Topic: Formation and Evolution of Cool Stars and Brown Dwarfs
By extending our self-consistent MHD simulations for the solar wind by Suzuki & Inutsuka (2005), we study the evolution of stellar winds of solar-type stars from early main sequence stage to red giant phase. As a star evolves from the early main sequence to later stages, the mass loss rate considerably decreases by a small decrease of the fluctuation amplitude associated with surface convection at the surface because of global instabilities involving dissipation and reflection of nonlinear Alfven waves in the atmosphere (Suzuki 2011). After the end main sequence phase when the stellar radius becomes ~ 10 times of the Sun, the steady hot corona with temperature 10^6 K, suddenly disappears. Instead, many hot and warm (10^5 - 10^6 K) bubbles are formed in cool (T < 2x10^4 K) chromospheric winds because of the thermal instability of the radiative cooling function; the red giant wind is not a steady stream but structured outflow. Also, the wind velocity is much smaller than the surface escape speed, because the wind starts to be accelerated from several stellar radii (Suzuki 2007).