Evolution of Stellar Wind form the Sun to Red Giants

Takeru Suzuki

School of Arts & Sciences, University of Tokyo, Japan

By performing global 1D MHD simulations, we investigate the heating and acceleration of solar and stellar winds in open magnetic field regions. Our simulation covers from photosphere to 20-60 stellar radii, and takes into account radiative cooling and thermal conduction. We do not adopt ad hoc heating function; heating is automatically calculated from the solutions of Riemann problem at the cell boundaries.

In the solar wind case we impose transverse photospheric motions with velocity ~1 km/s and period between 20 seconds and 30 minutes, which generate outgoing Alfven waves. We have found that the dissipation of Alfven waves through compressive wave generation by decay instability is quite effective owing to the density stratification, which leads to the sufficient heating and acceleration of the coronal plasma.

Then, we study the evolution of stellar winds from main sequence to red giant phases. When the stellar radius becomes ~10 times of the Sun, the steady hot corona with temperature 106 K, suddenly disappears. Instead, many hot and warm (105 - 106 K) bubbles are formed in cool (T < 2 104 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