Polar Magnetic Activity and Spin‐down on the Lower Main Sequence

Abstract

On the Sun, magnetic activity is essentially restricted to latitudes within 45° of the equator. However, active stars, such as RS CVn systems, and T Tauri stars generally show polar activity. I model the evolution of a toroidal magnetic flux tube as it rises from the base of the convection zone in various lower main-sequence stars and show that, on the main sequence, polar activity is more likely for less massive stars. For example, a 1 M☉ star must rotate with ω > 5 ω☉ in order to develop significant polar magnetic flux, while a 0.4 M☉ star will generate predominantly polar flux for ω > 0.5 ω☉. This leads to the expectation that essentially all M stars should display polar, rather than equatorial, magnetic activity. A similar effect is expected for RS CVn and T Tauri systems, and for giant stars. An important consequence of this polar activity is the inhibition of angular momentum loss from these systems. On the lower main sequence, angular momentum is lost via the interaction of the stellar magnetic field with the outflowing stellar wind. However, if regions of strong magnetic field are confined to the poles, stellar spin-down rates can be reduced dramatically, by up to a factor of 3 or more for early M stars. Examination of open clusters should show the presence of rapidly rotating M stars at ages beyond those previously expected.