THE EFFECT OF DIFFUSION ON SOLAR EVOLUTION

We examine the effect of element diffusion on the evolution of the Sun . We include partial ionization and diffusion of all elements, a new n etwork of nuclear reactions which includes the reactions of the light elements like Li-7 and Be-9, and we follow the full evolution and depl etion of these elements from the pre-main-sequence phase to the presen t stage. Our major conclusions are: The effect of diffusion on the pre dicted primordial hydrogen and helium abundances is DELTAX = +0.025, D ELTAY = -0.024. However, the global effect of diffusion on the structu re of the Sun is small, as was found by Bahcall & Pinsonneault. More s pecifically, we find that diffusion leads to DELTAT(c)/T = +0.003, DEL TArho(c)/rho = 0.01 [where DELTAQ(c) = Q(with diffusion) - Q(no diffus ion) for a quantity Q at the center] at the center and DELTAT(s)/T = 0.05%, DELTArho(s)/rho = 0.09% at the surface. [Here DELTAQ(s) = Q(wit h diffusion) - Q(no diffusion) for a quantity Q on the surface]. We ob tain the depletion of light elements Li-7 and Be-9 due to both diffusi on and nuclear reactions and find that diffusion enhances the depletio n at the surface by 1.08.