2026ApJS..282...67Z

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 T_k, 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.