Galactic Interstellar Sulfur Isotopes: A Radial <sup>32</sup>S/<sup>34</sup>S Gradient?
Hongzhi Yu, J. S. Zhang, C. Henkel, Y. T. Yan, Wenhui Liu, Xindi Tang, N. Langer, Tianxiang Luan, J.L. Chen, Y. X. Wang, Gaopi Deng, Y. P. Zou
Abstract
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.