HD/H2 ratio in the diffuse interstellar medium
We present a semi-analytical description of the relative hydrogen deuteride (HD)/H2 abundance in the diffuse interstellar medium. We found three asymptotics of the relative HD/H2 abundance for different parts of the medium and their dependence on the physical parameters, namely, number density, intensity of the ultraviolet field, cosmic ray ionization rate, and metallicity. Our calculations are in a good agreement with the full network calculations using MEUDON PDR code. We found that in the case of low metallicity and/or higher cosmic ray ionization rate, HD formation rate is significantly enhanced, HD/H2 ratio increases, and the D I/HD transition occurs at lower penetration depth of ultraviolet radiation than the H I/H2 transition. This can explain the observed difference in the HD/H2 abundance between the local and high-redshift measurements.