Probing sulfur chemistry in oxygen-rich AGB stars with ALMA

Abstract

Sulfur and its isotopic ratios play a crucial role in understanding astrophysical environments, providing insights into nucleosynthesis, ISM processes, star formation, planetary evolution, and galactic chemistry. We investigate the distribution of sulfur bearing species $\rm{SO_2}$, $\rm{^{34}SO_2}$, SO, and $\rm{^{34}SO}$ towards five oxygen rich Asymptotic Giant Branch (AGB) stars ($o$ Ceti, R Dor, W Hya, R Leo, and EP Aqr), along with their excitation temperatures, column densities, and isotopic ratios. Using ALMA Band 6,7,8 data and CASSIS, we detect these species and estimate excitation temperature and column density via the rotational diagram and MCMC methods under LTE. Line imaging of various transitions is used to infer spatial distributions. The excitation temperatures of $\rm{SO_2}$ range from $\sim$200-600 K with column densities of $\rm{1-7\times10^{16}\ cm^{-2}}$, while $\rm{^{34}SO_2}$ shows comparable or slightly lower values and about an order of magnitude lower column densities. The $\rm{^{32}S/^{34}S}$ ratios for R Dor and W Hya are near solar, slightly higher for $o$ Ceti, and lower for EP Aqr and R Leo. Most detected lines exhibit centralized emission: high excitation $\rm{SO_2}$ traces compact hot gas in inner CSEs, whereas low-excitation lines trace more extended structures. Morphological differences, irregular emission in $o$ Ceti, circular in R Leo and W Hya, clumpy in R Dor, and unresolved in EP Aqr may arise from variations in physical conditions, multiplicity, outflows, rotation, desorption processes, UV or cosmic ray effects, or observational resolution. Overall, the centralized SO and $\rm{SO_2}$ emissions support previous findings for low mass-loss rate AGB stars, and the $\rm{^{32}S/^{34}S}$ ratios likely reflect natal cloud composition, with deviations linked to metallicity or excitation conditions.