Abstract
Using NICMOS on the Hubble Space Telescope, we have performed imaging polarimetry of proto–planetary nebulae (PPNs). Our objective is to study the structure of optically thin circumstellar shells of post–asymptotic giant branch (AGB) stars by separating dust-scattered, linearly polarized starlight from unpolarized direct starlight. This unique technique allows us to probe faint reflection nebulae around the bright central star, which can be buried under the point-spread function of the central star in conventional imaging. Our observations and archival search have yielded polarimetric images for five sources: IRAS 07134+1005 (HD 56126), 06530-0213, 04296+3429, (Z)02229+6208, and 16594-4656. These images have revealed the circumstellar dust distribution in unprecedented detail via polarized intensity maps, providing a basis for understanding the three-dimensional structure of these dust shells. We have observationally confirmed the presence of the inner cavity caused by the cessation of AGB mass loss, and the internal shell structures, which are strongly tied to the progenitor star's mass-loss history on the AGB. We have also found that equatorial enhancement in these circumstellar shells comes with various degrees of contrast, suggesting a range of optical depths in these optically thin shells. Our data support the interpretation that the dichotomy of PPN morphologies is due primarily to differences in optical depth and secondarily to the inclination effect. The polarization maps reveal a range of inclination angles for these optically thin reflection nebulae, dispelling the notion that elliptical nebulae are pole-on bipolar nebulae.
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