Computer Science
Scientific paper
Apr 2002
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002phdt........28b&link_type=abstract
Thesis (PhD). THE JOHNS HOPKINS UNIVERSITY, Source DAI-B 62/10, p. 4585, Apr 2002, 150 pages.
Computer Science
Scientific paper
The far-ultraviolet absorption and scattering properties of interstellar dust are not well determined, yet they are responsible for the regulation of the penetration of far-UV light into interstellar clouds. This energetic radiation can ionize atomic species and photodissociate molecules, and thus plays an important role in the ionization, chemical, and thermal balances of the interstellar medium. Two methods for determining the properties of dust are the search for correlations between dust extinction and chemical tracers along various sightlines, and the measurement of the brightness of scattered light in reflection nebulae. This dissertation presents the results from two such studies. A sample of 59 sightlines to reddened Galactic OB stars was examined for the correlations of the strength of the CO Fourth Positive (A 1Π - X1Σ +) absorption band system with the morphology of the ultraviolet interstellar extinction curve. The CO absorption was measured in archival high-dispersion IUE spectra and compared to parametric fits of the extinction curves from the literature. A correlation with the non-linear far-UV curvature term was found with greater absorption, normalized to E( B - V), being associated with stronger curvature. The reflection nebula NGC 2023 was observed by a rocket- borne long-slit imaging spectrograph in the 900 1400 Å bandpass. Through the use of a Monte Carlo modeling process, the scattering properties of the dust were derived. The albedo is low, 0.2 0.4, and decreasing toward shorter wavelengths, while the phase function asymmetry parameter is consistent with highly forward-scattering grains, g ˜ 0.85. The decrease in albedo, while the optical depth increases to shorter wavelengths, implies that the far-UV rise in the extinction curve is due to an increase in absorption efficiency.
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