THE EFFECTS OF HELIUM DIFFUSION ON SOLAR P-MODE FREQUENCIES

Precise solar models are computed with and without including helium diffusion. We show that the inclusion of helium diffusion according to the Bahcall-Loeb prescription causes systematic shifts of order 1-5 muHz in the predicted p-mode oscillation frequencies. Calculations with the diffusion coefficients multiplied and divided by two show that the inferred effects are linearly proportional to the magnitude of the diffusion. Relative to a model that does not include helium diffusion, the models with diffusion increase the frequencies of low-l modes and decrease the frequencies of high-l modes. The effect on the frequencies of intermediate-l modes depends on whether or not the modes penetrate significantly below the convection zone. Future observations may reveal this characteristic signature of diffusion. Unfortunately, systematic discrepancies of order 10 muHz out of 3000 muHz prevent, at present, a direct test of the signature of helium diffusion. Improvements in the treatment of nonadiabatic effects and in the opacity and equation of state may remove the largest discrepancies and permit measurements that are specifically sensitive to helium diffusion.