M-dwarf's Chromosphere, Corona and Wind Connection via the Nonlinear Alfvén Wave

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 Alfvén wave is the promising mechanism for both heating stellar atmosphere and driving stellar wind. Based on this Alfvén 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 Alfvén 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 Alfvén 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 Alfvén wave energy input from the M dwarf's photosphere are remarkable.

31 pages, 24 figures. accepted for publication in The Astrophysical Journal. for associated python code, see https://www.kwasan.kyoto-u.ac.jp/%7Esakaue/awsaws/awsaws.py

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