Journal
ASTROPHYSICAL JOURNAL
Volume 799, Issue 1, Pages -Publisher
IOP PUBLISHING LTD
DOI: 10.1088/0004-637X/799/1/66
Keywords
astrochemistry; cosmic rays; dust, extinction; ISM: clouds; ISM: molecules
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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).
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