EFFECTS OF ACCRETION ONTO MASSIVE MAIN-SEQUENCE STARS

We investigate the effect of mass accretion onto massive main sequence stars on their internal structure and evolution. Adopted accretion ra tes and accretion time scales are in the range applying to massive Cas e B binary systems. For the first time, we incorporate the influence o f molecular weight gradients on convection in this context. In contras t to earlier studies, we find that the so called ''rejuvenation'' of t he mass accreting star does not always take place. Rather, stellar mod els with a chemical structure unlike that of single stars may be obtai ned. We investigate which physical parameters determine whether rejuve nation occurs, namely the fraction of the core hydrogen burning time s pent until the onset of accretion, the amount of matter accreted, the initial mass of the accreting star, and the efficiency of convection i n regions of stabilizing mean molecular weight gradients. Further an, the evolution of accretion stars until central carbon ignition is inve stigated. We find that the main result of non-rejuvenation is a much s maller helium core mass and accordingly longer core helium burning tim es compared to single stars. This may lead to blue supergiant position s adjacent to the main sequence band in the HR diagram, and to blue su pergiant pre-supernova configurations. Possible relations to the obser ved distribution of luminous stars in the HR diagram of the LMC and to the progenitor df supernova 1987 A are discussed.