Apparent Non-Coevality among the Stars in Upper Scorpio: Resolving the Problem using a Model of Magnetic Inhibition of Convection

Two eclipsing binaries in the USco association have recently yielded precise values of masses and radii for 4 low-mass members of the association. Standard evolution models would require these dM4.5 - dM5 stars to have ages which are younger than the ages 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 that the 4 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.