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

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.