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Session 50 - Disks and Bipolar Outflows from Hot Stars - I.
Oral session, Wednesday, June 12
In the last decade it has become clear that hypersonic bipolar outflows are a ubiquitous phenomena associated with evolved stars. High resolution observations of Planetary Nebulae, the nebulae surrounding Luminous Blue Variables such as Eta Carinae, Wolf Rayet bubbles, and the circumstellar environment of SN 1987A have all revealed high velocity outflows with a considerable range of shapes. In this talk we will focus on the current state of our theoretical understanding of these outflows. In particular we will raise two questions:
1) What does the ubiquity of bipolar outflows tell us about fundamental astrophysical processes associated with stellar evolution and stellar winds.
2) How can these outflows be used as fossils or tracers of the evolution of particular stars or classes of stars.
Beginning with Planetary Nebulae considerable progress has been made in understanding bipolar outflows as the result of concentric interacting stellar winds. In what has be called the "Generalized Interacting Stellar Wind" (GISW) scenario a fast tenuous wind from the hot central star expands into a slower, more dense wind ejected earlier with a aspherical (toroidal) density distribution. The asphericity of the latter wind quickly leads to an expanding prolate or bipolar bubble of swept-up gas bounded by strong shock waves. Numerical simulations of the GISW model have revealed a surprisingly rich variety of gas-dynamical behavior in these systems allowing the GISW model to recover many of the observed properties of stellar Bipolar Outflows including the development of collimated supersonic jets.
As a partial answer to the two questions posed above we will review the basic GISW model. We will discuss the results of recent theoretical studies in light of what has been learned from observations. While we will focus on what has been learned so far from theory, the emphasis of the talk will be on what might be learned if a unified picture of the star, its winds, and the broader environment (evolution, binaries, etc) can be developed across the HR diagram.
Program listing for Wednesday