Probing sulphur chemistry in oxygen-rich asymptotic giant branch stars with ALMA

¹ Rosseland Centre for Solar Physics, University of Oslo, PO Box 1029 Blindern, 0315 Oslo, Norway
² Institute of Theoretical Astrophysics, University of Oslo, PO Box 1029 Blindern, 0315 Oslo, Norway
³ Dep. of Space, Earth and Environment, Chalmers University of Technology, Onsala Space Observatory, 43992 Onsala, Sweden
⁴ School of Physics & Astronomy, Monash University, Wellington Road, Clayton 3800, Victoria, Australia
⁵ Institute of Astronomy, KU Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium

^★ Corresponding author: prasanta.astro@gmail.com

Received: 17 June 2025
Accepted: 6 October 2025

Abstract

Context. Sulphur and its isotopic ratios play a crucial role in our understanding of the physical properties of astrophysical environments; in particular, providing key insights into nucleosynthesis, interstellar medium processes, star formation, planetary system evolution, and galactic chemical evolution.

Aims. We aim to investigate the distribution of sulphur species - SO₂, ³⁴SO₂, SO, and ³⁴SO - towards a sample of five oxygen-rich asymptotic giant branch (AGB) stars, along with measurements of excitation temperature, column density, and isotopic ratios.

Methods. We used ALMA Band 6, 7, and 8 data of o Ceti, R Dor, W Hya, R Leo, and EP Aqr. SO₂,³⁴SO₂, SO, and ³⁴SO were detected towards AGB stars using the CASSIS software. To estimate the gas temperature and column density of these species, we applied the rotational diagram method (when applicable) and the Markov chain Monte Carlo method, assuming local thermodynamic equilibrium (LTE). Finally, line imaging of different transitions was performed to infer the distributions of the detected sulphur-bearing species in our sample.

Results. The measured excitation temperatures of SO₂ for our sample sources range from ∼200 to 600 K, with estimated column densities in the range of 1-7 × 10¹⁶ cm⁻². The excitation temperatures estimated using ³⁴SO₂ are comparable or slightly lower, while the column densities are about an order of magnitude lower than those of SO₂. Our measured ³²S/³⁴S ratios for R Dor and W Hya are close to the solar value; however, the measured value for o Ceti is slightly higher, and the measured values for EP Aqr and R Leo are lower. Finally, spatial analysis shows that most detected lines appear as centralized emissions. Moreover, the high excitation transitions of SO₂ show compact emission and probe hot gas of the inner region circumstellar envelopes (CSEs), whereas low-excitation transitions trace slightly extended structures. However, we find some differences in the emission of detected species across our sample.

Conclusions. The excitation temperature of the observed regions of the CSE can be probed using the SO₂ molecule. The morphological correlation between SO and SO₂ emissions suggests that they are chemically linked. Differences in the emission distributions of the detected species across our sample of low mass-loss rate AGB stars such as (i) centralized emission towards o Ceti with irregular emission shapes, (ii) centralized emission with ordered circular features towards R Leo and W Hya, (iii) clumpy emission features in R Dor, and (iv) unresolved emission in Ep Aqr may arise from several factors, i.e. the physical conditions of the sources (e.g. density and temperature structures of the CSEs), source multiplicity, outflows, rotation, or other associated physical processes such as thermal and nonthermal desorption, the effects of UV photons and cosmic rays, and finally the resolution of our observations. Nonetheless, the predominantly centralized distributions of SO and SO₂ in our sample support previous findings for low mass-loss rate AGB stars. Our measured ³²S/³⁴S ratios for the two stars R Dor and W Hya agree well with solar values within uncertainties, indicating that these ratios likely reflect the isotopic composition of the stars’ natal clouds and deviate for three stars (o Ceti, R Leo, and EP Aqr), which could be due to the metallicity and/or excitation conditions within various sources.