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 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 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/2H _ { 2 } ratio , range from 3.7 \times 10 ^ { -7 } to 4.3 \times 10 ^ { -6 } .
Our results also suggest that the HD/H _ { 2 } ratio increases with the molecular fraction f ( H _ { 2 } ) 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 .