A chemical survey of molecules in `spiral arm' clouds.

Details A chemical survey of molecules in `spiral arm' clouds. A chemical survey of molecules in `spiral arm' clouds.

Jan 1996

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996a%26a...305..950g&link_type=abstract

Astronomy and Astrophysics, v.305, p.950

Astronomy and Astrophysics

Astrophysics

25

Ism: Abundances, Ism: Clouds, Ism: Molecules, Radio Lines: Ism

Scientific paper

We have used the SEST 15m telescope to make a spectral survey of molecular absorption lines at frequencies of 82-113 GHz. The background source 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 and their variants. N_2_H^+^ and CH_3_OH were detected for the first time in absorption, and SO was confirmed along this line of sight. The major result of this survey is a unique, uniformly calibrated dataset of all the species known to produce molecular absorption at millimetre wavelengths. We have estimated the abundances of the 11 species (HCO^+^, HCN, HNC, CN, CCH, C_3_H_2_, CS, SiO, N_2_H^+^, CH_3_OH and SO) in each of the foreground regions. These estimates are believed to be very reliable, as data from isotopic lines were used to reduce the effects of excitation variations in the more optically thick transitions, and the abundances for the most chemically stable species are found to vary by only 30%, close to the minimum errors. Overall, the chemistry of the absorption clouds is very similar to that found in the dark cloud TMC1, with most abundances agreeing within factors of 2. The exceptions are C_3_H_2_, 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. However, the absorption clouds have HCN/HNC ratios intermediate between those of low A_V_ clouds and TMC1. These similarities and differences are discussed in the context of chemical models. The high SiO abundance appears to be a result of temperature dependence of the formation mechanism, and the largest abundances are found in clouds which are known to be warmer than average. The CS and C_3_H_2_ abundances are not temperature-dependent, but instead show a trend of decreasing with increasing gas shielding. This may provide evidence for the recently proposed two-phase chemistry in cool molecular clouds, where abundances fluctuate with the C/CO ratio.

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