  The structure of the dusty circumstellar envelopes (CSEs) of proto-planetary
nebulae (PPNs) reveals the mass-loss history of these sources and how such
histories may differ for elliptical (SOLE) and bipolar (DUPLEX) PPNs. To study
the PPN structures via dust-scattered linearly polarized starlight, we have
compiled the imaging-polarimetric data for all 18 evolved stars that have 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 stars. The new perspective
gained from the imaging-polarimetric data has revealed several new aspects to
the structures of PPNs. Point-symmetry is a prevalent imaging-polarimetric
characteristic resulting from the azimuthal density gradient in the CSEs. Among
these point-symmetric nebulae, three detailed morphological types can be
differentiated by their polarized intensity, I_pol, 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 gradient
reversal in optically thinner elliptical nebulae. The equatorial plane of the
system defined by the integrated angle of polarization is not necessarily
orthogonal to the axis of the apparent bipolar structure in the total intensity
data.
