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R. Sahai (JPL/ Caltech), A. Dayal (IPAC/ Caltech), S. Kwok, K.Y.L. Su (Univ. of Calgary), B.J. Hrivnak (Valparaiso Univ.)
As part of a comprehensive study of the origins of aspherical structure in protoplanetary (PPN) and planetary nebulae (PN), we report here high-resolution H (F160W) and K (F222M) images of 3 bipolar PPN, Roberts 22, Hen 401, and the Boomerang Nebula, obtained with NICMOS/HST. All 3 objects have substantial cold, dense molecular envelopes seen in mm-wave CO lines. Amongst these, the Boomerang Nebula is specially noteworthy for harboring the coldest (~1K) gas in the Universe (Sahai & Nyman 1997, ApJ, 487, L1).
The Hubble images reveal a rich and complex morphological structure of the circumstellar material for each of these objects. The morphology of our PN at H and K-band is found to be very similar to that seen at optical wavelengths (0.6\mum) with WFPC2, supporting our view that we are seeing scattered light from dust at all 3 wavelengths in these images. Although all 3 PPN are bipolar, their morphologies differ substantially in detail. The bipolar lobes in Hen 401 resemble very long, thin-walled cylinders, separated by a darker waist. The lobes in Roberts 22 are more squat, and characterised by a remarkable degree of point-symmetry. The waist region is significantly more clumpy than in Hen 401, presumably due to disintegration by fast collimated outflows (Sahai et al 1999, AJ, in press). Two bright compact clumps are seen in the vicinity of the waist, which are redder than the surrounding nebulosity. The bipolar lobes in the Boomerang (which extend far beyond the NICMOS 20'' field-of-view) have a ``figure-8'' morphology, and show the presence of partial shells and filaments embedded within a diffuse component. Roberts 22 and Hen 401 were also observed in the narrow-band F212N and F215N filter to isolate the H2 v=0, S(1) 2.1\mum line emission, which typically traces regions of shocked gas in PPN. The H2 emission is found to be similar to the distribution of scattered light in the bright lobes, consistent with the hypothesis that the lobes are overdense shocked structures produced by the expansion of fast outflow(s) within the dense progenitor AGB envelope. We discuss the structures of these 3 PPN in the context of current theories for the formation and shaping of PN.
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