Polarimetry of 53 post-AGB stars

A statistical analysis shows four distinctive groups of polarized post-AGB stars: unpolarized or very lowly polarized (degree of polarization or DoP<1%), lowly polarized (1%<DoP<4%), moderately polarized (4%<DoP<8%) and highly polarized (DoP>8%). 23 out of the 53 (66%) belong to the first group, 10 (19%) to the second, five ( %) to the third and only three (6%) to the last group.

Approximately 34%of our sample was found to be unpolarized objects, which is close to the percentage of round planetary nebulae. On average, the low and moderate groups show a wavelength-dependent polarization that increases towards shorter wavelengths, implying an intrinsic origin of the polarization, which signifies a Rayleigh-like scattering spectrum typical for non-symmetrical envelopes composed principally of small dust grains.