Considerable effort has gone into observing mass loss rates from stars with masses from 0.7 to 8 times the Sun's mass. Most of this mass is lost when the star is a luminous red giant, and most in fact during the last 200,000 years of a much longer nuclear-burning lifetime. The mass loss process is however hard to model, because it involves dynamical processes often far from equilibrium. This work is focused on providing a reliable prescription of this mass loss for oxygen-rich stars at the end of the Asymptotic Giant Branch. Of fundamental concern are recent results from European mass loss codes where adding the wavelength dependence of the opacity of silicate grains made them unable to drive winds (until this point it had always been thought that such dust played a major role in enabling winds and mass loss). Here Dr. Willson and her team will take hydrodynamical modeling codes already developed or in use by the team and improve their treatment of dust opacities and non-local thermodynamic equilibrium cooling/reheating rates by radiation as well as explore the effects of interior-atmosphere feedback during pulsations. The dust opacity problem will be directly addressed by this work and through collaboration with dust researchers at NASA and in Europe. Extensive testing of the resulting models/prescriptions for mass loss will be done with a variety of observations, including empirical mass loss studies, interferometric measurements of atmospheric features including 'molecular shells' near the inner dust radius, and a century of data for over 500 stars collected by mostly amateur astronomers and archived at the American Association of Variable Star Observers.
This project will educate and train a graduate student whose Ph.D. thesis will be based on this work. An additional 2 or 3 undergraduate students at Iowa State University will also be involved. The resulting model grids will be placed on a web site where they will be accessible to other astronomers through an 'intelligent interface.' These will be useful for population studies and the analysis of data from interferometry and infrared observations. The models and data on the website will also be included in exercises for introductory and advanced courses in astronomy. Beyond this, the question of the ultimate fate of the Earth and the conditions that Earth will encounter as the Sun evolves have public appeal, and this work will provide material for planetarium shows, public lectures, and popular articles as well as targeted publications for amateur astronomers.
