Astronomy and Astrophysics – Astronomy
Scientific paper
Sep 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992pasp..104..857a&link_type=abstract
Publications of the Astronomical Society of the Pacific, v.104, p.857
Astronomy and Astrophysics
Astronomy
Scientific paper
This thesis presents a combined observational and theoretical study of the halos of molecular clouds. It consists of five parts. In parts 1 and 2 we present and interpret a study of H I 21-cm and CO (J=1-0) emission through which we show that molecular clouds generally have extended, heated H I halos, in addition to local warming of the CO at the edge of the molecular cloud. It is suggested that the heating mechanism is due to UV light through the photoelectric effect. A detailed study, in part 3, shows the B5 molecular cloud to be surrounded by an almost complete H I halo. The velocity structure of the H I emission, as compared to the molecular cloud, seems to indicate that the halo is expanding away from the cloud. In the fourth part of the thesis we have performed an observational study of OH emission in the transitional zone, and find the OH emission to be exnteded significantly relative to the observed CO. In combination with the modeling presented in the fifth part of the thesis we show how the OH abundance can be explained by the modeled halo structures. A physical model is used in conjunction with a chemistry code. The observed extents of the H I envelopes imply that the generally accepted rate for H2 formation on grains is too large. The OH in the halos is produced through the H+ route made possible by the increased abundance of hydrogen and elevated temperatures close to the cloud. An analysis is given of the distribution of the halos in the galaxy. We conclude that the warm neutral halos are not gravitationally bound to the underlying molecular clouds and are probably isobaric. The vertical pressue distribution of the halo gas above the galactic plane follows the e^-(z/h)^2 distribution expected for a gas in hydrostatic equilibrium. The pressure is larger than that inferred for the diffuse atomic gas, and the scale height is that of the molecular, rather than the atomic, medium. Thus they are likely to be expanding into the diffuse gas. This conclusion is in accord with the observations of the B5 cloud. (SECTION: Dissertation Abstracts)
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