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Session 103 - Cool Stars.
Display session, Thursday, January 18
North Banquet Hall, Convention Center
In populations with ages between about 1 and 10 Gyr, the brightest stars include vigorously pulsating and mass-losing Miras. Their location at the tip of the AGB coincides closely with the superwind phase that ends their ascent. The (L, T, M) where the superwind sets in is governed by a complex interplay of physical effects, as has been demonstrated by detailed numerical models by Bowen (Bowen 1988; Bowen and Willson 1991): Pulsation alters the temperature and density structure of the atmosphere, and this allowst grains to form relatively close to the star. The position of the "superwind cliff" is not very sensitive to the most uncertain parameters of the models, but is sensitive to the metallicity that is assumed, because for a given mass and luminosity lower metallicity stars have higher T_eff; and (2) lower mass stars are less well able to make the quantities of dust needed to drive the highest mass loss rates. Fairly rapid mass loss occurs even in models with no dust present , because of temperature and pressure gradients in the dynamic atmosphere. The reduction in the mass loss rate for stars of reduced metallicity is sufficient, however, to reduce substantially the minimum supernova progenitor mass in very low-metallicity populations. When these models are compared with observational constraints they are found to match extremely well; one consequence is a new interpretation of the significance of the Reimers relation that is often used to estimate mass loss rates in stellar evolution calculations.
