2019ApJ...876...44F

The magnetic fields of low-mass stars are observed to be variable on decadal timescales, ranging in behavior from cyclic to stochastic. The changing strength and geometry of the magnetic field should modify the efficiency of angular momentum loss by stellar winds, but this has not been well quantified. In Finley et al. (2018), we investigated the variability of the Sun and calculated the time-varying angular momentum-loss rate in the solar wind. In this work, we focus on four low-mass stars that have all had their surface magnetic fields mapped for multiple epochs. Using mass-loss rates determined from astrospheric Lyα absorption, in conjunction with scaling relations from the MHD simulations of Finley & Matt (2018), we calculate the torque applied to each star by their magnetized stellar winds. The variability of the braking torque can be significant. For example, the largest torque for ε Eri is twice its decadal averaged value. This variation is comparable to that observed in the solar wind, when sparsely sampled. On average, the torques in our sample range from 0.5 to 1.5 times their average value. We compare these results to the torques of Matt et al. (2015), who use observed stellar rotation rates to infer the long-time-averaged torque on stars. We find that our stellar wind torques are systematically lower than the long-time-averaged values, by a factor of ∼3-30. Stellar wind variability appears unable to resolve this discrepancy, implying that there remain some problems with observed wind parameters, stellar wind models, or the long-term evolution models, which have yet to be understood.