2020ApJ...899..145Y

We present observations of ¹²C³²S, ¹²C³⁴S, ¹³C³²S, and ¹²C³³S J = 2-1 lines toward a large sample of massive star-forming regions by using the Arizona Radio Observatory 12 m telescope and the IRAM 30 m. Taking new measurements of the carbon ¹²C/¹³C ratio, the ³²S/³⁴S isotope ratio was determined from the integrated ¹³C³²S/¹²C³⁴S line intensity ratios for our sample. Our analysis shows a ³²S/³⁴S gradient from the inner Galaxy out to a galactocentric distance of 12 kpc. An unweighted least-squares fit to our data yields ³²S/³⁴S = (1.56 ± 0.17)D_GC + (6.75 ± 1.22) with a correlation coefficient of 0.77. Errors represent 1σ standard deviations. Testing this result by (a) excluding the Galactic center region, (b) excluding all sources with C³⁴S opacities >0.25, (c) combining our data and old data from previous study, and (d) using different sets of carbon isotope ratios leads to the conclusion that the observed ³²S/³⁴S gradient is not an artifact but persists irrespective of the choice of sample and carbon isotope data. A gradient with rising ³²S/³⁴S values as a function of galactocentric radius implies that the solar system ratio should be larger than that of the local interstellar medium. With the new carbon isotope ratios, we indeed obtain a local ³²S/³⁴S isotope ratio about 10% below the solar system one, as expected in the case of decreasing ³²S/³⁴S ratios with time and increased amounts of stellar processing. However, taking older carbon isotope ratios based on a lesser amount of data, such a decrease is not seen. No systematic variation of ³⁴S/³³S ratios along galactocentric distance was found. The average value is 5.9 ± 1.5, the error denoting the standard deviation of an individual measurement.