Imaging and Spatially Resolved Spectroscopy of AFGL 2688 in the Thermal Infrared Region

Abstract

We present ground-based high-resolution (~0".3) imaging of AFGL 2688 at L' (3.8 um) and M'(4.7um). A wealth of structure in the central region is revealed due to less extinction in the thermal infrared. A clear border in the southern lobe at L' corresponds to the edge of the heavily obscured region in visible, indicating there is a dense material surrounding the central region. The images also show a narrow dark lane oriented to 140 deg east of north with the normal at 50 deg. The normal position angle is inconsistent with the optical polar axis (PA = 15 deg), but is aligned to the high-velocity CO components found in the radio wavelength observations. The central star remains invisible at L' and M'. Several clumpy regions in the north lobe dominate in L' and M' luminosity. In particular a pointlike source (peak A) at 0".5 northeast of the center of the nebula exhibits the highest surface brightness with a very red spectral energy distribution (SED). Based on the almost identical SED as adjacent regions, we suggest that the pointlike source is not self-luminous, as was proposed, but is a dense dusty blob reflecting thermal emission from the central star. We also present spatially resolved slit spectroscopy of the bright dusty blobs. An emission feature at 3.4 um as well as at 3.3 um is detected everywhere within our field of view. There is no spatial variation in the infrared emission feature (IEF) throughout the observed area (0".2-1".5, or 240-1800 AU from the central source). The constant flux ratio of the emission feature relative to the continuum is consistent with the view that the blobs are mostly reflecting the light from the central star in the 3 um region.