HOPKINS ULTRAVIOLET TELESCOPE OBSERVATIONS OF FAR-ULTRAVIOLET SCATTERING IN NGC-7023 - THE DUST ALBEDO

We have obtained the first sub-Lyalpha spectroscopic observations of the reflection nebula NGC 7023 and its illuminating star, HD 200775, using the Hopkins Ultraviolet Telescope during the Astro-1 mission in 1990 December. The ratio of the nebular to stellar flux is virtually flat between 1100 and 1860 angstrom, indicating that sigma(a), the cross section for absorption, must rise sharply with decreasing wavelength. Independent of any model, this means that much of the far-ultraviolet rise in the extinction curve is due to an increase in absorption rather than scattering. If, in addition, we assume a spherical geometry, we derive an albedo of 0.5 at 1100 angstrom with somewhat higher values at longer wavelengths. If the geometry is not spherical, lower values of the albedo may be obtained.