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Session 15 - Interstellar Medium I.
Display session, Monday, January 15
North Banquet Hall, Convention Center
In March 1992, the Far-Ultraviolet Space Telescope (FAUST) was used to obtain measurements of the intensity of the diffuse sky background at intermediate and high galactic latitudes (Bowyer et al.\ 1993 ApJ, 415, 875; Sasseen amp; Deharveng 1995 ApJ, in press). A large fraction of the total detected background was found to be of galactic origin, resulting from scattering by interstellar dust. We have used a new Monte-Carlo radiative transfer model, representing a cloudy galaxy with a realistic radiation field based on the observed locations and UV fluxes for some 58,000 stars, to severely constrain possible values for the dust albedo and the scattering phase function of dust grains present in the diffuse interstellar medium. The observed correlations of the measured background intensity with cosec |b|, N(H I), and the IRAS 100 \mum galactic background provided semi-independent constraints for the model. The best solution, which is based on the combined analysis of 14 separate 4\deg by 4\deg fields, and which applies to the effective wavelength of 1600 Åyields most likely values for the dust albedo a = 0.43 \pm 0.05, the phase function asymmetry g = 0.68 \pm 0.10, and the sum of average airglow and extragalactic background of 700 \pm 50 photons cm^-2 s^-1 Åsr^-1. A separate constraint on the extragalactic background alone was not possible in this analysis. While the phase function asymmetry found in this study of diffuse dust is in very close agreement with that observed in dense reflection nebulae and star forming regions, the UV dust albedo in the diffuse medium appears to be about 50% lower than that found in dense clouds.
