CHEMISTRY AND STRUCTURE OF THE OMC1 RADICAL REGION

A 7-arcmin strip along the OMC1 ridge has been mapped in the transitions (CO)-C-13 J = 2-1, (CS)-C-13 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 approximately 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.