Galactic Interstellar Sulfur Isotopes: A Radial 32 S/ 34 S Gradient?

Abstract

We present observations of 12 C 32 S, 12 C 34 S, 13 C 32 S, and 12 C 33 S 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 12 C/ 13 C ratio, the 32 S/ 34 S isotope ratio was determined from the integrated 13 C 32 S/ 12 C 34 S line intensity ratios for our sample. Our analysis shows a 32 S/ 34 S gradient from the inner Galaxy out to a galactocentric distance of 12 kpc. An unweighted least-squares fit to our data yields 32 S/ 34 S = (1.56 ± 0.17) D GC  + (6.75 ± 1.22) with a correlation coefficient of 0.77. Errors represent 1 σ standard deviations. Testing this result by (a) excluding the Galactic center region, (b) excluding all sources with C 34 S 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 32 S/ 34 S gradient is not an artifact but persists irrespective of the choice of sample and carbon isotope data. A gradient with rising 32 S/ 34 S 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 32 S/ 34 S isotope ratio about 10% below the solar system one, as expected in the case of decreasing 32 S/ 34 S 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 34 S/ 33 S ratios along galactocentric distance was found. The average value is 5.9 ± 1.5, the error denoting the standard deviation of an individual measurement.