Charting circumstellar chemistry of carbon-rich asymptotic giant branch stars

III. SiO and SiS abundances

¹ Department of Space, Earth and Environment, Chalmers University of Technology, 412 96 Gothenburg, Sweden
² Joint ALMA Observatory (JAO), Alonso de Córdova 3107, Vitacura 763-0355, Casilla 19001, Santiago, Chile
³ European Southern Observatory (ESO), Alonso de Córdova 3107, Vitacura 763-0355, Santiago, Chile
⁴ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
⁵ Astrophysics Research Centre, School of Mathematics and Physics, Queen’s University Belfast, University Road, Belfast BT7 1NN, UK
⁶ School of Physics & Astronomy, Monash University, Wellington Road, Clayton 3800, Victoria, Australia
⁷ Institute of Astronomy, KU Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium
⁸ NASA Goddard Space Flight Center, 8800 Greenbelt Road, Greenbelt, MD 20771, USA
⁹ Gemini Observatory / NSF NOIRLab, 670 N. A’ohoku Place, Hilo, HI 96720, USA

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 16 December 2025
Accepted: 6 April 2026

Abstract

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 (²⁹SiO, ³⁰SiO, ²⁹SiS, ³⁰SiS, and Si³⁴S) 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 × 10⁻⁶–4.7 × 10⁻⁶), we find that the SiO peak abundances of the rest of the stars are a factor of ~5 larger than that of IRC +10 216. The e-folding radii (R_e) are in the range 1.3 × 10¹⁶ − 7.0×10¹⁶cm for SiO and 6.0 × 10¹⁵ − 1.0×10¹⁷ 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.