Speaker: Henry Prager, New Mexico Tech and Los Alamos National Laboratory

Host: Lee Anne Willson

Title: Studies of Mass-Loss Rates and Stellar Atmospheres on the Asymptotic Giant Branch

Abstract:  Asymptotic Giant Branch (AGB) stars are the final stage of stellar evolution for stars between 0.7 to about 8 Msun. These stars have dynamic stellar atmospheres with periodic opacity-driven pulsations and complicated chemistry that leads to dust formation. The combination of pulsations and the formation of dust in the atmosphere drives the process of mass loss in these stars. Through mass loss, stellar material is recycled back into the interstellar medium; this recycling of material is essential to understanding the chemical evolution of galaxies and understanding how the material needed to make planets is formed.

    
    The first part of this work is focused on determining a consistent mass-loss rate formula for AGB stars using stellar parameters. These stellar parameters can either be observed or determined from stellar models. The observed properties and mass-loss rates for AGB stars in the LMC have been determined by previous research, and we have used that data to determine the radius and mass of the stars with pulsation-mass-radius and radius-mass-luminosity relations. We determined that standard fitting procedures were insufficient because of the significant, unavoidable intrinsic scatter and uncertainty in the luminosity and mass-loss rate data.If we assume that the mass-loss rate can be accurately described by a power law, it is possible to use the distribution of stars in pulsation period-luminosity space to determine the parameters of a formula. We used this method to determine formulae for oxygen-rich and carbon-rich stars pulsating in both the fundamental and first-overtone modes. These formulae show much stronger dependence on stellar parameters than typically determined from observations, bringing the formulae into much better agreement with what is expected from stellar models and the synthetic stellar populations.
    
    The second part of this work is a parameter study of atmospheric pulsation models of AGB stars. Using George Bowen's atmospheric pulsation code, we have generated a number of grids of atmospheric models. With these grids, we can quantify how adjusting the physical parameters of the model affects the properties of the grid, most importantly the skew and position of the grid in luminosity-mass space. Notably, standard input parameters fail to reproduce observations, but with the parameter study, we have determined the ranges of parameters necessary to reproduce observed populations of AGB stars with a one-dimensional code.