The Perseus B5 molecular cloud halo: measurements of pressure, temperature, and composition.
WANNIER P., ANDERSSON B.-G., PENPRASE B.E. and FEDERMAN S.R.
Abstract (from CDS):
We present GHRS observations of interstellar C I and CO toward three stars located in the direction of the Perseus region near the B5 molecular cloud and discuss these together with published results from a fourth sight line. The column densities of the three fine-structure levels of C I are used together with ground-based optical data and published ultraviolet data to yield several properties of the gas along the stellar sight lines. The selected stars probe gas located in the neutral envelope surrounding the dense molecular cloud. The gas pressure is tightly constrained by our observations, yielding a value of ~2200 K.cm–3 in the vicinity of B5, somewhat higher than in the surrounding interstellar medium. We also present estimates of the kinetic temperature, based on observations of H2 and C2, yielding 20-60 K in the vicinity of B5. Together with the pressure determination, this implies a local gas density of 35-100 cm–3, very close to that which would be implied by the observed H2 column density assuming a filling factor of unity. This result provides additional support to the notion that the gas is part of an extensive, uniform gaseous halo surrounding the B5 cloud. The combination of temperature and pressure suggests that the diffuse gas may be part of a general outflow from the dense molecular cloud.