Unraveling the structure of aspherical proto-planetary nebulae. I. Hubble Space Telescope imaging and hydroxyl maser line observations of Roberts 22

We have obtained high-resolution wide-band, narrowband, and polarimetric im ages of the bipolar proto-planetary nebula Roberts 22 with the Wide Field P lanetary Camera 2 on the Hubble Space Telescope. OH maser-line emission has also been observed using the Australia Telescope Compact Array. The wide-b and images at 0.6 mu m show bright bipolar lobes shaped Like a butterfly's "wings," separated by a dark equatorial "body" of dense dust that completel y obscures the central star. The material within each lobe appears to be or ganized into an amazingly complex pattern of a very large number of interse cting loops and filaments. The bright lobes are surrounded by a fainter hal o whose inner region contains multiple thin shell structures, reminiscent o f those seen in the prototype proto-planetary nebula, CRL 2688 (Egg Nebula) . The halo can be traced to a distance of about 25" from the nebular center . The radial surface brightness of the halo is inconsistent with that expec ted for time-invariant mass loss at a constant expansion velocity. A simple model of the radial scattered light distribution supports the hypothesis t hat the bright lobes are cavities with thin (<0."5), dense walls that are o ptically thick in scattered light. The amount of mass in the cavity walls i s quite large (0.3 M.) and the dynamical timescale for their formation is s hort (less than or similar to 440 yr). The bright lobes show high fractiona l polarization (40%-50%) and a centrosymmetric polarization pattern, typica l of a singly scattering reflection nebula. In contrast, the polarization i n the dark equatorial region is small, and the polarization vectors do not show a simple orientation pattern. The polarization data have been compared with published models to set rough constraints on the dust albedo and scat tering phase function and the nebular optical depths. The OH maser-line emi ssion at 1665 and 1667 MHz is found to be concentrated in the dark equatori al region; the kinematics of the OH features indicates that they arise in l ow-latitude outflows. The nebular morphology in Roberts 22 is characterized by a high degree of point symmetry, which may result from the interaction of collimated bipolar outflows with the progenitor AGE envelope.