Magnetic flux transport on active cool stars and starspot lifetimes

Abstract

Context. Many rapidly rotating cool stars show signatures of large magnetic regions at all latitudes. Mid-latitude starspots and magnetic regions have characteristic lifetimes of 1 month or less, as indicated by observations using (Zeeman-) Doppler imaging techniques. Aims. We aim to estimate the lifetimes of bipolar magnetic regions and starspots on the surfaces of cool stars. We consider different possible configurations for starspots and compare their flux variations and lifetimes based on a magnetic flux transport model. Methods. We carry out numerical simulations of the surface evolution of bipolar magnetic regions (BMRS) and magnetic spots on stars, which have radii and surface rotational shears of AB Doradus, the Sun, and the HR 1099 primary. The surface flux transport model is based on the magnetic induction equation for radial fields under the effects of surface differential rotation, meridional flow, and turbulent diffusion due to convective flow patterns. We calculate the flux evolution and the lifetimes of BMRs and unipolar starspots, varying the emergence latitude, surface shear rate, and tilt angle. Results. For BMRs comparable to the largest observed on the Sun, we find that varying the surface flows and the tilt angle modifies the lifetimes over a range of one month. For very large BMRs (area ∼ 10% of the stellar surface) the assumption of a tilt angle increasing with latitude leads to a significant increase of lifetime, as compared to the case without tilt. Such regions can evolve to polar spots that live more than a year. Adopting the observed weak latitudinal shear and the radius of the active subgiant component of HR 1099, we find longer BMR lifetimes as compared to the more strongly sheared AB Dor case. Random emergence of six additional tilted bipoles in an activity belt at 60° latitude enhanced the lifetimes of polar caps up to 7 years. We have also compared the evolution and lifetime of monolithic starspots with those of conglomerates of smaller spots of similar total area. We find similar decay patterns and lifetimes for both configurations.