A Far Ultraviolet Spectroscopic Explorer Survey of Interstellar Molecular Hydrogen in the Galactic Disk

Abstract

We report results from a Far Ultraviolet Spectrographic Explorer (FUSE) survey of interstellar molecular hydrogen (H 2 ) in the Galactic disk toward 139 O-type and early B-type stars at Galactic latitudes ∣ b ∣ ≤ 10°, with updated photometric and parallax distances. H 2 absorption is measured using the far-UV Lyman and Werner bands, including strong R(0), R(1), and P(1) lines from rotational levels J = 0 and J = 1 and excited states up to J = 5 (sometimes J = 6 and 7). For each sight line, we report column densities N H2 , N HI , N ( J ), and N H = N HI + 2 N H2 and the molecular fraction f H2 = 2 N H2 / N H . Our survey extends the 1977 Copernicus H 2 survey up to N H ≈ 5 × 10 21 cm −2 . The lowest rotational states have excitation temperatures and rms dispersions 〈 T 01 〉 = 88 ± 20 K and 〈 T 02 〉 = 77 ± 18 K, suggesting that J = 0, 1, 2 are coupled to the gas kinetic temperature. Populations of higher- J states exhibit mean excitation temperatures 〈 T 24 〉 = 237 ± 91 K and 〈 T 35 〉 = 304 ± 108 K, produced primarily by UV radiative pumping. Correlations of f H2 with E ( B − V ) and N H show a transition to f H2 ≥ 0.1 at N H ≳ 10 21 cm −2 and E ( B − V ) ≳ 0.2, interpreted with an analytic model of H 2 formation–dissociation equilibrium and attenuation of the far-UV radiation field by self-shielding and dust opacity. Results of this disk survey are compared to previous FUSE studies of H 2 in translucent clouds, at high Galactic latitudes, and in the Magellanic Clouds. Using updated distances to the target stars, we find average sight-line values 〈 f H2 〉 = 0.20 and 〈 N H / E ( B − V )〉 = 6.07 × 10 21 cm −2 mag −1 .