SO AND H2S IN LOW-DENSITY MOLECULAR CLOUDS

The Effelsberg 100-m and the IRAM 30-m telescopes have been used to ob serve low excitation lines of sulfur-bearing molecules in absorption t oward the background continuum sources W49 and SgrB2. Both H2S and SO were detected in the 40 km s(-1) feature toward W49 as well as in seve ral negative velocity features toward SgrB2. NH3(1,1) and (2,2) have a lso been observed in these lines of sight, which allows us to make est imates of the temperature in the low density absorption line clouds. F rom these data, we have estimated molecular column densities and compa red the derived abundances with those observed in the nearby dust clou ds TMC1 and L183. We have also compared these results with predictions from chemical models. We find that the abundance distribution in the low density (less than or equal to 10(4)cm(-3)) spiral arm clouds is i n general similar to that observed in TMC1 and L183. However, our data show that SO and NH3 are more than an order of magnitude underabundan t relative to local ''standards''. From the model calculations, we als o find that the data for [CS]/[SO] in W49 (v=40km s(-1)) and SgrB2 (v= -40km s(-1)) are consistent with these clouds being ''somewhat translu cent'' with a total visual extinction of roughly 8 magnitudes, but the results depend sensitively upon assumptions concerning the gas phase abundances. Moreover, we find difficulty in producing the observed H2S column density using current gas phase models and we discuss possible solutions for this problem.