Galactic Interstellar ³² S/ ³⁴ S Isotopic Ratio Traced by Sulfur Monoxide

Abstract To date, the Galactic interstellar radial 32 S/ 34 S gradient has only been studied with the CS isotopologues, 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 (∼82%), and 34 SO 2 2 –1 1 in eight sources (∼10%). With the SMT 10 m, SO 5 5 –4 4 was detected in 136 of 184 sources (∼74%), and 34 SO 5 5 –4 4 in 55 of 77 strong SO sources (∼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.