Standard Solar Models in the Light of New Helioseismic Constraints. I. The Solar Core

Abstract

In this paper, we examine a new, updated solar model that takes advantage of the recent reexamination of nuclear reaction rates and the microscopic diffusion of helium and heavy elements. Our best model fits the helioseismic data reasonably well, giving the base of the convective zone at Rbcz = 0.715, the photospheric helium in mass fraction as 0.243, and the sound-speed square difference between the Sun and the model as δc2/c2 < 1%. This model leads to a reestimate of neutrino fluxes, giving 7.18 SNU for the chlorine experiment, 127.2 SNU for the gallium detector, and 4.82 106 cm-2 s-1 for the 8B neutrino flux. Acoustic-mode predictions are also estimated. We then consider the radiative zone and discuss what we learn from such a model when confronted with the present helioseismic constraints from space experiments aboard SOHO. We present three models that respect these constraints and better fit the seismic observations by taking advantage of the known physical uncertainties—nuclear reaction rates, CNO abundances, and microscopic diffusion. We also study some current questions, such as the possibility of mixing in the nuclear core, the revision of the solar radius, and the influence of the solar age. We conclude that the standard model, inside its inherent uncertainties, is robust in light of the present acoustic-mode detection and that mixing in the core is not really favored, even though a proper understanding of the angular momentum evolution with time has not yet been reached. The initial solar helium abundance seems more and more constrained; this study supports an initial abundance between 0.273 and 0.277 in mass fraction. This analysis allows us to define minimal values for neutrino predictions, compatible with present seismic results. We note that a reduction of about 30% in chlorine and water detectors, which is more than half the discrepancy with the experimental results, is still supported by the present study. This work also emphasizes the fact that acoustic-mode determination does not put strong constraints on the nuclear plasma characteristics. Finally, we estimate g-mode frequencies in a range that may be accessible to the satellite SOHO; these results emphasize the substantially improved sensitivity of these modes to details of the nuclear solar core, and show the frequency dependence of these modes for the different models previously discussed.