Red Giant Winds Driven by Alfv\'en Waves with Magnetic Diffusion

Abstract

We investigate the driving mechanism of Alfv\'en wave-driven stellar winds from red giant stars, Arcturus ($\alpha$ Boo; K1.5 III) and Aldebaran ($\alpha$ Tau; K5 III), with nonideal MHD simulations in 1D super-radially open flux tubes. Since the atmosphere is not fully ionized, upward propagating Alfv\'enic waves excited by surface convection are affected by ambipolar diffusion. Our fiducial run with the nonideal MHD effect for $\alpha$ Boo gives a time-averaged mass-loss rate, $\dot{M}=3.3\times 10^{-11}M_{\odot}$/yr, which is more than one order of magnitude reduced from the result in the ideal MHD run and nicely explains the observational value. Magnetized hot bubbles with $T\gtrsim 10^6$ K are occasionally present simultaneously with cool gas with a few $10^3$ K in the atmosphere because of the thermal instability triggered by radiative cooling; there coexist fully ionized plasma emitting soft X-rays and molecules absorbing/emitting infrared radiations. The inhomogeneity in the atmosphere also causes large temporal variations in $\dot{M}$ within an individual magnetic flux tube. We also study the effect of magnetic field strength and metallicity, and find that the wind density, and accordingly the mass-loss rate, positively and sensitively depends on both of them through the ambipolar diffusion of Alfv\'enic waves. The nonideal MHD simulation for $\alpha$ Tau, which is slightly more evolved than $\alpha$ Boo and has weaker magnetic field, results in weaker wind with $\dot{M}=1.5\times 10^{-12}M_{\odot}$/yr with $T\lesssim 10^5$ K throughout the simulation time. However, given the observations implying the presence of locally strong magnetic fields on the surface of $\alpha$ Tau, we also conduct a simulation with a field strength twice as strong. This results in $\dot{M}=2.0\times 10^{-11}M_{\odot}$/yr - comparable to the observed value - with transient magnetized hot bubbles.