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HD/H2 as a probe of the roles of gas, dust, light, metallicity and cosmic rays in promoting the growth of molecular hydrogen in the diffuse interstellar medium

We modelled recent observations of UV absorption of HD and \HH\ in the Milky Way and toward damped/sub-damped Lyman alpha systems at z=0.18 and z $>$ 1.7. N(HD)/N(\HH) ratios reflect the separate self-shieldings of HD and \HH\ and the coupling introduced by deuteration chemistry. Locally, observations are explained by diffuse molecular gas with $ 16 \pccc \la$ n(H) $\la 128 \pccc $ if the cosmic-ray ionization rate per H-nucleus \zetaH $= 2\times 10^{-16}\ps$ as inferred from \H3\p\ and OH\p. The dominant influence on N(HD)/N(\HH) is the cosmic-ray ionization rate with a much weaker downward dependence on n(H) at Solar metallicity, but dust-extinction can drive N(HD) higher as with N(\HH). At z $>$ 1.7, N(HD) is comparable to the Galaxy but with 10x smaller N(\HH) and somewhat smaller N(\HH)/N(H I). Comparison of our Galaxy and the Magellanic Clouds shows that smaller \HH/H is expected at sub-Solar metallicity and we show by modelling that HD/\HH\ increases with density at low metallicity, opposite to the Milky Way. Observations of HD would be explained with higher n(H) at low metallicity but high-z systems have high HD/\HH\ at metallicity 0.04 $\la$ Z $\la$ 2 Solar. In parallel we trace dust-extinction and self-shielding effects. The abrupt \HH\ transition to \HH/H $\approx$ 1-10% occurs mostly from self-shielding although it is assisted by extinction for n(H) $\la 16 \pccc$. Interior \HH\ fractions are substantially increased by dust extinction below $\la 32\pccc$. At smaller n(H), \zetaH, small increases in \HH\ triggered by dust extinction can trigger abrupt increases in N(HD).