H-ALPHA IMAGING POLARIMETRY OF THE PROTOPLANETARY NEBULA M2-9

After correction for interstellar polarization, our Halpha polarizatio n map of die protoplanetary nebula M2-9 shows that a significant fract ion of the emission-line radiation from the nebular lobes is scattered radiation originating in the central core. We find no evidence for a spatially extended illuminating source on scales of greater-than-or-eq ual-to 1-2 arcsec. In order to explain our polarization results, we pr esent a geometrical model for the system which has diametrically oppos ed cavities previously created in a preexisting dusty envelope. The il lumination of the cavity walls leads to the reflection nebula properti es of M2-9. We find that the high-excitation emission-line knots, supp osedly undergoing regular rotation about the major axis on a time-scal e of decades, have not moved significantly since 1977. This, taken in conjunction with our observation of the absence of concentrations of s cattering material at the locations of the knots, suggests that the kn ots arise from the sporadic illumination of the cavity walls by ionizi ng radiation from the central region.