Unraveling the Structure of Aspherical Proto-Planetary Nebulae. I. Hubble Space Telescope Imaging and Hydroxyl Maser Line Observations of Roberts 22

Details Unraveling the Structure of Aspherical Proto-Planetary Nebulae. I. Hubble Space Telescope Imaging and Hydroxyl Maser Line Observations of Roberts 22 Unraveling the Structure of Aspherical Proto-Planetary Nebulae. I. Hubble Space Telescope Imaging and Hydroxyl Maser Line Observations of Roberts 22

Mar 1999

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999aj....117.1408s&link_type=abstract

The Astronomical Journal, Volume 117, Issue 3, pp. 1408-1420.

Astronomy and Astrophysics

Astronomy

48

Stars: Circumstellar Matter, Ism: Planetary Nebulae: General, Stars: Agb And Post-Agb, Stars: Mass Loss

Scientific paper

We have obtained high-resolution wide-band, narrowband, and polarimetric images of the bipolar proto-planetary nebula Roberts 22 with the Wide Field Planetary Camera 2 on the Hubble Space Telescope. OH maser-line emission has also been observed using the Australia Telescope Compact Array. The wide-band images at 0.6 μm show bright bipolar lobes shaped like a butterfly's ``wings,'' separated by a dark equatorial ``body'' of dense dust that completely obscures the central star. The material within each lobe appears to be organized into an amazingly complex pattern of a very large number of intersecting loops and filaments. The bright lobes are surrounded by a fainter halo whose inner region contains multiple thin shell structures, reminiscent of 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 expected for time-invariant mass loss at a constant expansion velocity. A simple model of the radial scattered light distribution supports the hypothesis that the bright lobes are cavities with thin (<0.5"), dense walls that are optically thick in scattered light. The amount of mass in the cavity walls is quite large (0.3 M_solar) and the dynamical timescale for their formation is short (<~440 yr). The bright lobes show high fractional polarization (40%-50%) and a centrosymmetric polarization pattern, typical of a singly scattering reflection nebula. In contrast, the polarization in 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 scattering phase function and the nebular optical depths. The OH maser-line emission at 1665 and 1667 MHz is found to be concentrated in the dark equatorial region; the kinematics of the OH features indicates that they arise in low-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 AGB envelope.

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