PHYSICS OF ACCRETION ONTO YOUNG STARS .2. STRUCTURE AND EVOLUTION OF ACCRETING STARS

We present new computations of pre-main sequence evolution for stars t hat accrete a substantial part of their mass. We compute several model s with different mass accretion rates, focusing on the effect of deute rium burning on stellar evolution during the accretion phase. We treat accretion not only as a surface boundary condition but as a global ph enomenon that affect also the inner parts of the star, using a formali sm developed by Siess & Forestini (1996a). We study the effect of vary ing the mass distribution of accreted matter into the star, and analyz e the influence of different chemical compositions for the accreted ma tter. We also take into account the possible deposition of accretion e nergy transported by the accreted matter. We show that accretion can l ead to substantial changes in the pre-main sequence evolutionary track s in the Hertzsprung-Russell diagram (HRD). When accretion stops, the star relaxes to its ''standard'' track in the HRD, and this, independe ntly of the previous accretion rate.