Astronomy and Astrophysics – Astrophysics
Scientific paper
Jul 1996
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996hst..prop.6856t&link_type=abstract
HST Proposal ID #6856
Astronomy and Astrophysics
Astrophysics
Hst Proposal Id #6856 Interstellar Medium
Scientific paper
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 telling example is provided by the triple-ring nebula around SN1987A, where our WFPC-2 images ruled out the possibility that the rings are simply limb-brightened bubbles. Using WFPC2, we propose to image 4 objects, which encompass the stellar evolution from the tip of the AGB to the planetary nebula phase, including the intermediate proto-planetary phase, which is short-lived, but in many ways is the most interesting. The high-angular resolution provided by WFPC-2 on the HST will allow us for the first time to image the circumstellar matter with sufficient resolution which can be used to find and resolve sharp and compact structures. The 4 objects, IRC+10216 {AGB star}, Boomerang Nebula {proto-planetary}, Mz-3 & IC4634 {young planetary nebulae} show indications of a variety of such structures in ground-based images. For example, IRC+10216, the best-studied mass-losing AGB star shows faint ring-like structures in scattered light at visual wavelengths. Linear-polarisation mapping of the Boomerang nebula shows two bipolar lobes on either side of a central star obscured by an equatorial disk. Mz-3 shows an extensive set of radial filaments in H-alpha+NII; and IC4634 shows a striking point-symmetric structure, both in its overall morphology, as well on small scales {in the form of compact knots}. The WFPC-2 imaging will allow us to study the above-mentioned features with sufficient angular detail for each nebula so that we can determine their true nature and origin. We also hope to discover new features {e.g. jets, neutral clumps in absorption} which we cannot see with the seeing-limited resolution of ground-based images. Finally, we will the WFPC2 images will allow us to study the hydrodynamics of interacting winds, a subject of considerable importance for our understanding of a broad range of astrophysical objects.
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