HST/NICMOS IMAGING POLARIMETRY OF PROTO-PLANETARY NEBULAE II: MACRO-MORPHOLOGY OF THE DUST SHELL STRUCTURE VIA POLARIZEDLIGHT

The structure of the dusty circumstellar envelopes (CSEs) o f proto-planetary nebulae (PPNs) reveals the mass-loss history of these sources and how such histories ma y differ for elliptical (SOLE) and bipolar (DUPLEX) PPNs. To study the PPN structures via dust-scattered l in arly polarized starlight, we have compiled the imaging-polarimetric data for all 18 evolved stars that hav e been obtained to date with NICMOS on-board the Hubble Space Telescope (HST). This alternative imaging technique provides a unique way to probe the distribution of dust grains that scatter light around evolved s tars. The new perspective gained from the imagingpolarimetric data has revealed several new aspects to the st ructures of PPNs. Point-symmetry is a prevalent imaging-polarimetric characteristic resulting from the a zimuthal density gradient in the CSEs. Among these point-symmetric nebulae, three detailed morphological ty pes can be differentiated by their polarized intensity, Ip, and polarization strength, P. While the azimuthal density gradient is reversed above and below the equatorial plane in optically thicker bipolar nebulae, there is no grad ient reversal in optically thinner elliptical nebulae. The equatorial plane of the system defined by the integrated a ngle of polarization is not necessarily orthogonal to the axis of the apparent bipolar structure in the total int ensity data. Subject headings: circumstellar matter — stars: AGB and post-AGB — stars: indi vidual (IRAS z02229+6208, IRAS 04296+3429, IRAS 04395 +3601, IRAS 06530 −0213, IRAS 07134 +1005, IRAS 09452+1330, IRAS 10131 +3049, IRAS 10178 −5958, IRAS 10197 −5750, IRAS 12419−5414, IRAS 16594 −4656, IRAS 17150 −3224, IRAS 17245 −3951, IRAS 17423−1755, IRAS 17441 −2411, IRAS 19343 +2926, AFGL 2688) — stars: mass loss — reflection nebulae