A CHEMICAL SURVEY OF MOLECULES IN SPIRAL ARM CLOUDS

We have used the SEST 15m telescope to make a spectral survey of molec ular absorption lines at frequencies of 82-113 GHz. The background sou rce used was the Sgr B2 'M' HII region complex, and the line of sight intercepted six distinct foreground regions with different velocities. A total of 17 transitions was detected, from 11 different molecules a nd their variants. N2H+ and CH3OH were detected for the first time in absorption, and SO was confirmed along this line of sight. The major r esult of this survey is a unique, uniformly calibrated dataset of all the species known to produce molecular absorption at millimetre wavele ngths. We have estimated the abundances of the 11 species (HCO+, HCN, HNC, CN, CCH, C3H2, CS, SiO, N2H+, CH3OH and SO) in each of the foregr ound regions. These estimates are believed to be very reliable, as dat a from isotopic lines were used to reduce the effects of excitation va riations in the more optically thick transitions, and the abundances f or the most chemically stable species are found to vary by only 30%, c lose to the minimum errors. Overall, the chemistry of the absorption c louds is very similar to that found in the dark cloud TMC1, with most abundances agreeing within factors of 2. The exceptions are C3H2, CS, and SiO, where the average abundances relative to TMC1 are 0.1, 4 and 500, respectively. The chemistry is also similar to that found in low- A(V) absorption clouds seen towards extragalactic continuum sources. H owever, the absorption clouds have HCN/HNC ratios intermediate between those of low Av clouds and TMC1. These similarities and differences a re discussed in the context of chemical models. The high SiO abundance appears to be a result of temperature dependence of the formation mec hanism, and the largest abundances are found in clouds which are known to be warmer than average. The CS and C3H2 abundances are not tempera ture-dependent, but instead show a trend of decreasing with increasing gas shielding. This may provide evidence for the recently proposed tw o-phase chemistry in cool molecular clouds, where abundances fluctuate with the C/CO ratio.