Physics
Scientific paper
Sep 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001phdt.........6l&link_type=abstract
Thesis (PhD). QUEEN'S UNIVERSITY OF BELFAST (NORTHERN IRELAND), Source DAI-C 62/04, p. 586, Winter 2001, 188 pages.
Physics
3
Scientific paper
In order to constrain the physical structure, the ionization and dust composition of the low density gas, this thesis presents the results of optical, ultraviolet and radio observations towards different sightlines in the Galactic halo and the Magellanic Bridge. The Galactic halo is studied both on small spatial and velocity scales towards the M15 globular cluster and Galactic halo B-type stars, respectively. The former is found to be clumpy, and the latter indicates clouds separated by only a few km s-1 . Additional tentative distance estimates are presented towards 15 field stars to obtain the distance to the high-velocity cloud. With the space-based HST telescope, a sightline towards the Magellanic Bridge shows few high-velocity clouds and the first detection in ultraviolet absorption of the Magellanic Bridge gas. Results from relative abundance determinations, in particular from CII and OI, and the presence of highly ionized elements, suggest that the high-velocity cloud HVC 291.5-41.2 + 80 consists of a warm, mainly ionized medium. Relative gas phase abundances for the Magellanic Bridge are similar to those found in the Galactic halo, despite a significantly lower metallicity and different dust-to-gas ratio in the Magellanic Bridge. The higher ionization species in the cloud have an abundance ratio N(C+3)/N(Si +3) ~ 1.9, similar to that inferred for collisional ionized Galactic cloud interfaces at temperatures ~ 105 K. The resolution of the data allows us to identify a cloud structure in the stronger lines, with a broad component (FWHM ~20 km s-1) at ~ 179 km s-1, consisting of a warm, low density, partially ionized (50 to 70%) gas, and a sharp component (FWHM ~ 11 km s -1) at 198 km s-1, consisting of a warm, low density, mainly neutral gas. Finally, using very high resolution ( R ~ 160000) spectroscopic observations, this thesis presents a LTE and non-LTE analysis of the sharpest main-sequence B-type star, HD 83206, discovered to date. Because it has very sharp metal lines, this star affords an opportunity to test theories of model atmospheres and line formation.
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