On the relationship of interstellar N2H+, HCO+, HCN, and CN

A survey of 73 sources has been made in the emission lines of N2H+ and HCO+, including detailed maps of four sources (Ori A, OMC-2, DR 21 OH, and NGC 6334). These data are combined with equally extensive data for HCN and CN to make a detailed study of the spatial relationship of these four species. Actual abundance ratios are shown to vary, often sharply, over small scale lengths (approx.1') within our mapped sources. Excitation temperatures vary also. All four species appear to be subthermally excited. The abundance ratios, and their spatial variations, are interpreted in terms of ion-molecular formation and destruction processes. It is concluded that the relative abundances of CO may be considerably lower in some regions than previously believed. No evidence is found for variations in the C/O ratio over small scale lengths in Ori A. The degree of ionization of the Ori A molecular cloud may be significantly different from that of other molecular clouds