A Survey of Interstellar Molecular Hydrogen with the Berkeley Extreme and Far-Ultraviolet Spectrometer on the ORFEUS-SPAS I and II Missions

Details A Survey of Interstellar Molecular Hydrogen with the Berkeley Extreme and Far-Ultraviolet Spectrometer on the ORFEUS-SPAS I and II Missions A Survey of Interstellar Molecular Hydrogen with the Berkeley Extreme and Far-Ultraviolet Spectrometer on the ORFEUS-SPAS I and II Missions

Dec 1999

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999aas...195.7303d&link_type=abstract

American Astronomical Society, 195th AAS Meeting, #73.03; Bulletin of the American Astronomical Society, Vol. 31, p.1476

Astronomy and Astrophysics

Astronomy

Scientific paper

We present measurements of interstellar H2 absorption lines in the continuum spectra of 54 early-type stars in the Galactic disk and halo and 3 stars in the Magellanic Clouds. The data were obtained with the Berkeley Extreme and Far-Ultraviolet Spectrometer (BEFS), part of the ORFEUS telescope, which flew on the ORFEUS-SPAS I and II space-shuttle missions in 1993 and 1996, respectively. The spectra extend from the interstellar cutoff at 912 Angstroms to about 1200 Angstroms with a resolution of 3000 and statistical signal-to-noise ratios between 10 and 65. Assuming a velocity profile derived from optical observations (when available), we model the column densities N(J" ) of the rotational levels J" = 0 through 5 for each line of sight. Our data reproduce the relationships among molecular and total hydrogen column density, fractional molecular abundance, and reddening first seen in Copernicus observations of nearby stars (Savage et al. 1977), indicating a common mechanism of H2 production throughout the Galactic disk and halo. Our halo sight lines (25 stars with |z| > 500 pc) probe a heretofore unexplored region of parameter space: their mean densities N(H 1+H2)/r are, on average, nearly an order of magnitude lower than those of our disk sample and lower than any sight line available to Copernicus. One early result: log [N(4) / N(0)], which reflects the strength of the ambient UV radiation field, shows a linear correlation with log f, the fractional molecular abundance, over nearly 5 orders of magnitude. This work is supported by NASA grant NAG5-696.

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