ORFEUS-I Observations of Molecular Hydrogen in the Galactic Disk

We present measurements of interstellar H2 absorption lines in the continuum spectra of seven early-type stars in the Galactic disk at distances between 1 and 4 kpc. Five of these stars provide lines of sight through the Sagittarius spiral arm. The spectra, obtained with the Berkeley EUV/FUV spectrometer on the ORFEUS telescope in 1993 September, have a resolution of 3000 and statistical signal-to-noise ratios between 20 and 80. We determine column densities for each observed rotational level and derive excitation temperatures and densities for the H2 clouds along each line of sight. Our data continue the relationships among H2 column density, fractional molecular abundance, and reddening apparent in Copernicus observations of nearby stars, indicating a common mechanism for H2 production. Estimates of cloud temperatures and densities are consistent with those derived from Copernicus data. The molecular fraction f is nearly constant over a wide range of distances and mean reddenings, consistent with a model in which a significant fraction of the neutral ISM is associated with H2-bearing molecular clouds, even along low-density lines of sight.