The emergence of magnetic flux loops in sunlike stars

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 1/3 and 10 times solar, injection latitudes φ_I 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, 〈 φ_E 〉, increases and its range decreases with higher rotation rates, that φ_E ≤ 45° for stars with rotational periods ≥27 days, that φ_E increases with B₀ in rapid rotators, while the reverse is true for slow rotators, that the dependence of φ_E on B₀ decreases with increasing φ_I, that tubes with higher flux emerge at larger φ_E, 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 star-spots to occur, either active stars must have a larger range of φ_I 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.