M-dwarf's Chromosphere, Corona and Wind Connection via the Nonlinear Alfv\'en Wave

TL;DR

This study uses 1D MHD simulations to explore how nonlinear Alfvén waves heat M dwarf atmospheres and drive stellar winds, revealing key differences from the Sun and the importance of slow shock dynamics.

Contribution

It demonstrates the role of nonlinear Alfvén wave propagation and slow shock formation in M dwarf atmospheric heating and wind acceleration, extending previous models to multiple stars.

Findings

M dwarf coronae are cooler and denser than the solar corona.

Stellar winds from M dwarfs are faster but have lower mass-loss rates than the solar wind.

Slow shocks from nonlinear Alfvén waves are crucial for chromosphere dynamics and wind acceleration.

Abstract

M dwarf's atmosphere is expected to be highly magnetized. The magnetic energy can be responsible for heating the stellar chromosphere and corona, and driving the stellar wind. The nonlinear propagation of Alfv\'en wave is the promising mechanism for both heating stellar atmosphere and driving stellar wind. Based on this Alfv\'en wave scenario, we carried out the one-dimensional compressive magnetohydrodynamic (MHD) simulation to reproduce the stellar atmospheres and winds of TRAPPIST-1, Proxima Centauri, YZ CMi, AD Leo, AX Mic, as well as the Sun. The nonlinear propagation of Alfv\'en wave from the stellar photosphere to chromosphere, corona, and interplanetary space is directly resolved in our study. The simulation result particularly shows that the slow shock generated through the nonlinear mode coupling of Alfv\'en wave is crucially involved in both dynamics of stellar chromosphere (stellar spicule) and stellar wind acceleration. Our parameter survey further revealed the following general trends of physical quantities of stellar atmosphere and wind. (1) The M dwarfs' coronae tend to be cooler and denser than solar corona. (2) M dwarfs' stellar winds can be characterized with relatively faster velocity and much smaller mass-loss rate compared to those of solar wind. The physical mechanisms behind these tendencies are clarified in this paper, where the stronger stratification of M dwarf's atmosphere and relatively smaller Alfv\'en wave energy input from the M dwarf's photosphere are remarkable.