Sulfur monoxide as a tracer of the Galactic $^{32}$S/$^{34}$S gradient

TL;DR

This study addresses the Galactic $^{32}$S/$^{34}$S gradient by using SO and $^{34}$SO as tracers to complement traditional CS-based measurements. It presents the first large, distance-calibrated survey of these sulfur-bearing species with the IRAM 30 m and SMT 10 m across a broad sample of molecular clouds. The results show that the ratios derived from the lower transition $2_2-1_1$ trace the increasing radial gradient previously inferred from CS, whereas the higher transition $5_5-4_4$ lies systematically lower; neither distance nor kinetic temperature nor optical depth effects dominate, per LTE and non-LTE analyses. The comparison with Galactic chemical evolution models suggests revisions to nucleosynthesis prescriptions at low metallicity, highlighting the need for more measurements in the Galaxy's outer regions.

Abstract

To date, the Galactic interstellar radial $^{32}$S/$^{34}$S gradient has only been studied with the CS isotopologs, which may be affected by uncertainties due to the use of a single tracer. As another abundant S-bearing molecules, SO and its isotopomer $^{34}$SO could be considered as tracers of the $^{32}$S/$^{34}$S ratio. We present the first systematic observations of SO and $^{34}$SO toward a large sample of molecular clouds with accurate distances, performed with the IRAM 30 m and the 10 m Submillimeter Telescope (SMT). With the IRAM 30 m, SO $2_2-1_1$ was detected in 59 of 82 sources ($\sim$82%), and $^{34}$SO $2_2-1_1$ in 8 sources ($\sim$10%). With the SMT 10 m, SO $5_5-4_4$ was detected in 136 of 184 sources ($\sim$74%), and $^{34}$SO $5_5-4_4$ in 55 of 77 strong SO sources ($\sim$72%). SO/$^{34}$SO ratios were derived for 8 ($2_2-1_1$) and 55 ($5_5-4_4$) sources. No correlation was found between the SO/$^{34}$SO ratio and heliocentric distance or $T_k$, suggesting negligible distance and fractionation effects. Both LTE and non-LTE analyses consistently suggest that the optical depth effect is also insignificant. $^{32}$S/$^{34}$S ratios from the $2_2-1_1$ transitions follow the increasing radial trend proposed by previous CS species measurements, while those from the $5_5-4_4$ lines are systematically lower. The lower transitions of SO and $^{34}$SO may be suitable tracers of $^{32}$S/$^{34}$S, though the detections are rare. Comparisons between measurements and Galactic chemical evolution model suggest that the nucleosynthesis prescriptions need to be revised in the low-metallicity regime, but more data for the outermost Galactic regions are crucial for drawing strong conclusions.