HD/H$_2$ ratio in the diffuse interstellar medium

TL;DR

This paper develops a semi-analytical model to describe the HD/H2 ratio in the diffuse interstellar medium, revealing how physical parameters influence molecular abundances and explaining observed variations across different environments.

Contribution

It introduces a semi-analytical framework for HD/H2 abundance, highlighting the effects of metallicity and cosmic rays, and aligns well with detailed PDR code calculations.

Findings

HD formation is enhanced at low metallicity and high cosmic-ray ionization rates.

HD/H2 ratio increases under certain physical conditions, affecting the DI/HD transition.

The model explains differences in HD/H2 ratios observed locally and at high redshift.

Abstract

We present a semi-analytical description of the relative HD/H2 abundance in the diffuse interstellar medium. We found three asymptotics of the relative HD/H$_2$ 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 DI/HD transition occurs at a lower penetration depth of UV radiation than the HI/H$_2$ transition. This can explain the observed difference in the HD/H$_2$ abundance between the local and high-redshift measurements.