Toward A Self Consistent MHD Model of Chromospheres and Winds From Late Type Evolved Stars

TL;DR

This paper introduces a pioneering 1.5D MHD model of chromospheric heating and stellar wind acceleration in cool evolved stars, emphasizing the role of Alfven waves and ion-neutral interactions in alpha Tau.

Contribution

It presents the first self-consistent MHD simulation incorporating partial ionization effects to model chromospheric heating and wind driving in late-type evolved stars.

Findings

Alfven wave dissipation explains UV line broadening.

Resistive heating rates match observational radiative losses.

Simulated wind properties agree with observations of alpha Tau.

Abstract

We present the first magnetohydrodynamic model of the stellar chromospheric heating and acceleration of the outer atmospheres of cool evolved stars, using alpha Tau as a case study. We used a 1.5D MHD code with a generalized Ohm's law that accounts for the effects of partial ionization in the stellar atmosphere to study Alfven wave dissipation and wave reflection. We have demonstrated that due to inclusion of the effects of ion-neutral collisions in magnetized weakly ionized chromospheric plasma on resistivity and the appropriate grid resolution, the numerical resistivity becomes 1-2 orders of magnitude smaller than the physical resistivity. The motions introduced by non-linear transverse Alfven waves can explain non-thermally broadened and non-Gaussian profiles of optically thin UV lines forming in the stellar chromosphere of alpha Tau and other late-type giant and supergiant stars. The calculated heating rates in the stellar chromosphere due to resistive (Joule) dissipation of electric currents, induced by upward propagating non-linear Alfven waves, are consistent with observational constraints on the net radiative losses in UV lines and the continuum from alpha Tau. 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. Our simulations also suggest that momentum deposition by non-linear Alfven waves becomes significant in the outer chromosphere at 1 stellar radius from the photosphere. The calculated terminal velocity and the mass loss rate are consistent with the observationally derived wind properties in alpha Tau.