Star spot models for M-dwarfs in NGC 2516

By combining rotation periods with spectroscopic determinations of projected rotation velocity, Jackson et al. (2009MNRAS.399L..89J) have found that the mean radii for low-mass M-dwarfs in the young, open cluster NGC 2516 are larger than model predictions at a given absolute I magnitude or I-K color and also larger than measured radii of magnetically inactive M-dwarfs. The relative radius difference is correlated with magnitude, increasing from a few percent at M_I_=7 to greater than 50% for the lowest luminosity stars in their sample at M_I_~9.5.

Jackson et al. have suggested that a two-temperature star spot model is capable of explaining the observations, but their model requires spot coverage fractions of at least 50% in rapidly rotating M-dwarfs. Here we examine these results in terms of stellar models that include the inhibiting effects of magnetic fields on convective energy transport, with and without the effects of star spots. We find that a pure spot model is inconsistent with the color-magnitude diagram.