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K. Y. L. Su (University of Calgary), B. J. Hrivnak (Valparaiso University), S. Kwok (University of Calgary)
Proto-planetary nebulae (PPN) provide the best opportunity to test the shaping mechanism of planetary nebulae. Here we report the results of our cycle 8 HST/WFPC2 wide-band imaging of 7 PPN. The PPN are viewed at a variety of orientations from pole-on (~0\arcdeg), to intermediate angles (30\arcdeg~60\arcdeg), to edge-on (~90\arcdeg). For the pole-on case, IRAS 20136+1309, both F606W and F814W images show point-source structure, but the FWHM of the object is wider than the field stars, suggesting nebulosity. For the edge-on cases, both IRAS 17245-3951 and IRAS 22574+6609 show bipolar structure with a dark lane separating the two reflection lobes. No central star was detected in our deep F814W images. The other four PPN display intermediate viewing angles. IRAS 17106-3046 and IRAS 20028+3910 show bipolarity with one reflection lobe much brighter than the other one; however, the central star was only detected in IRAS 17106-3036. IRAS 16594-4656 shows approximately point-symmetric structure in our F814W image, similar to that seen in our previous F606W image (Hrivnak et al. 1999). In the most intriguing case, IRAS 19477+2401, our F814W image shows a faint point source associated with clover leaf-like nebulosity. Concentric arc structures similar to those seen in the Egg Nebula, the Cotton Candy Nebula, and IRC+10216 are also detected in our deep F814W images of IRAS 16594-4656 and IRAS 20028+3910.
The existence of bipolar reflection nebulosity observed in the PPN phase suggests the presence of asymmetry in the circumstellar dust shell. A 2-D dust radiation-transfer model is used to fit the SED and the optical morphology of the PPN. We found that the differences in morphology can be explained by orientation effects (Su et al. 1998).
\begin{flushleft} Hrivnak, B. J., Kwok, S. & Su, K. Y. L. 1999, 524, 849. \\
Su, K. Y. L., Volk, K., Kwok, S. & Hrivnak, B. J. 1998, ApJ, 508, 744. \\
\end{flushleft}