Astronomy and Astrophysics – Astrophysics
Scientific paper
Jun 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006a%26a...452..269l&link_type=abstract
Astronomy and Astrophysics, Volume 452, Issue 1, June II 2006, pp.269-272
Astronomy and Astrophysics
Astrophysics
4
Ism: Abundances, Ism: Atoms, Ism: Molecules
Scientific paper
Aims.To clarify the circumstances under which it is acceptable to approximate the interstellar gas-phase D/H ratio by D I/H I. Methods: .Observed column densities of H I, D I, H2 and HD are compared for six lines of sight having appreciable fractions of H2. Results: .The overall fraction of deuterium in HD varies by a factor 3-4 but is found to be much smaller than the fraction of H in H2 in all cases, implying that deuterium appears as D I and N(D I)/N(H I) exceeds the gas-phase D/H ratio in H2-bearing gas. Conclusions: .Deuterium in H2-bearing gas contributes to the observed D I absorption and the D/H ratio should be inferred from N(D)/N(H) where N(D) = (N(D I) + N(HD)), N(H) = N(H I) + 2N(H2): failure to do so biases the resulting D/H ratio upward, typically by 5%-15% in present data. Along sightlines with multiple kinematic components having different molecular fractions, fractionation can cause velocity differences between D I and H I profiles. Shifts between H2 and HD velocity centroids may arise when the molecule-bearing gas has kinematic substructure reflecting regions of different ionization balance and HD/H2 ratios.
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