1994A&A...289..579T

The Effelsberg 100-m and the IRAM 30-m telescopes have been used to observe low excitation lines of sulfur-bearing molecules in absorption toward the background continuum sources W49 and SgrB2. Both H₂S and SO were detected in the 40 km/s feature toward W49 as well as in several negative velocity features toward SgrB2. NH₃(1,1) and (2,2) have also been observed in these lines of sight, which allows us to make estimates of the temperature in the low density absorption line clouds. From these data, we have estimated molecular column densities and compared the derived abundances with those observed in the nearby dust clouds TMC1 and L183. We have also compared these results with predictions from chemical models. We find that the abundance distribution in the low density (≤10⁴/cm³) spiral arm clouds is in general similar to that observed in TMC1 and L183. However, our data show that SO and NH₃ are more than an order of magnitude underabundant relative to local "standards". From the model calculations, we also find that the data for [CS]/[SO] in W49 (v=40km/s) and SgrB2 (v=-40km/s) are consistent with these clouds being "somewhat translucent" 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.