CN and HCN in dense interstellar clouds

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 10(2) to 10(6) cm(-3). We study clouds that are exposed to FUV fields with intensities (at 1000 angstrom) from 1 x 10(-18) to 1 x 10(-14) ergs s(-1) cm(-2) Hz(-1) sr(-1), or 20 to 2 x 10(5) times the mean interstellar FUV intensity. We assume cosmic-ray H-2 ionization rates ranging from 5; 10(-17) s(-1) to an enhanced value of 5; 10(-16) s(-1). We also examine the sensitivity of the density profiles to the gas-phase sulfur abundance.