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