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Issue A&A
Volume 452, Number 1, June II 2006
Page(s) 269 - 272
Section Interstellar and circumstellar matter
DOI http://dx.doi.org/10.1051/0004-6361:20054785



A&A 452, 269-272 (2006)
DOI: 10.1051/0004-6361:20054785

Measuring interstellar gas-phase D/H ratios in the presence of $\mathsf{H_2}$

H. S. Liszt

National Radio Astronomy Observatory, 520 Edgemont Road, 22903-2475 Charlottesville, VA, USA
    e-mail: hliszt@nrao.edu

(Received 28 December 2005 / Accepted 21 February 2006 )

Abstract
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.


Key words: ISM: abundances -- ISM: atoms -- ISM: molecules

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© ESO 2006


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