SEISMOLOGICAL COMPARISONS OF SOLAR MODELS WITH ELEMENT DIFFUSION USING THE MHD, OPAL, AND SIREFF EQUATIONS OF STATE

We compare the thermodynamic and helioseismic properties of solar mode ls evolved using three different equation of state (EOS) treatments: t he Mihalas, Dappen & Hummer EOS tables (MHD); the latest Rogers, Swens on, & Iglesias EOS tables (OPAL), and a new analytical EOS (SIREFF) de veloped by Swenson et al. All of the models include diffusive settling of helium and heavier elements. The models use updated OPAL opacity t ables based on the 1993 Grevesse & Noels solar element mixture, incorp orating 21 elements instead of the 14 elements used for earlier tables . The properties of solar models that are evolved with the SIREFF EOS agree closely with those of models evolved using the OPAL or MHD table s. However, unlike the MHD or OPAL EOS tables, the SIREFF in-line EOS can readily account for variations in overall Z abundance and the elem ent mixture resulting from nuclear processing and diffusive element se ttling. Accounting for Z abundance variations in the EOS has a small, but nonnegligible, effect on model properties (e.g., pressure or squar ed sound speed), as much as 0.2% at the solar center and in the convec tion zone). The OPAL and SIREFF equations of state include electron ex change, which produces models requiring a slightly higher initial heli um abundance, and increases the convection zone depth compared to mode ls using the MHD EOS. However, the updated OPAL opacities are as much as 5% lower near the convection zone base, resulting in a small decrea se in convection zone depth. The calculated low-degree nonadiabatic fr equencies for all of the models agree with the observed frequencies to within a few microhertz (0.1%). The SIREFF analytical calibrations ar e intended to work over a wide range of interior conditions found in s tellar models of mass greater than 0.25 M-. and evolutionary states fr om pre-main-sequence through the asymptotic giant branch (AGB). It is significant that the SIREFF EOS produces solar models that both measur e up to the stringent requirements imposed by solar oscillation observ ations and inferences, and are more versatile than EOS tables.