Galactic interstellar sulfur isotopes: A radial \(^{32}\)S$/$$^{34}$S gradient?

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}\(S = (1.56 \)\pm\( 0.17)\)\rm D_{\rm GC}\( + (6.75 \)\pm\( 1.22) with a correlation coefficient of 0.77. Errors represent 1\)\sigma\( standard deviations. Testing this result by (a) excluding the Galactic center region, (b) excluding all sources with C\)^{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}\(S gradient is not an artefact but persists irrespective of the choice of the sample and carbon isotope data. A gradient with rising \)^{32}\(S\)/$$^{34}\(S values as a function of galactocentric radius implies that the solar system ratio should be larger than that of the local interstellar medium. With the new carbon isotope ratios we obtain indeed a local \)^{32}\(S\)/$$^{34}\(S isotope ratio about 10\)\%\( below the solar system one, as expected in case of decreasing \)^{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.