Probing sulfur chemistry in oxygen-rich AGB stars with ALMA

TL;DR

We investigate sulfur chemistry in five oxygen-rich AGB stars using ALMA Band 6–8, mapping SO$_2$, $^{34}$SO$_2$, SO, and $^{34}$SO and deriving $T_{ex}$ and $N$ under LTE via rotational diagrams and MCMC. The study finds $T_{ex}\sim 200-600$ K and $N_{SO_2}\sim (1-7)\times10^{16}$ cm$^{-2}$ with $^{34}$SO$_2$ ~10× less abundant, and $^{32}$S/$^{34}$S ratios near solar for some sources but deviating for others, reflecting natal-cloud composition and excitation effects. Emission is predominantly centralized, with high-$E_{up}$ SO$_2$ tracing inner hot gas and low-$E_{up}$ lines mapping extended regions; o Ceti shows a notable arc-like extended structure likely due to shocks or binarity. Together these results illuminate how sulfur chemistry in the circumstellar envelopes of low mass-loss-rate AGB stars is shaped by local physical conditions and initial isotopic composition, informing nucleosynthesis, dust formation, and galactic chemical evolution.

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.

Figures (18)

• Figure 1: Observed and modeled spectra of SO$_2$ transitions towards $o$ Ceti. The black line represents the observed spectrum, while the blue line represents the modeled spectrum. The strong line at 296.5500 GHz, shown in the second subplot of the third row from the bottom, represents the SO line. The narrow line in the last subplot of the same row corresponds to TiO$_2$ (331.5996 GHz).
• Figure 2: Rotational diagram of SO$_2$ and $^{34}$SO$_2$ for o Ceti. Blue filled squares represent the data points, and the vertical lines on each data point indicate the error bars. The solid red lines represent the fitted line.
• Figure 3: Rotational diagram of SO$_2$ and $^{34}$SO$_2$ for R Leo. Blue filled squares represent the data points, and the vertical lines on each data point indicate the error bars. The solid red lines represent the fitted straight line.
• Figure 4: Rotational diagram of SO$_2$ for R Dor. Blue filled squares represent the data points, and the vertical lines on each data point indicate the error bars. The solid red line represents the fitted straight line.
• Figure 5: Rotational diagram of SO$_2$ for W Hya. Blue filled squares represent the data points, and the vertical lines on each data point indicate the error bars. The solid red line represents the fitted straight line.