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Session 16 - Interstellar Grains, PAH's and the Interstellar Bands.
Display session, Monday, January 13
IC 63, located about 20 arcmin NE of the B0.5IVpe star \gamma Cas, is an example of a rare class of reflection nebulae which are externally illuminated and which scatter light within a very narrow range of scattering angles near 90\arcdeg. Thus, IC 63 affords a unique opportunity to examine the wavelength dependence of the large-angle scattering amplitude of interstellar grains and to constrain the shape of the scattering phase function. We used low-resolution IUE spectra of IC 63 in combination with ground-based CCD images as input to a Monte Carlo radiative transfer treatment to derive possible combinations of the dust albedo and the asymmetry of the scattering phase function. Our model consists of a conical nebula with an optically thick tip, which is illuminated by a single source. The model contains the same orientation, dimension, and distance parameters as observed in the IC 63/\gamma Cas system. Since it is not known where IC 63 is located with respect to the plane of the sky, a range of scattering angles between 60\arcdeg and 120\arcdeg was explored. The IUE spectral data were binned into 200 Å\ wide intervals so that the wavelength dependence of the scattering behavior could be examined. For each wavelength interval and for each assumed scattering angle, a monotonic relation between possible values of the dust albedo and the phase function asymmetry could be derived. All results regarding specific scattering properties for dust in IC 63 are therefore conditional. We find, if the dust albedo is as high in IC 63 in the far-UV as that found in other reflection nebulae, the phase function must be highly forward directed. Second, if the phase function asymmetry is approximately constant throughout the UV, the dust albedo displays a minimum coinciding with the position of the 2175 Å\ bump in the extinction curve, confirming the absorption nature of this feature. We acknowledge support from the IUE program and from NASA LTSA grants NAGW3168 and NAG5-3367 to the University of Toledo.
Program listing for Monday