2025/11/28 by J. S. Zhang, Zou, Yipeng, Zhang, Jiangshui +13
Physics and Astronomy · #Astrophysics and Star Formation Studies #Stellar, planetary, and galactic studies #Astronomy and Astrophysical Research
paper · pdf · doi:10.48550/arxiv.2511.23147
To date, the Galactic interstellar radial 32S/34S 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 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 8 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.