AAS Meeting #193 - Austin, Texas, January 1999
Session 65. Interstellar Dust and Gas
Display, Friday, January 8, 1999, 9:20am-6:30pm, Exhibit Hall 1

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[65.14] The Perseus/B5 Molecular Cloud Halo: Pressure, Temperature and Composition

P. G. Wannier (JPL/Caltech), B-G Andersson (Johns Hopkins University), S. L. Federman (University of Toledo), B. E. Penprase (Pomona College)

We have used the GHRS instrument on the HST to observe interstellar C I and CO, in order to derive the properties of the gas in the neutral envelope surrounding the the Perseus/B5 molecular cloud. The gas pressure is tightly constrained by our observations, yielding a value of 2200 K cm-3, somewhat higher than in the surrounding ISM. We also present estimates of the kinetic temperature, based on complementary UV observations of H2, and optical observations of C2, yielding values of 20-60K in the vicinity of B5. Together with the pressure determination, these modest kinetic temperatures yield a local gas density of 35-100 cm-3, very close to that which would be implied by the observed H2 column density and assuming a filling factor of unity. During the course of our analysis of the GHRS CO data, we found that the rotational ladder of CO is more likely to be radiatively excited than collisionally excited, both in our sightlines and in others previously reported in the literature. These prior analyses have therefore led to a general over-estimate of the density of the cool atomic and molecular medium. The combination of temperature, pressure and filling factor suggest that the diffuse gas around the Perseus/B5 cloud may be part of a general outflow from the dense molecular cloud

The author(s) of this abstract have provided an email address for comments about the abstract: peter@wannier.jpl.nasa.gov

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