Deuterated molecular hydrogen in the Galactic ISM. New observations along seven translucent sightlines

TL;DR

This study measures HD molecules in 17 Galactic sightlines, revealing correlations with other molecules and implications for the interstellar D/H ratio, using FUSE and Copernicus data.

Contribution

It provides new HD column density measurements along multiple sightlines and explores their relationship with other molecules and the D/H ratio in the interstellar medium.

Findings

HD correlates most tightly with CH.

Lower limits to D/H ratio range from 3.7e-7 to 4.3e-6.

HD/H2 ratio increases with molecular fraction f(H2).

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 KI 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 KI 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 10^(-7) to 4.3 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(H2) = 1. Finally, we note an apparent relationship between the molecular fractions of hydrogen and deuterium.