The effect of starspots on the radii of low-mass pre-main-sequence stars

Abstract

Apolytropicmodel is used to investigate the effects of dark photospheric spots on the evolution and radii of magnetically active, low-mass (M < 0.5M⊙), pre-main-sequence (PMS) stars. Spots slow the contraction along Hayashi tracks and inflate the radii of PMS stars by a factor of (1-β)-N compared to unspotted stars of the same luminosity, where β is the equivalent covering fraction of dark starspots and N ≃ 0.45 ± 0.05. This is a much stronger inflation than predicted by Spruit & Weiss for main-sequence stars with the same β, where N ~ 0.2-0.3. These models have been compared to radii determined for very magnetically active K- and Mdwarfs in the young Pleiades and NGC 2516 clusters, and the radii of tidally locked, low-mass eclipsing binary components. The binary components and zero-age main-sequence K-dwarfs have radii inflated by ~10 per cent compared to an empirical radius-luminosity relation that is defined by magnetically inactive field dwarfs with interferometrically measured radii; lowmass M-type PMS stars, that are still on their Hayashi tracks, are inflated by up to~40 per cent. If this were attributable to starspots alone, we estimate that an effective spot coverage of 0.35< β < 0.51 is required. Alternatively, global inhibition of convective flux transport by dynamogenerated fields may play a role. However, we find greater consistency with the starspot models when comparing the loci of active young stars and inactive field stars in colour-magnitude diagrams, particularly for the highly inflated PMS stars, where the large, uniform temperature reduction required in globally inhibited convection models would cause the stars to be much redder than observed. © 2014 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society.