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 destructionreactions among 100 molecular species formed from the elements H,He, C, N, O, Mg, Si, S, and Fe. Only gas-phase reactions are consideredexcept for grain formation of H2. Results are presented for gas densitiesof 10 to 1 million per cu cm and a wide range of optical depth. Thedependence of the molecule abundances on cosmic element abundances,the interstellar radiation field, kinetic temperature, and cosmic-rayionization rate is investigated. A comparison with observations ofinterstellar clouds shows that the standard model employed is clearlyinadequate only for CH(+) and HCO, that agreement for H2CO is goodat intermediate and high densities, and that the CN abundances atlow densities agree well with optical observations. Agreement withobservations is found to be satisfactory for CH, C2H, CO, OH, HCN,NH3, HCO(+), and N2H(+).