[ITAL]Infrared Space Observatory[/ITAL] Polarimetric Imaging of the Egg Nebula (RAFGL 2688)

We present polarimetric imaging of the protoplanetary nebula RAFGL 2688obtained at 4.5 microns with the Infrared Space Observatory (ISO). We havedeconvolved the images to remove the signature of the point spread function ofthe ISO telescope, to the extent possible. The deconvolved 4.5 micron image andpolarimetric map reveal a bright point source with faint, surroundingreflection nebulosity. The reflection nebula is brightest to thenorth-northeast, in agreement with previous ground- and space-based infraredimaging. Comparison with previous near-infrared polarimetric imaging suggeststhat the polarization of starlight induced by the dust grains in RAFGL 2688 ismore or less independent of wavelength between 2 microns and 4.5 microns. This,in turn, indicates that scattering dominates over thermal emission atwavelengths as long as ~5 microns, and that the dust grains have characteristicradii < 1 micron.Comment: 27 pages, 9 figures; to appear in the Astronomical Journal, May 2002 issu