2006A&A...452..269L

To clarify the circumstances under which it is acceptable to approximate the interstellar gas-phase D/H ratio by DI/HI. Observed column densities of HI, DI, H₂ and HD are compared for six lines of sight having appreciable fractions of H₂. 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 DI and N(DI)/N(HI) exceeds the gas-phase D/H ratio in H₂-bearing gas. Deuterium in H₂-bearing gas contributes to the observed DI absorption and the D/H ratio should be inferred from N(D)/N(H) where N(D)=(N(DI)+N(HD)), N(H)=N(HI)+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 DI and HI 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.