Hubble Space Telescope NICMOS Imaging
Polarimetry of Proto-Planetary Nebulae. II. Macromorphology of the Dust
Shell Structure via Polarized Light
Ueta, Toshiya; Murakawa, Koji; Meixner,
Margaret
The Astronomical Journal, Volume 133, Issue 4, pp.
1345-1360
Abstract
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. 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, 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 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..
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