On the relationship between the size and surface coverage of starspots on magnetically active low-mass stars

Abstract

We present a model that predicts the light curve amplitude distribution for\nan ensemble of low-mass magnetically active stars, under the assumptions that\nstellar spin axes are randomly orientated and that cool starspots have a\ncharacteristic scale length and are randomly distributed across the stellar\nsurfaces. The model is compared with observational data for highly magnetically\nactive M-dwarfs in the young cluster NGC 2516. We find that the best fitting\nstarspot scale length is not constrained by these data alone, but requires\nassumptions about the overall starspot filling factor and starspot temperature.\nAssuming a spot coverage fraction of 0.4+/-0.1 and a starspot to unspotted\nphotosphere temperature ratio of 0.7+/-0.05, as suggested by the inflated radii\nof these stars compared to evolutionary model predictions and by TiO band\nmeasurements on other active cool stars of earlier spectral type, the\nbest-fitting starspot angular scale length is 3.5(+2)(-1) degrees, or a linear\nscale length of \\sim 25000 km. This linear scale length is similar to large\nsunspot groups, but 2--5 times smaller than the starspots recently deduced on\nan active G-dwarf using eclipse mapping by a transiting exoplanet. However, the\nbest-fitting spot scale length in the NGC 2516 M-dwarfs increases with the\nassumed spot temperature ratio and with the inverse square root of the assumed\nspot filling factor. Hence the light curve amplitude distribution might equally\nwell be described by these larger spot scale lengths if the spot filling\nfactors are <0.1 or the spot temperature ratio is >0.9.\n