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

To date, the Galactic interstellar radial 32S/34S 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 34SO could be considered as tracers of the 32S/34S ratio. We present the first systematic observations of SO and 34SO 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 22–11 was detected in 59 of 82 sources (∼82%), and 34SO 22–11 in eight sources (∼10%). With the SMT 10 m, SO 55–44 was detected in 136 of 184 sources (∼74%), and 34SO 55–44 in 55 of 77 strong SO sources (∼72%). SO/34SO ratios were derived for 8 (22–11) and 55 (55–44) sources. No correlation was found between the SO/34SO ratio and heliocentric distance or Tk, suggesting negligible distance and fractionation effects. Both LTE and non-LTE analyses consistently suggest that the optical-depth effect is also insignificant. 32S/34S ratios from the 22–11 transitions follow the increasing radial trend proposed by previous CS species measurements, while those from the 55–44 lines are systematically lower. The lower transitions of SO and 34SO may be suitable tracers of 32S/34S, 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.