Astronomy and Astrophysics – Astronomy
Scientific paper
Dec 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992aas...181.9302a&link_type=abstract
American Astronomical Society, 181st AAS Meeting, #93.02D; Bulletin of the American Astronomical Society, Vol. 24, p.1268
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 HI 21cm and CO (J=1-0) emission through which we show that molecular clouds generally have extended, heated HI 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 through the photo-electric effect. A detailed study shows the B5 molecular cloud to be surrounded by an almost complete HI halo. The velocity structure of the HI emission, as compared to the molecular cloud seem to indicate the 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 transition zone finding the OH emission to be significantly extended 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 extent of the HI 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 atomic 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 isobaric. The vertical pressure 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.
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