Our Sun. I. The Standard Model

We have computed a number of standard solar models. Our best Standard Sun has Y = 0.278, Z = 0.0194, and α = 2.1, using L_⊙ ≡ 3.86 × 10³³ ergs s^–1, R_⊙ ≡ 6.96 × 10¹⁰ cm, t _⊙ = 4.54 Gyr, and the value of Z/X = 0.02766 published by Grevesse in 1984; we used LAOL opacities, including molecular opacities, nuclear rates published by Caughlin and Fowler in 1988, and neutrino capture cross sections, published by Bahcall and Ulrich in 1988. We predicted a ³⁷Cl neutrino capture rate of 7.7 SNUs, which would be observed if all solar neutrinos reach Earth (i.e., in the absence of such effects as the MSW neutrino oscillation effect which could reduce the flux of electron neutrinos). This is in agreement with results of other authors, but a factor of 4 larger than the average observed rate. We predict neutrino capture rates for other targets: 26 SNUs for ⁸¹Br, 17 SNUs for ⁹⁸Mo, 125 SNUs for ⁷¹Ga, 615 SNUs for ¹¹⁵In, and 47 SNUs for ⁷Li.

We have investigated the sensitivity of the standard solar model to uncertainties in the solar luminosity, solar age, and observed Z/X ratio as well as to changes in molecular opacities, pressure ionization effects in the envelope, and nonequilibrium ³He energy contributions. Of these, only the uncertainty in Z/X has a significant effect on the solar Y value and the ³⁷Cl neutrino capture rate: use of the older value published by Ross and Aller in 1976 of Z/X = 0.02282 decreases Y by 0.014 and decreases the ³⁷Cl rate by 1.5 SNUs. While the 1989 work of Guenther, Jaffe, and Demarque has shown that Y can be significantly affected by the choice of stellar interior opacities, we find that even large changes in the low-temperature molecular opacities have no effect on Y, nor even on conditions at the base of the convective envelope. The large molecular opacities do cause a large increase in the mixing-length parameter α but do not cause the convective envelope to reach deeper. The temperature remains too low for lithium burning, and there is no surface lithium depletion, let alone the observed depletion of a factor of 100: the lithium problem of the standard solar model remains.

Subject headings: neutrinos — nucleosynthesis — Sun: interior