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 modeled recent observations of UV absorption of HD and H-2 in the Milky Way and toward damped/subdamped Ly alpha systems at z = 0.18 and z > 1.7. N(HD)/N(H-2) ratios reflect the separate self-shieldings of HD and H-2 and the coupling introduced by deuteration chemistry. Locally, observations are explained by diffuse molecular gas with 16 cm(-3) less than or similar to n(H) less than or similar to 128 cm(-3) if the cosmic-ray ionization rate per H nucleus zeta(H) = 2 x 10(-16) s(-1), as inferred from H-3(+) and OH+. The dominant influence on N(HD)/N(H-2) 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(H-2). At z > 1.7, N(HD) is comparable to the Galaxy but with 10 times smaller N(H-2) and somewhat smaller N(H-2)/N(H I). Comparison of our Galaxy with the Magellanic Clouds shows that smaller H-2/H is expected at subsolar metallicity, and we show by modeling that HD/H-2 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/H-2 at metallicity 0.04 less than or similar to Z less than or similar to 2 solar. In parallel, we trace dust extinction and self-shielding effects. The abrupt H-2 transition to H-2/H approximate to 1%-10% occurs mostly from self-shielding, although it is assisted by extinction for n(H) less than or similar to 16 cm(-3). Interior H-2 fractions are substantially increased by dust extinction below less than or similar to 32 cm(-3). At smaller n(H), zeta(H), small increases in H-2 triggered by dust extinction can trigger abrupt increases in N(HD).