Winds from Luminous Late-Type Stars: II. Broadband Frequency Distribution of Alfv\'en Waves

TL;DR

This paper uses advanced MHD simulations to explore how broadband Alfvén waves drive stellar winds in late-type giant stars, successfully matching observed wind properties and revealing their complex, anisotropic, and time-dependent nature.

Contribution

It introduces a new broadband Alfvén wave spectrum into non-linear MHD wind models, improving the understanding of stellar wind acceleration in evolved giants.

Findings

Simulations reproduce observed wind velocities and mass loss rates.

Broadband non-linear Alfvén waves are effective in driving stellar winds.

Winds are anisotropic and vary over time.

Abstract

We present the numerical simulations of winds from evolved giant stars using a fully non-linear, time dependent 2.5-dimensional magnetohydrodynamic (MHD) code. This study extends our previous fully non-linear MHD wind simulations to include a broadband frequency spectrum of Alfv\'en waves that drive winds from red giant stars. We calculated four Alfv\'en wind models that cover the whole range of Alfv\'en wave frequency spectrum to characterize the role of freely propagated and reflected Alfv\'en waves in the gravitationally stratified atmosphere of a late-type giant star. Our simulations demonstrate that, unlike linear Alfv\'en wave-driven wind models, a stellar wind model based on plasma acceleration due to broadband non-linear Alfv\'en waves, can consistently reproduce the wide range of observed radial velocity profiles of the winds, their terminal velocities and the observed mass loss rates. Comparison of the calculated mass loss rates with the empirically determined mass loss rate for alpha Tau suggests an anisotropic and time-dependent nature of stellar winds from evolved giants.