Chemistry and structure of the OMC1 radical region

Greaves, J. S. and White, Glenn J. (1992). Chemistry and structure of the OMC1 radical region. Monthly Notices of the Royal Astronomical Society, 259 pp. 457–464.



A 7-arcmin strip along the OMC1 ridge has been mapped in the transitions 13CO J = 2-1, 13CS J = 5-4 and CN N = 2-1. The cyanogen radical is found not to have an exceptionally high abundance in the so-called 'Radical Region'. Strong CN N = 1-0 emission detected previously instead results from a low excitation temperature. The N = 2-1 transition is subthermalized, in spite of optical depths as high as 3, and this is explained in terms of non-LTE effects. The chemistry of the Radical Region is examined using results from a recent spectral survey. The abundances of HCN, CS, C3H2 and CH3CCH are found to be similar to those in the cloud core, while SO, HCO+ and HCS+ are enhanced in the Radical Region by factors of ~ 5-50. It is concluded that this part of the cloud does not have very unusual chemistry. Previous results suggesting anomalous abundances are misleading, because the low-J transitions observed were enhanced in cool, dense gas.

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