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.
Categories: astro-ph.GA