Physics
Scientific paper
Jul 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995hst..prop.6221t&link_type=abstract
HST Proposal ID #6221
Physics
Hst Proposal Id #6221 Interstellar Medium
Scientific paper
We propose to image 3 post-AGB objects, including two planetary nebulae (PNe) and one proto-planetary nebula, using WFPC-2. Planetary nebulae show a dazzling variety of structures not apparent in the circumstellar envelopes of the AGB stars from which they evolve. It is believed that the physical phenomena which shape the PNe occur early in their formation history, involving the hydrodynamic interaction of one or more fast winds with the slower AGB mass-outflow. Clumping of the nebular gas is expected theoretically to strongly affect the nature of the hydrodynamic interaction, and one or two of the closest PNe show the presence of such neutral clumps. The axisymmetric morphology, found in 50% (or more) of all PNe, is believed to be due to the presence of an equatorial density enhancement in the slow AGB outflow. The presence of point-symmetric structure (about the central star) on both small and large-scales in many PNe strongly suggests the presence of collimated jet-like outflows which can provide the imprint of such structures in the circumstellar envelopes of the progenitors. The seeing-limited resolution of ground-based observations broadens and merges together intrinsically sharp structures such as rings, jets, knots, neutral clumps, and shells making it extremely difficult to understand the detailed morphology of these nebulae. Without such an understanding, it is not possible to make definitive tests of theoretical models for the formation of planetary nebulae from AGB stars. A
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