ULTRAVIOLET IMAGING TELESCOPE IMAGES OF THE REFLECTION NEBULA NGC-7023 - DERIVATION OF ULTRAVIOLET SCATTERING PROPERTIES OF DUST GRAINS

The Ultraviolet Imaging Telescope, as part of the Astro-1 mission, was used to obtain high-resolution surface brightness distribution data in six ultraviolet wavelength bands for the bright reflection nebula NGC 7023. From the quantitative comparison of the measured surface brightness gradients, ratios of nebular to stellar flux, and detailed radial surface brightness profiles with corresponding data from the visible, two major conclusions result: (1) the scattering in the near- and far-ultraviolet in this nebula is more strongly forward-directed than in the visible; (2) the dust albedo in the ultraviolet for lambda greater-than-or-equal-to 140 nm is identical to that in the visible, with the exception of the 220 nm bump in the extinction curve. In the wavelength region of the bump, the albedo is reduced by 25% to 30% in comparison with wavelength regions both shorter and longer. This lower albedo is expected, if the bump is a pure absorption feature.