USING SOLAR P-MODES TO DETERMINE THE CONVECTION ZONE DEPTH AND CONSTRAIN DIFFUSION-PRODUCED COMPOSITION GRADIENTS

Low- and intermediate-degree (1 = 5-60) solar p-mode frequencies, whic h are sensitive to the solar structure near the bottom of the convecti on zone, are used to find the convection zone depth and to constrain t he size and shape of composition gradients produced by diffusive settl ing of helium and heavier elements. We have calculated the evolution a nd nonadiabatic oscillation frequencies of solar models incorporating the latest Mihalas, Dappen, Hummer, and Mihalas (MHD) equation of stat e, OPAL opacities, and diffusion of hydrogen, helium, and several heav ier elements. Comparisons between observed and calculated p-mode frequ encies indicate (1) the solar convection zone base is at 0.712 +/- 0.0 01 R.; (2) the solar convection zone helium mass fraction Y is very ne ar 0.240 (+/-0.005), or about 0.024 less than the initial abundance re quired to match the solar luminosity; (3) the diffusion-produced Y and Z composition gradients below the convection zone are fairly steep an d not smoothed significantly by turbulence. This implies that turbulen t mixing below the convection zone cannot cause most of the solar lith ium destruction during the main sequence.