Abundances of interstellar molecules

Equilibrium abundances are calculated for various molecules in interstellar H I gas clouds using rates from a previous paper for molecule formation on grain surfaces, and rates from other authors for gas-phase reactions and for photodissociation. Results depend on a parameter (n/100 H ) exp (2.5r ), where n is gas density and r is the visual optical depth from points inside the cloud to its surface. olecules probably are not depleted by condensation if < tO (hut the situation is uncertain if grains with ice mantles are present). For lightly shielded clouds ( < 10) where hydrogen is mainly atomic and carbon is mainly C+, the combined abundances of various molecules containing C, N, or 0 are estimated (relative to atomic C, N, and 0) to be 1O- . When exceeds some critical value 300, the neutral C in the gas eliminates photons beyond 11.3 eV; because of their high binding energy, CO molecules are then not dissociated and are expected to be almost as abundant as C atoms. Abundances (with even greater uncertainties) are also estimated for the more complex molecules.