Structured Red Giant Winds with Magnetized Hot Bubbles and the Corona/Cool Wind Dividing Line

By performing MHD simulations, we investigate the mass loss of intermediate-and low-mass stars from main sequence (MS) to red giant branch (RGB) phases.Alfven waves, which are excited by the surface convections travel outwardly anddissipate by nonlinear processes to accelerate and heat the stellar winds. Wedynamically treat these processes in open magnetic field regions from thephotospheres to 25 stellar radii. When the stars evolve to slightly bluewardpositions of the dividing line (Linsky & Haisch), the steady hot corona withtemperature, ~ 1MK, suddenly disappears. Instead, many hot (~1MK) and warm(~10^5K) bubbles are formed in cool (T<~2x10^4K) chromospheric winds because ofthermal instability; the red giant wind is not a steady stream but structuredoutflow. As a result, the mass loss rates, \dot{M}, largely vary in time by 3-4orders or magnitude in the RGB stars. Supported by magnetic pressure, thedensity of hot bubbles can be kept low to reduce the radiative cooling and tomaintain the high temperature long time. Even in the stars redward of thedividing line, hot bubbles intermittently exist, and they can be sources ofUV/soft X-ray emissions from hybrid stars. Nearly static regions are formedabove the photospheres of the RGB stars, and the stellar winds are effectivelyaccelerated from several stellar radii. Then, the wind velocity is much smallerthan the surface escape speed, because it is regulated by the slower escapespeed at that location. We finally derive an equation that determines \dot{M}from the energetics of the simulated wave-driven winds in a forward manner. Therelation explains \dot{M} from MS to RGB, and it can play a complementary roleto the Reimers' formula, which is mainly for more luminous stars.