A New FUSE Survey of Interstellar HD

TL;DR

This study uses archival FUSE data to survey interstellar HD across 41 lines of sight, analyzing its formation, relation to other interstellar properties, and implications for cosmology.

Contribution

It provides new insights into HD formation processes, its correlation with interstellar properties, and clarifies its limited use as a cosmological probe.

Findings

HD forms in dense cloud regions.

The HD/H2 ratio is lower than the atomic D/H ratio.

HD destruction rate declines more slowly than H2.

Abstract

We have used archival FUSE data to complete a survey of interstellar HD in 41 lines of sight with a wide range of extinctions. This follow up to an earlier survey was made to further assess the utility of HD as a cosmological probe; to analyze the HD formation process; and to see what trends with other interstellar properties were present in the data. We employed the curve-of-growth method, supported by line profile fitting, to derive accurate column densities of HD. We find that the N(HD)/2N(H2) ratio is substantially lower than the atomic D/H ratio and conclude that the molecular ratio has no bearing on cosmology, because local processes are responsible for the formation of HD. Based on correlations with E(B-V), H2, CO, and iron depletion, we find that HD is formed in the densest portion of the clouds; the slope of the logN(HD)/log(H2) correlation is greater than 1.0, caused by the destruction rate of HD declining more slowly than that of H2; and, as a sidelight, that the depletions are density dependent.