ELECTRON-DENSITIES AND THE EXCITATION OF CN IN MOLECULAR CLOUDS

In molecular clouds of modest density and relatively high fractional ionization [n(e)/n(H-2) > 10(-5)], the rotational excitation of CN is controlled by a competition among electron impact, neutral impact (primarily H-2), and the interaction with the cosmic background radiation. The degree of excitation can be measured through optical absorption lines and millimeter-wave emission lines. The available, accurate data on CN in diffuse and translucent molecular clouds are assembled and used to determine electron densities. The derived values, n(e) almost-equal-to 0.02-0.5 cm-3, imply modest neutral densities in the range 10(2.5) - 10(3.5) cm-3, which generally agree well with determinations by other techniques. The absorption- and emission-line measurements of CN both exclude densities higher than n(H-2) almost-equal-to 10(3.5) cm-3 on scales varying from 10(-3) to 60" in these clouds.