2005ApJ...632..302B

We present a theoretical investigation of CN and HCN molecule formation in dense interstellar clouds. We study the gas-phase CN and HCN production efficiencies from the outer photon-dominated regions (PDRs) into the opaque cosmic-ray-dominated cores. We calculate the equilibrium densities of CN and HCN and of the associated species C⁺, C, and CO, as functions of the far-ultraviolet (FUV) optical depth. We consider isothermal gas at 50 K, with hydrogen particle densities from 10² to 10⁶/cm3. We study clouds that are exposed to FUV fields with intensities (at 1000 Å) from 1x10^–18 to 1x10^–14 ergs/s/cm2/Hz/sr, or 20 to 2x10⁵ times the mean interstellar FUV intensity. We assume cosmic-ray H₂ionization rates ranging from 5x10^–17/s to an enhanced value of 5x10^–16/s. We also examine the sensitivity of the density profiles to the gas-phase sulfur abundance.