Imaging polarimetry of proto-planetary and planetary nebulae

Imaging polarimetry maps are presented for a sample of bipolar proto-planetary and planetary nebulae (Frosty Leo, Roberts 22, Hen 401, MZ 3, NGC 2346, IC 4406 and J 320). Each of the highly polarized proto-planetary nebulae possesses a ‘polarization disc’ which has been observed more frequently in nebulae associated with star forming regions. In order to account for the observed high levels of polarization in protoplanetary nebulae we consider the effects of a thin coating of a volatile material on refractory grains with an original size distribution typical of the interstellar medium. The planetary nebulae are seen in a mixture of reflected and emission light and their polarization patterns suggest that, in many instances, they are emission nebulae surrounded by an extensive envelope of reflection nebulosity. The origin of the skew-symmetry and ansae in the isophotal maps of proto-planetary and planetary nebulae are discussed in terms of binary stars and magnetic fields.