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تسجيل مستخدم جديدAlfven-wave driven magnetic rotator winds from low-mass stars I: rotation dependences of magnetic braking and mass-loss rate
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Observations of stellar rotation show that low-mass stars lose angular momentum during the main sequence. We simulate the winds of Sun-like stars with a range of rotation rates, covering the fast and slow magneto-rotator regimes, including the transition between the two. We generalize an Alfven-wave driven solar wind model that builds on previous works by including the magneto-centrifugal force explicitly. In this model, the surface-averaged open magnetic flux is assumed to scale as $B_ast f^{rm open}_ast propto {rm Ro}^{-1.2}$, where $f^{rm open}_ast$ and ${rm Ro}$ are the surface open-flux filling factor and Rossby number, respectively. We find that, 1. the angular momentum loss rate (torque) of the wind is described as $tau_w approx 2.59 times 10^{30} {rm erg} left( Omega_ast / Omega_odot right)^{2.82}$, yielding a spin-down law $Omega_ast propto t^{-0.55}$. 2. the mass-loss rate saturates at $dot{M}_w sim 3.4 times 10^{-14} M_odot {rm yr^{-1}}$, due to the strong reflection and dissipation of Alfven waves in the chromosphere. This indicates that the chromosphere has a strong impact in connecting the stellar surface and stellar wind. Meanwhile, the wind ram pressure scales as $P_w propto Omega_ast^{0.57}$, which is able to explain the lower-envelope of the observed stellar winds by Wood et al. 3. the location of the Alfven radius is shown to scale in a way that is consistent with 1D analytic theory. Additionally, the precise scaling of the Alfven radius matches previous works which used thermally-driven winds. Our results suggest that the Alfven-wave driven magnetic rotator wind plays a dominant role in the stellar spin-down during the main-sequence.
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