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Session 50 - Disks and Bipolar Outflows from Hot Stars - I.
Oral session, Wednesday, June 12
When a star rotates rapidly in comparison to the outflow velocity of its wind, the flow is no longer purely radial. For the line-driven winds of hot stars, the initial outward acceleration by radiation pressure is smaller than gravity. Consequently, the flow ``falls'' toward the equatorial plane where it collides with the flow from the opposite hemisphere of the star. Since the flow velocites are supersonic, a pair of standing shocks form above and below the equatorial plane. These shocks compress the flow and produce a thin dense equatorial disk, called the wind-compressed disk (WCD). In this talk, I will review the WCD model, present the conditions required for disk formation, and discuss the observational evidence for disks in the winds of rapidly rotating hot stars. In addition to hot star winds, it also has been postulated that similar equatorial density enhancements in AGB winds could explain the morphology of planetary nebulae, as well as the production of the mid-plane ring around SN1987a. Therefore, I will also discuss the possible application of the WCD mechansim to AGB winds. Finally, if there is a pre-existing Keplerian disk (such as may be present around massive young stars or mass transfer binaries), the equatorially directed wind will either mix with the Keplerian disk, leading to wind-induced accretion, or entrain the Keplerian disk as it flows outward over its surface. I will discuss the timescale for removing the Keplerian disk and the conditions required for its survival.
Program listing for Wednesday