MOLECULAR HYDROGEN IN THE <i>FAR ULTRAVIOLET SPECTROSCOPIC EXPLORER</i> TRANSLUCENT LINES OF SIGHT: THE FULL SAMPLE

Abstract

We report total abundances and related parameters for the full sample of the\nFUSE survey of molecular hydrogen in 38 translucent lines of sight. New results\nare presented for the "second half" of the survey involving 15 lines of sight\nto supplement data for the first 23 lines of sight already published. We assess\nthe correlations between molecular hydrogen and various extinction parameters\nin the full sample, which covers a broader range of conditions than the initial\nsample. In particular, we are now able to confirm that many, but not all, lines\nof sight with shallow far-UV extinction curves and large values of the\ntotal-to-selective extinction ratio, $R_V$ = $A_V$ / $E(B-V)$ -- characteristic\nof larger than average dust grains -- are associated with particularly low\nhydrogen molecular fractions ($f_{\\rm H2}$). In the lines of sight with large\n$R_V$, there is in fact a wide range in molecular fractions, despite the\nexpectation that the larger grains should lead to less H$_2$ formation.\nHowever, we see specific evidence that the molecular fractions in this\nsub-sample are inversely related to the estimated strength of the UV radiation\nfield and thus the latter factor is more important in this regime. We have\nprovided an update to previous values of the gas-to-dust ratio, $N$(H$_{\\rm\ntot}$)/$E(B-V)$, based on direct measurements of $N$(H$_2$) and $N$(H I).\nAlthough our value is nearly identical to that found with Copernicus data, it\nextends the relationship by a factor of 2 in reddening. Finally, as the new\nlines of sight generally show low to moderate molecular fractions, we still\nfind little evidence for single monolithic "translucent clouds" with $f_{\\rm\nH2}$ $\\sim$ 1.\n