Abstract
We present observations of(12)C(32)S,(CS)-C-12-S-34,(CS)-C-13-S-32, and(12)C(33)SJ = 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/C-13 ratio, the(32)S/S-34 isotope ratio was determined from the integrated(13)C(32)S/(CS)-C-12-S-34 line intensity ratios for our sample. Our analysis shows a(32)S/S-34 gradient from the inner Galaxy out to a galactocentric distance of 12 kpc. An unweighted least-squares fit to our data yields(32)S/S-34 = (1.56 0.17)D-GC + (6.75 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 (CS)-S-34 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/S-34 gradient is not an artifact but persists irrespective of the choice of sample and carbon isotope data. A gradient with rising(32)S/S-34 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/S-34 isotope ratio about 10% below the solar system one, as expected in the case of decreasing(32)S/S-34 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/S-33 ratios along galactocentric distance was found. The average value is 5.9 1.5, the error denoting the standard deviation of an individual measurement.