Two Böhm-Vitense Gaps in the Main Sequence of the Hyades

Hipparcos proper motions and trigonometric parallaxes allow the derivation of secular parallaxes that fix the distances to individual stars in the Hyades to an accuracy of ~2%. The resulting color-absolute magnitude diagram for 92 high-fidelity single members of the cluster displays a very narrow main sequence, with two turnoffs and associated gaps. These occur at the locations where the onset of surface convection affects the B - V colors, as predicted by Böhm-Vitense 30 years ago. The new distances provide stringent constraints on the transformations of colors and absolute magnitudes to effective temperatures and luminosities and on models of stellar interiors.