Measuring interstellar gas-phase D/H ratios in the presence of H₂

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.