Formulating Mass-Loss Rates for Sun-like Stars: A Hybrid Model Approach

Abstract

We observe an enhanced stellar wind mass-loss rate from low-mass stars exhibiting higher X-ray flux. This trend, however, does not align with the Sun, where no evident correlation between X-ray flux and mass-loss rate is present. To reconcile these observations, we propose a hybrid model for the stellar wind from solar-type stars, incorporating both Alfv\'en wave dynamics and flux emergence-driven interchange reconnection, an increasingly studied concept guided by the latest heliospheric observations. For establishing a mass-loss rate scaling law, we perform a series of magnetohydrodynamic simulations across varied magnetic activities. Through a parameter survey concerning the surface (unsigned) magnetic flux ($\Phi^{\rm surf}$) and the open-to-surface magnetic flux ratio ($\xi^{\rm open} = \Phi^{\rm open}/\Phi^{\rm surf}$), we derive a scaling law of the mass-loss rate given by $\dot{M}_w/\dot{M}_{w,\odot} = \left( \Phi^{\rm surf} / \Phi^{\rm surf}_\odot \right)^{0.52}\left( \xi^{\rm open} / \xi^{\rm open}_\odot \right)^{0.86}$, where $\dot{M}_{w,\odot} = 2.0 \times 10^{-14} \ M_\odot {\rm \ yr}^{-1}$, $\Phi^{\rm surf}_\odot = 3.0 \times 10^{23} {\rm \ Mx}$, and $\xi^{\rm open}_\odot = 0.2$. By comparing cases with and without flux emergence, we find that the increase in the mass-loss rate with the surface magnetic flux can be attributed to the influence of flux emergence. Our scaling law demonstrates an agreement with solar wind observations spanning 40 years, exhibiting superior performance when compared to X-ray-based estimations. Our findings suggest that flux emergence may play a significant role in the stellar winds of low-mass stars, particularly those originating from magnetically active stars.