The Distribution of Interstellar Molecular Hydrogen in the Galactic Disk and Halo

Details The Distribution of Interstellar Molecular Hydrogen in the Galactic Disk and Halo The Distribution of Interstellar Molecular Hydrogen in the Galactic Disk and Halo

Dec 2001

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001aas...199.1108d&link_type=abstract

American Astronomical Society, 199th AAS Meeting, #11.08; Bulletin of the American Astronomical Society, Vol. 33, p.1324

Astronomy and Astrophysics

Astronomy

Scientific paper

Dixon et al. (BAAS, 31, 1476, 1999) derive molecular-hydrogen column densities from the spectra of 57 early-type stars observed 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. We have analyzed their results, investigating the distribution of molecular hydrogen with |z|, the distance from the Galactic plane. We find that the majority of interstellar H2 clouds follow an exponential density distribution with a scale height of 50 +/- 20 pc. Toward stars with very low projected H2 column densities, we find evidence for a second population of low-density molecular clouds for which n(H2) increases linearly with |z|. If not an artifact of the small sample size or a selection effect, this density profile suggests either an increase in the rate of molecular formation or a decrease in the rate of molecular destruction in these clouds with distance from the plane. We consider a model in which a decrease in the rate of molecular destruction results from a drop in the Galactic FUV background with |z| and show that it can qualitatively reproduce the observed distribution of cloud densities. This research is supported by NASA grant NAG5-696.

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