Molecular clouds HD/H2 in the early Universe

Abstract

Abstract We present a simplified semi-analytical description of the relative HD/H2 abundance in the cold neutral interstellar medium. With this description we was able to obtain three asymptotics of the relative HD/H2 abundance and its dependence on physical parameters in the medium, namely, the number density of the gas, the intensity of the ultraviolet field, the cosmic ray ionization rate and metallicity. Our calculations in presented simple formalism are in the reasonable agreement with the calculations using the Meudon PDR code. We found that in the case of low metallicity and a higher cosmic ray ionization rate, the relative abundance of HD/H2 is significantly enhanced, which can explain the observed difference between the local and high-z measurements of relative HD/H2 abundance.