Enhancement of surface helium abundance in intermediate-mass main-sequence stars

Abstract

The evolution of rapidly rotating 8, 4, and 2 M⊙ main-sequence stars is considered together with hydrodynamical transfer in their interiors. The conditions under which turbulent erosion, semiconvection, and shear turbulence lead to partial mixing of the matter in the radiative envelope and central regions of the stars are determined. The enhancement of the surface helium abundance with time depends on both the intensity of partial mixing in their interiors and mass loss by the stellar wind. The ratio of the number densities of helium and hydrogen at the surface can rise by the end of main-sequence stage by ∼30% for a 8 M⊙ star and ∼10−20% for a 4 M⊙ star, depending on the mass-loss rate. Partial mixing of the matter in the radiative envelope and in the central region of the star can provide an explanation for the observed enhancement of the atmospheric helium abundances of early B stars toward the end of their main-sequence evolution. The enhancement of the surface helium abundance in a 2 M⊙ star is so small that it cannot be detected, and is appreciably lower than the enhancement beneath the surface.