Magnetic Braking Revisited

We present a description for the angular momentum loss rate due to magnetic braking for late-type stars, taking into account recent observational data on the relationship between stellar activity and rotation. The analysis is based on an idealized two-component coronal model subject to constraints imposed on the variation of the coronal gas density, with a rotation period inferred from the observed variation of X-ray luminosity L_X with rotation rate Ω (L_X ∝ Ω²) for single rotating dwarfs. An application of the model to high rotation rates leads to a gradual turnover of the X-ray luminosity that is similar to the saturation recently observed in rapidly rotating dwarfs. The resulting angular momentum loss rate, , depends on Ω in the form ∝ Ω^β, where β ~ 3 for slow rotators and ~1.3 for fast rotators. The relation at high rotation rates significantly differs from the power-law exponent for slowly rotating stars, depressing the angular momentum loss rate without necessarily requiring the saturation of the magnetic field. The application of this model to the evolution of cataclysmic variable binary systems leads to mass transfer rates that are more in accordance with those observed compared to rates based on either a Skumanich law or an empirical law based on β = 1.