2017ApJ...834...67M

Two eclipsing binaries in the USco association have recently yielded precise values of masses and radii for four low-mass members of the association. Standard evolution models would require these dM4.5-dM5 stars to have ages which are younger than those of more massive stars in the association by factors which appear (in extreme cases) to be as large as ∼3. Are the stars in the association therefore non-coeval? We suggest that the answer is no: by incorporating the effects of magnetic inhibition of convective onset, we show that the stars in USco can be restored to coevality provided the four low-mass member stars have vertical surface fields in the range 200-700 G. Fields of such magnitude have already been measured on the surface of certain solar-type stars in other young clusters.