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

Abstract

Abstract We report results from a Far Ultraviolet Spectrographic Explorer (FUSE) survey of interstellar molecular hydrogen (H2) in the Galactic disk toward 139 O-type and early B-type stars at Galactic latitudes ∣b∣ ≤ 10°, with updated photometric and parallax distances. H2 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 + 2N H2 and the molecular fraction f H2 = 2N H2/N H. Our survey extends the 1977 Copernicus H2 survey up to N H ≈ 5 × 1021 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 ≳ 1021 cm−2 and E(B − V) ≳ 0.2, interpreted with an analytic model of H2 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 H2 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 × 1021 cm−2 mag−1.