# Alfv\'en wave-driven wind from RGB and AGB stars

## TL;DR

This paper presents a magnetohydrodynamical model of Alfvén wave-driven stellar winds in RGB and AGB stars, revealing different wind types and their properties throughout stellar evolution, with implications for observed mass loss.

## Contribution

It introduces a detailed MHD model for Alfvén wave-driven winds in RGB and AGB stars, classifying wind types and linking them to stellar evolution stages.

## Key findings

- Four distinct wind types identified during stellar evolution.
- Mass-loss rates are lower than empirical estimates in some wind types.
- Massive, slow winds in TP-AGB stars may be driven by Alfvén waves.

## Abstract

We develop a magnetohydrodynamical model of Alfv\'en wave-driven wind in open magnetic flux tubes piercing the stellar surface of Red Giant Branch (RGB) and Asymptotic Giant Branch (AGB) stars, and investigate the physical properties of the winds. The model simulations are carried out along the evolutionary tracks of stars with initial mass in the range of 1.5 to 3.0 $M_{\odot}$ and initial metallicity $Z_{\rm ini}$=0.02. The surface magnetic field strength being set to be 1G, we find that the wind during the evolution of star can be classified into the following four types; the first is the wind with the velocity higher than 80 km s$^{-1}$ in the RGB and early AGB (E-AGB) phases; the second is the wind with outflow velocity less than 10 km s$^{-1}$ seen around the tip of RGB or in the E-AGB phase; the third is the unstable wind in the E-AGB and thermally pulsing AGB (TP-AGB) phases; the fourth is the stable massive and slow wind with the mass-loss rate higher than 10$^{-7} M_{\odot}$ yr$^{-1}$ and the outflow velocity lower than 20 km s$^{-1}$ in the TP-AGB phase. The mass-loss rates in the first and second types of wind are two or three orders of magnitude lower than the values evaluated by an empirical formula. The presence of massive and slow wind of the fourth type suggests the possibility that the massive outflow observed in TP-AGB stars could be attributed to the Alfv\'en wave-driven wind.