HELIUM DIFFUSION IN THE SUN

We calculate improved standard solar models using the new Livermore (OPAL) opacity tables, an accurate (exportable) nuclear energy generation routine which takes account of recent measurements and analyses, and the recent Anders-Grevesse determination of heavy element abundances. We also evaluate directly the effect of the diffusion of helium with respect to hydrogen on the calculated neutrino fluxes, on the primordial solar helium abundance, and on the depth of the convective zone. Helium diffusion increases the predicted event rates by about 0.8 SNU, or 11 % of the total rate, in the chlorine solar neutrino experiment, by about 3.5 SNU, or 3 %, in the gallium solar neutrino experiments, and by about 12 % in the Kamiokande and SNO solar neutrino experiments. The best standard solar model including helium diffusion and the most accurate nuclear parameters, element abundances, and radiative opacity predicts a value of 8.0 SNU +/- 3.0 SNU for the Cl-37 experiment and 132(-17)+21 SNU for the Ga-71 experiment, where the uncertainties include 3-sigma errors for all measured input parameters. The inferred primordial solar helium abundance is Y = 0.273, increased 0.4 % by taking account of helium diffusion, and the calculated depth of the convective zone is R = 0.707 R., decreased 2.0 % by taking account of helium diffusion.