Molecular Hydrogen in the FUSE Translucent Lines of Sight: The Full Sample

TL;DR

This study analyzes molecular hydrogen in 38 translucent lines of sight using FUSE data, revealing correlations with dust properties and UV radiation, and updating gas-to-dust ratios, while challenging the notion of uniform translucent clouds.

Contribution

It provides new measurements for the full FUSE sample, confirms the influence of dust and radiation on molecular fractions, and refines gas-to-dust ratio estimates, expanding understanding of interstellar molecular hydrogen.

Findings

Molecular fractions inversely related to UV radiation strength.

Large dust grains associated with low molecular hydrogen fractions.

Updated gas-to-dust ratio consistent with previous data, extending to higher reddening.

Abstract

We report total abundances and related parameters for the full sample of the FUSE survey of molecular hydrogen in 38 translucent lines of sight. New results are presented for the "second half" of the survey involving 15 lines of sight to supplement data for the first 23 lines of sight already published. We assess the correlations between molecular hydrogen and various extinction parameters in the full sample, which covers a broader range of conditions than the initial sample. In particular, we are now able to confirm that many, but not all, lines of sight with shallow far-UV extinction curves and large values of the total-to-selective extinction ratio, $R_V$ = $A_V$ / $E(B-V)$ -- characteristic of larger than average dust grains -- are associated with particularly low hydrogen molecular fractions ($f_{\rm H2}$). In the lines of sight with large $R_V$, there is in fact a wide range in molecular fractions, despite the expectation that the larger grains should lead to less H$_2$ formation. However, we see specific evidence that the molecular fractions in this sub-sample are inversely related to the estimated strength of the UV radiation field and thus the latter factor is more important in this regime. We have provided an update to previous values of the gas-to-dust ratio, $N$(H$_{\rm tot}$)/$E(B-V)$, based on direct measurements of $N$(H$_2$) and $N$(H I). Although our value is nearly identical to that found with Copernicus data, it extends the relationship by a factor of 2 in reddening. Finally, as the new lines of sight generally show low to moderate molecular fractions, we still find little evidence for single monolithic "translucent clouds" with $f_{\rm H2}$ $\sim$ 1.