Abstract
Deep Hubble Space Telescope (HST) F814W images were obtained of six proto–planetary nebulae (PPNs), each of which shows a bipolar morphology but with a difference in orientation. Two of these (IRAS 17245-3951 and IRAS 22574+6609) show a dark lane separating bipolar lobes and appear to be seen approximately edge-on. The other four (IRAS 16594-4656, 17106-3046, 19477+2401, and 20028+3910) appear to be at intermediate orientations, and in all but one case, the central star is seen. In addition, one bright PPN (IRAS 20136+1309) was observed that is slightly extended, and we suggest that it may be a bipolar nebula seen edge-on. Visible-band HST images also exist for six of these, and color images were formed to analyze the dust distribution. New ground-based photometry was combined with satellite data to delineate the spectral energy distribution (SED) from 0.5 to 100 μm for each of these. The orientation effects on the optical morphologies and the SEDs are discussed. In general, the ratio of dust to photospheric flux is higher as the orientation increases toward edge-on, although there are some exceptions. Some numerical models were constructed and used to show quantitatively the effects that differences in the asymmetry of the circumstellar envelope or in the size of the cavity opening angle can have on this ratio. The general differences in appearance and SED of these PPNs are attributed primarily to the different viewing orientations. These results, when combined with those of previous imaging studies of PPNs, strengthen the idea that PPNs possess a basic bipolar structure due to an asymmetric circumstellar dust shell.
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