The association of high-latitude molecular clouds with H I gas

Details The association of high-latitude molecular clouds with H I gas The association of high-latitude molecular clouds with H I gas

Oct 1994

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994apj...434..162g&link_type=abstract

Astrophysical Journal, Part 1 (ISSN 0004-637X), vol. 434, no. 1, p. 162-183

Other

32

H I Regions, Interstellar Gas, Milky Way Galaxy, Molecular Clouds, Spatial Distribution, Velocity Distribution, Abundance, Carbon Monoxide, Stellar Winds, Supernovae

Scientific paper

The relationhip between the High Galactic latitude molecular clouds (HCLs) and associated atomic hydrogen gas is investigate. In a global study, we examine a total of 75 HLCs observed in the CO (J=1-0) transition and found that all HCLs are associated with H I gas in both position and velocity with most of the HCLs located along filamentary or looplike H I structures. A small-scale comparison, using Green Bank 43 m H I data toward 10 regions associated with 18 HLCs, shows that the positions of the CO and H I peaks are typically offset from one another by approximately 1.5 deg, comparable to the linear size of the HCLs. No obvious pattern for these CO-H I offsets was found either in relation to the Galactic plane or to an arbitrary explosion center. Typical velocity dispersions of the CO and its associated H I are found to be 0.6 and 3.0 km/s, with a mean H I/CO linewidths ratio of 5.7, which is close to the square root of the ratio of mass of Co/H I possibly indicating energy equipartition between the two phases. The data suggest that the molecular clouds condensed from the atomic gas in situ, rather than having been entrained in the H I. Moreover, the data are consistent with formation from the H I by means of a phase transition in pressure equilibrium. A few cloud complexes are anomalous, perhaps indicating variants in how the clouds formed. We find a minimum total hydrogen column density of 5 x 1020/sq cm and a minimum relative abundance of H2 nucleons of 50% in the 18 clouds we observed. Two of the 10 observed H I clouds, those associated with the molecular clouds MBM 27-29 and MBM 53-55, show clear arclike structures and the latter appear to be expanding with a velocity of about 18 km/s. The kinetic energy of this expanding shell is estimated to be about 4 x 1049 ergs, consistent with having been produced as a result of a supernova explosion or a stellar wind.

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