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| Paper: |
Mass Loss at the Tip of the AGB |
| Volume: |
313, Asymmetrical Planetary Nebulae III |
| Page: |
394 |
| Authors: |
Willson, L.A.; Kim, A. |
| Abstract: |
The AGB terminates with an episode of mass loss lasting a few times 105 years. The mechanism for this mass loss is pulsation combined with radiation pressure on grains for most stars with metallicity greater than about 30% of solar. The mass loss rate increases very steeply with increasing luminosity and radius and decreasing mass. The mass loss rate (and in some cases also the outflow velocity) on the AGB is modulated by (a) shell flash cycles and (b) an as-yet unidentified mechanism with a timescale of decades to centuries. The mechanism for AGB mass loss ceases to be effective when the envelope mass becomes small enough that the star begins to shrink. At the same time, a planet or low-mass companion may spin up or distort the envelope when Menv is less than about ten times the mass of the companion. The interaction of the envelope and any close body provides a natural mechanism for an event that would give rise to an asymmetric proto-planetary nebula inside a spherically symmetric distribution of material. In this review, we describe what is now reliably known about the events ending the AGB evolution of intermediate mass stars, and discuss some of the areas of uncertainty most relevant to the formation of APN. |
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