THE SCREENED NUCLEAR-REACTION RATES AND THE SOLAR-NEUTRINO PUZZLE

This paper reexamines the nuclear reaction rates used for hydrogen burning in the classical framework of stellar evolution. The screening effect is discussed, and the formalisms generally used appear not accurate enough for the determination of the solar neutrino fluxes. So we analyze screening with the more realistic potential suggested by Mitler. We compare the first-order description which allows factorization of the screening correction with a complete calculation in which an ellipsoidal charge distribution around reacting ions is introduced. Since the radial deformation of the charge distribution appears to have a small effect and is of the same order as and of opposite sign to the dynamical effect of the surrounding ions, we conclude that the factorized prescription of Mitler seems really easy to implement and appropriate within 2% accuracy for the whole hydrogen-burning phase and stellar masses between 0.6 and 20 M.. It leads to an enhancement factor of the reaction rates located between those of Salpeter weak screening and Graboske et al, intermediate screening. Then we consider each reaction rate of the p-p chain, and it appears that two reactions may still be modified significantly beyond the ''conventional error bar'': the electronic capture on Be-7 and the proton interaction on the same nucleus, both of which influence the B-8 neutrino flux. Considering reasonable variations of the nuclear reaction rates of the p-p chains, compatible with the present experimental and theoretical situation, the solar neutrino discrepancy between experiments and predictions could be considerably reduced.