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W. V. Dixon (JHU), M. Hurwitz (UCB)
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.