ORFEUS-I Observations of Molecular Hydrogen in the Galactic Disk

Details ORFEUS-I Observations of Molecular Hydrogen in the Galactic Disk ORFEUS-I Observations of Molecular Hydrogen in the Galactic Disk

1997-09-03

arxiv.org/abs/astro-ph/9709027v1

Astrophys.J. 492 (1998) 569

Astronomy and Astrophysics

Astrophysics

LaTeX file, 19 pages, 6 encapsulated PostScript figures. Uses aaspp4.sty and astrobib.sty. (Astrobib is available from http:

Scientific paper

10.1007/BFb0104730

We present measurements of interstellar H_2 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 H_2 clouds along each line of sight. Our data continue the relationships among H_2 column density, fractional molecular abundance, and reddening apparent in Copernicus observations of nearby stars, indicating a common mechanism for H_2 production. Estimates of cloud temperatures and densities are consistent with those derived from Copernicus data. We find that the molecular fraction of hydrogen 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 H_2-bearing molecular clouds, even along low-density lines of sight.

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