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 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 similar to 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.