The Emergence of Magnetic Flux Loops in Sunlike Stars

Abstract

We explore the latitude of emergence of flux tubes at the surface of G stars as a function of the rotation rate, magnetic flux, and injection latitude at the bottom of the convective zone. Our analysis is based on a thin flux tube evolution code that has been developed to study the emergence of magnetic flux in the Sun and is well calibrated by detailed comparisons with solar observations. We study solar models with rotation rates between and 10 times solar, injection latitudes ϕI between 1° and 40°, and tubes with a range of field strengths, B0, and fluxes. For our range of input parameters, we find that the mean latitude of emergence, ⟨ϕE⟩, increases and its range decreases with higher rotation rates, that ϕE ≤ 45° for stars with rotational periods ≥27 days, that ϕE increases with B0 in rapid rotators, while the reverse is true for slow rotators, that the dependence of ϕE on B0 decreases with increasing ϕI, that tubes with higher flux emerge at larger ϕE, and that the footpoint separation depends linearly on B0. We compare our results to other calculations and with observations of magnetic features on stars and suggest future observations and extensions of this research. Our results suggest that for near-polar starspots to occur, either active stars must have a larger range of ϕI than inferred for the Sun, or differential rotation and meridional flows are more important for magnetic flux redistribution in these stars. Our models also imply that flux appearing near the equator of active stars may be generated by a distributed, rather than a boundary layer, dynamo.