Studying stellar spin-down with Zeeman-Doppler magnetograms

Magnetic activity and rotation are known to be intimately linked for low-mass stars. Understanding rotation evolution over the stellar lifetime is therefore an important goal within stellar astrophysics. In recent years, there has been increased focus on how the complexity of the stellar magnetic field affects the rate of angular-momentum loss from a star. This is a topic that Zeeman-Doppler imaging (ZDI), a technique that is capable of reconstructing the large-scale magnetic field topology of a star, can uniquely address. Using a potential field source surface model, we estimate the open flux, mass-loss rate and angular-momentum-loss rates for a sample of 66 stars that have been mapped with ZDI. We show that the open flux of a star is predominantly determined by the dipolar component of its magnetic field for our choice of source surface radius. We also show that, on the main sequence, the open flux, mass-loss and angular-momentum-loss rates increase with decreasing Rossby number. The exception to this rule is stars less massive than 0.3M degrees. Previous work suggests that low-mass M dwarfs may possess either strong, ordered and dipolar fields or weak and complex fields. This range of field strengths results in a large spread of angular-momentum-loss rates for these stars and has important consequences for their spin-down behaviour. Additionally, our models do not predict a transition in the mass-loss rates at the so-called wind-dividing line noted from Lya studies.