ORFEUS-I OBSERVATIONS OF MOLECULAR-HYDROGEN IN THE GALACTIC DISK

We present measurements of interstellar H-2 absorption lines in the co ntinuum spectra of seven early-type stars in the Galactic disk at dist ances between 1 and 4 kpc. Five of these stars provide lines of sight through the Sagittarius spiral arm. The spectra, obtained with the Ber keley EUV/FUV spectrometer on the ORFEUS telescope in 1993 September, have a resolution of 3000 and statistical signal-to-noise ratios betwe en 20 and 80. We determine column densities for each observed rotation al level and derive excitation temperatures and densities for the H-2 clouds along each line of sight. Our data continue the relationships a mong H-2 column density, fractional molecular abundance, and reddening apparent in Copernicus observations of nearby stars, indicating a com mon mechanism for H-2 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 f raction of the neutral ISM is associated with H-2-bearing molecular cl ouds, even along low-density lines of sight.