A NEW NEAR-INFRARED IMAGING STUDY OF AFGL-2688

The object AFGL 2688 is a bipolar nebula surrounding a post-asymptotic giant branch star and is in transition toward becoming a planetary ne bula. We present new near-infrared images in the I, J, H, and K bandpa sses. These images reveal a wealth of structure in the nebula not prev iously seen. We find that the northern lobe is not smooth, but clumpy. There is evidence in the I-band image for periodic variations in mass -loss rate. In addition, we present narrowband images centered on the v = 1 --> 0 S ( 1 ) line of molecular hydrogen (lambda = 2.121 mum) an d nearby continuum, obtained with a new variable-wavelength narrowband -filter mechanism. The narrowband images reveal that broadband flux fo und in the equatorial region is dominated by line emission from vibrat ionally excited molecular hydrogen. H-2 emission is also present in th e lobes. Our K-band images show the equatorial emission to be in the f orm of a ring or torus extending around the object at a distance R app roximately 7 X 10(16) (D/1 kpc) cm. Mechanisms for exciting the equato rial H-2 emission are discussed; we conclude that shocks are the only plausible source. We have analyzed the broadband images using models w hich assume single scattering of photons from the central star. We fin d that the bipolar axis is inclined to the plane of the sky by i appro ximately 5-degrees. The images are best reproduced by models in which the density of scattering dust decreases fairly rapidly with increasin g stellar latitude, with little or no dust near the poles. Furthermore , the optical depth is found to have very little wavelength dependence , which implies that the dust in AFGL 2688 is different from that in t he interstellar medium. This result is consistent with polarimetric st udies, which find that the scattering grains are likely to be large gr aphite particles.