociation rate in chemical models is underestimated.