Charting circumstellar chemistry of carbon-rich asymptotic giant branch stars

II. Abundances and spatial distributions of CS

¹ Department of Space, Earth and Environment, Chalmers University of Technology, 412 96 Gothenburg, Sweden
² Theoretical Astrophysics, Division for Astronomy and Space Physics, Department of Physics and Astronomy, Uppsala University, Box 516, 751 20 Uppsala, Sweden
³ Joint ALMA Observatory (JAO), Alonso de Córdova 3107, Vitacura 763-0355, Casilla 19001, Santiago, Chile
⁴ European Southern Observatory (ESO), Alonso de Córdova 3107, Vitacura 763-0355, Santiago, Chile
⁵ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
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 Astrophysics Research Centre, School of Mathematics and Physics, Queen’s University Belfast, University Road, Belfast BT7 1NN, UK
⁹ NASA Goddard Space Flight Center, 8800 Greenbelt Road, Greenbelt, MD 20771, USA
¹⁰ Gemini Observatory / NSF’s NOIRLab, 670 N. A’ohoku Place, Hilo, HI 96720, USA

^★ Corresponding author: ramlal.unnikrishnan@chalmers.se

Received: 1 April 2025
Accepted: 16 May 2025

Abstract

Context. The circumstellar envelopes (CSEs) of asymptotic giant branch (AGB) stars harbour a rich variety of molecules and are sites of complex chemistry. Our current understanding of the circumstellar chemical processes of carbon-rich AGB stars is predominantly based on observations of a single star, IRC +10 216, often regarded as an archetypical carbon star.

Aims. We aim to estimate stellar and circumstellar properties for five carbon stars, and constrain their circumstellar CS abundances. This study compares the CS abundances among the sources, informs circumstellar chemical models, and helps to assess if IRC+10 216 is a good representative of the physics and chemistry of carbon star CSEs.

Methods. We modelled the spectral energy distributions (SEDs) and CO line emission to derive the stellar and outflow properties. Using these, we then retrieved CS abundance profiles with detailed radiative transfer modelling, imposing spatial and excitation constraints from ALMA and single-dish observations.

Results. We obtain good fits to the SEDs and CO lines for all sources and reproduce the CS line emission across various transitions and apertures, yielding robust estimates of the CS abundance profiles. Peak CS fractional abundances range from 1×10⁻⁶−4×10⁻⁶, with e-folding radii of 1.8×10¹⁶−6.8×10¹⁶ cm. We also derive reliable ¹²C/¹³C and ³²S/³⁴S ratios from CS isotopologue modelling.

Conclusions. Our results refine previous single-dish CS abundance estimates and improve the relative uncertainty on the CS e-folding radius for IRAS 07454-7112 by a factor of ~2.5. Chemical models reproduce our estimates of the CS radial extent, corroborating the CS photodissociation framework used therein. We find no significant differences between the derived CS abundance profiles for IRC +10 216 and the rest of the sample, apart from the expected density-driven variations.