Galactic Interstellar Sulfur Isotope Ratios

a large sample of massive star forming regions by using the Arizona Radio Observatory12-m telescope.Our detection rate is high,including 61 sources out of 95 targets with detections of all four lines.Among those 61 sources,6 sources were also observed by the IRAM 30-m.Comparing results from the ARO 12-m and the IRAM 30-m for these6 sources,we find that beam size effects are negligible for our measured isotope ratios.Taking new measurements of the carbon¹²C/¹³C ratio^[1],the³²S/³⁴S isotope ratio was determined from the integrated¹³C³²S/¹²C³⁴S line intensity ratios for our sample.Our analysis shows a significant³²S/³⁴S gradient from the Galactic center out to a galactocentric distance of 12 kpc.A least-squares fit to our data yields³²S/³⁴S=?1.58±0.44?D_GC+?6.59±2.20?with a correlation coefficient of 0.77,which is consistent with an inside-out formation scenario for our Galaxy.We also obtain a local³²S/³⁴S isotope ratio about 10%below the solar system one,as expected in case of decreasing³²S/³⁴S 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 of³⁴S/³³S ratios along galactocentric distance was found.