Mid-infrared imaging of the bipolar nebulae AFGL 618, AFGL 2688, and AFGL 915

We have obtained nearly diffraction-limited images of three bipolar proto-planetary nebulae: AFGL 618, AFGL 2688 (the ''Egg nebula''), and AFGL 915 (the ''Red Rectangle''), Images were taken at unidentified infrared (UIR) emission feature wavelengths and at several continuum wavelengths in the 10 and 20 mu m atmospheric windows. In all three nebulae the emission is dominated by a central point source, In AFGL 618, evidence for a slight (0.1 ''-0.2 '') extension of the core in the E-W direction is seen. In AFGL 2688 and AFGL 915, the cores are extended and 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 to follow the optical and near-IR bicone morphology. The ''spikes'' that have been observed at 2 mu m and give the nebula its rectangular appearance are also visible at 10 mu m. The UIR feature emission is spatially 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-infrared 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 presumably is collimating the flow. (C) 1996 American Astronomical Society.