Galactic interstellar sulfur isotopes: A radial ${}^{32}$S$\mathrm{/}$${}^{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$\mathrm{/}$${}^{34}$S isotope ratio was determined from the integrated ${}^{13}$C${}^{32}$S/${}^{12}$C${}^{34}$S line intensity ratios for our sample. Our analysis shows a ${}^{32}$S$\mathrm{/}$${}^{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$\mathrm{/}$${}^{34}$S = (1.56 $\pm$ 0.17)${\mathrm{D}}_{\mathrm{G}\mathrm{C}}$

• (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, © 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$\mathrm{/}$${}^{34}$S gradient is not an artefact but persists irrespective of the choice of the sample and carbon isotope data. A gradient with rising ${}^{32}$S$\mathrm{/}$${}^{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$\mathrm{/}$${}^{34}$S isotope ratio about 10$\mathrm{\%}$ below the solar system one, as expected in case of decreasing ${}^{32}$S$\mathrm{/}$${}^{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$\mathrm{/}$${}^{33}$S ratios along galactocentric distance was found.