Evolution of solar-type stellar wind

TL;DR

This study models the evolution of solar-type stellar winds from young active stars to red giants, revealing how magnetic activity influences wind properties and the transition from steady corona to structured outflows.

Contribution

It extends MHD simulations to explore how stellar wind characteristics change over stellar evolution, especially the effects of magnetic activity and stellar expansion.

Findings

Mass loss rates can be up to 100 times higher in young stars.

Wind energy saturation occurs due to radiative losses.

Red giant winds are structured, not steady, with embedded hot bubbles.

Abstract

By extending our self-consistent MHD simulations for the solar wind, we study the evolution of stellar winds of solar-type stars from early main sequence stage to red giant phase. Young solar-type stars are active and the mass loss rates are larger by up to ~ 100 times than that of the present-day sun. We investigate how the stellar wind is affected when the magnetic field strength and fluctuation amplitude at the photosphere increase. While the mass loss rate sensitively depends on the input energy from the surface because of the global instability related to the reflection and nonlinear dissipation of Alfven waves, it saturates at ~ 100 times because most of the energy is used up for the radiative losses rather than the kinetic energy of the wind. After the end of the main sequence phase when the stellar radius expands by ~ 10 times, the steady hot corona with temperature $10^6$ K, suddenly disappears. Chromospheric materials, with hot bubbles embedded owing to thermal instability, directly stream out; the red giant wind is not a steady stream but structured outflow.