Poster Abstracts

Name/Affiliation:  Vladimir Airapetian (NASA)

Title:  Toward Self Consistent MHD Model of Chromospheres of Late Type Evolved Stars

The chromospheres of cool, evolved stars represent the interface layers between the photospheres and their outer atmospheres, in which the stellar wind forms, and, therefore, should play a critical role in specifying the amount of mechanical energy dissipating into atmospheric heating and in depositing the momentum needed to drive stellar winds. We present a self consistent magnetohydrodynamic (MHD) model of the chromospheric heating and acceleration of the outer atmospheres of cool evolved stars, using alpha Tau as a case study. We use a 1D MHD code with a generalized Ohmís law that accounts for the effects of partial ionization in the stellar atmosphere to study Alfven wave dissipation and wave reflection as well as mode conversion effects. In addition, we apply a high resolution grid to fully resolve resistive heating in our model. We conclude that resistive (Joule) dissipation of electric currents, induced by upward propagating non-linear Alfven waves, is the major factor contributing to the heating throughout the stellar chromosphere. We also find that as Alfven waves reach the low chromosphere at 0.05Rstar with plasma beta close to unity, they are efficiently coupled to longitudinal modes, transferring about 1/3 of their energy flux into non-linear slow magnetosonic waves. At the top of the chromosphere, Alfven waves experience significant reflection, producing downward propagating transverse waves that interact with upward propagating waves and produce velocity shear in the chromosphere. The heating rates due to resistive heating and momentum deposition due to non-linear Alfven waves reflected from the top of the chromosphere are consistent with observational constraints on the net radiative losses and mass loss rates from the wind from cool evolved stars.