The Gap in the Two-Color Diagram of Main-Sequence Stars

Abstract

in the two-color diagrams of main-sequence stars a gap of A(B - V) 0.07 is expected at B - V 0.25 due to the abrupt onset of convection. This gap is observed for field stars. it is also indicated for many clusters; however, it appears at different B - V and is less pronounced the more it is shifted to the red. Since in these clusters the stars on the blue side of the gap show large rotation, we interpret the shift as showing that large rotational velocities shift the onset of convection to lower Te . The abrupt decrease of the rotational velocities occurs at still lower Teff when convective energy transport becomes important in the helium ionization zones. Subject headings: convection - interiors, stellar - open clusters - rotation, stellar