Alfven-wave-driven Magnetic Rotator Winds from Low-mass Stars. I. Rotation Dependences of Magnetic Braking and Mass-loss Rate

Observations of stellar rotation show that low-mass stars lose angular momentum during the main sequence. We simulate the winds of sunlike 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-*integral(open)(*) proportional to Ro(-1.2), where integral(*) (open) and 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) approximate to 2.59 x 10(30) erg (Omega(*)/Omega(circle dot))(2.82), yielding a spin-down law Omega(*) proportional to t(-0.55). (2) The mass-loss rate saturates a (M) over dot(w) similar to 3.4 x 10(-14)M(circle dot) 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 proportional to Omega(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 one-dimensional 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.