A Far Ultraviolet Spectroscopic Explorer Survey of Interstellar
  Molecular Hydrogen in the Galactic Disk
  We report results from a FUSE survey of interstellar molecular hydrogen (H2)
in the Galactic disk toward 139 O-type and early B-type stars at Galactic
latitudes $|b| < 10^{\circ}$, with updated photometric and parallax distances.
The H2 absorption is measured using the far-ultraviolet 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 $N_{H2}$, $N_{HI}$, $N(J)$, $N_H =
N_{HI} + 2N_{H2}$, and molecular fraction, $f_{H2} = 2N_{H2}/N_H$. Our survey
extends the 1977 Copernicus H2 survey up to $N_H \sim 5\times10^{21}$
cm$^{-2}$. The lowest rotational states have mean excitation temperatures and
rms dispersions, $T_{01} = 88\pm 20$ K and $T_{02} = 77\pm18$ K, suggesting
that J = 0,1,2 are coupled to the gas kinetic temperature. Populations of
higher-J states exhibit mean excitation temperatures, $T_{24} = 237\pm91$ K and
$T_{35} = 304\pm108$ K, produced primarily by UV radiative pumping.
Correlations of $f_{H2}$ with E(B-V) and N_H show a transition to $f_{H2} \geq
0.1$ at $N_ H \geq 10^{21}$ 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 $\langle f_{H2} \rangle
\geq 0.20$ and $\langle N_H/E(B-V) \rangle = (6.07\pm1.01)\times10^{21}$
cm$^{-2}$ mag$^{-1}$.
