Deuterated molecular hydrogen in the Galactic ISM

Abstract

We present column density measurements of the HD molecule in the interstellar gas toward 17 Galactic stars. The values for the seven most heavily reddened sightlines, with $E(B-V) = 0.38{-}0.72$, are derived from observations with the Far Ultraviolet Spectroscopic Explorer (FUSE). The other ten values are from a reanalysis of spectra obtained with Copernicus. In all cases, high-resolution ground-based observations of \ion{K}{i} and/or the CH molecule were used to constrain the gas velocity structure and to correct for saturation effects. Comparisons of the column densities HD, CH, CN, and \ion{K}{i} in these 17 sightlines indicate that HD is most tightly correlated with CH. Stringent lower limits to the interstellar D/H ratio, derived from the HD/2H2 ratio, range from $3.7 \times 10^{-7}$ to $4.3 \times 10^{-6}$. Our results also suggest that the HD/H2 ratio increases with the molecular fraction f(H2) and that the interstellar D/H ratio might be obtained from HD by probing clouds with f(H$_2) \sim 1$. Finally, we note an apparent relationship between the molecular fractions of hydrogen and deuterium.