A steady-state calculation of molecule abundances in interstellar clouds

Abstract

Steady-state abundances are calculated from 455 ion-neutral, neutral-neutral, photodestruction, ion-electron recombination, and cosmic-ray destruction reactions among 100 molecular species formed from the elements H, He, C, N, O, Mg, Si, S, and Fe. Only gas-phase reactions are considered except for grain formation of H2. Results are presented for gas densities of 10 to 1 million per cu cm and a wide range of optical depth. The dependence of the molecule abundances on cosmic element abundances, the interstellar radiation field, kinetic temperature, and cosmic-ray ionization rate is investigated. A comparison with observations of interstellar clouds shows that the standard model employed is clearly inadequate only for CH(+) and HCO, that agreement for H2CO is good at intermediate and high densities, and that the CN abundances at low densities agree well with optical observations. Agreement with observations is found to be satisfactory for CH, C2H, CO, OH, HCN, NH3, HCO(+), and N2H(+).