MIDINFRARED IMAGING OF THE BIPOLAR NEBULAE AFGL-618, AFGL-2688, AND AFGL-915

We have obtained nearly diffraction-limited images of three bipolar pr oto-planetary nebulae: AFGL 618, AFGL 2688 (the ''Egg nebula''), and A FGL 915 (the ''Red Rectangle''), Images were taken at unidentified inf rared (UIR) emission feature wavelengths and at several continuum wave lengths in the 10 and 20 mu m atmospheric windows. In all three nebula e the emission is dominated by a central point source, In AFGL 618, ev idence for a slight (0.1 ''-0.2 '') extension of the core in the E-W d irection is seen. In AFGL 2688 and AFGL 915, the cores are extended an d fainter emission is detected several arcsec from the central peaks. In AFGL 2688, the mid-IR emission is extended in the same direction as the main optical and near-IR lobes, In AFGL 915, the nebula is seen t o follow the optical and near-IR bicone morphology. The ''spikes'' tha t have been observed at 2 mu m and give the nebula its rectangular app earance are also visible at 10 mu m. The UIR feature emission is spati ally separate from the central source and is enhanced along the walls of the bicone. In all three nebulae, the direction of largest spatial extension is along the major axis as defined by the optical and near-i nfrared morphology. Therefore the mid-infrared emission appears to be tracing material in the bipolar outflow regions, rather than detecting structure related to an equatorial density enhancement that presumabl y is collimating the flow. (C) 1996 American Astronomical Society.