English (United Kingdom)

https://curated-unify.zendy.io/wp-json/zendy-region/v1/featured_content/oa?rat=en

https://curated-unify.zendy.io/wp-json/zendy-region/v1/highlighted_journal/

Global: Zendy Plus annual

Presents the access of premium content as premium feature

Premium Content

Presents the keyphrase highlighting as premium feature

Keyphrase Highlighting

Presents the summarisation as premium feature

Summarisation

Insights

Presents the pdf analysis as premium feature

PDF Analysis

Presents the zaia usage as premium feature

ZAIA

Global: Zendy Tools annual

Global: Zendy Free Plan

Global: Zendy Tools monthly

Global: Zendy Plus monthly

We modelled recent observations of UV absorption of HD and \HH\ in the MilkyWay 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 thecoupling introduced by deuteration chemistry. Locally, observations areexplained 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 thecosmic-ray ionization rate with a much weaker downward dependence on n(H) atSolar 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) andsomewhat smaller N(\HH)/N(H I). Comparison of our Galaxy and the MagellanicClouds shows that smaller \HH/H is expected at sub-Solar metallicity and weshow by modelling that HD/\HH\ increases with density at low metallicity,opposite to the Milky Way. Observations of HD would be explained with highern(H) at low metallicity but high-z systems have high HD/\HH\ at metallicity0.04 $\la$ Z $\la$ 2 Solar. In parallel we trace dust-extinction andself-shielding effects. The abrupt \HH\ transition to \HH/H $\approx$ 1-10%occurs mostly from self-shielding although it is assisted by extinction forn(H) $\la 16 \pccc$. Interior \HH\ fractions are substantially increased bydust extinction below $\la 32\pccc$. At smaller n(H), \zetaH, small increasesin \HH\ triggered by dust extinction can trigger abrupt increases in N(HD).

astrophysical journal/the astrophysical journal

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