Red giant winds driven by Alfvén waves with magnetic diffusion

Abstract We investigate the driving mechanism of Alfvén wave-driven stellar winds from red giant stars, Arcturus ($\alpha$ Boo; K1.5 III) and Aldebaran ($\alpha$ Tau; K5 III), with non-ideal magnetohydrodynamics (MHD) simulations in one-dimensional super-radially open flux tubes. As the atmosphere is not fully ionized, upward propagating Alfvénic waves excited by surface convection are affected by ambipolar diffusion. Our fiducial run with the non-ideal MHD effect for $\alpha$ Boo gives a time-averaged mass-loss rate, $\langle \dot{M}\rangle =3.3\times 10^{-11}$ M$_{\odot }$ yr$^{-1}$, 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 temperature $T\gtrsim 10^6$ K, are occasionally present simultaneously with cool gas with $T\sim$ a few thousand 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 the mass-loss rate 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 Alfvénic waves. The non-ideal MHD simulation for $\alpha$ Tau, which is slightly more evolved than $\alpha$ Boo and has a weaker magnetic field, results in weaker wind with $\langle \dot{M}\rangle =1.5\times 10^{-12}$ M$_{\odot }$ yr$^{-1}$ with the atmospheric temperature ${\lesssim }10^5$ 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 $\langle \dot{M}\rangle =2.0\times 10^{-11}$ M$_{\odot }$ yr$^{-1}$—comparable to the observed value—with transient magnetized hot bubbles.