AND ASTROPHYSICS Helioseismic constraints on new solar models from the MoSEC code

Abstract. Evolutionary solar models are computed using a new stellar evolution code, MoSEC (Modular Stellar Evolution Code). This code has been designed with carefully controlled truncation errors in order to achieve a precision which reflects the increasingly accurate determination of solar interior struc-ture by helioseismology. A series of models is constructed to investigate the effects of the choice of equation of state (OPAL or MHD-E, the latter being a version of the MHD equation of state recalculated by the author), the inclusion of helium and heavy-element settling and diffusion, and the inclusion of a sim-ple model of mixing associated with the solar tachocline. The neutrino flux predictions are discussed, while the sound speed of the computed models is compared to that of the sun via the latest inversion of SOI-MDI p-mode frequency data. The com-parison between models calculated with the OPAL andMHD-E equations of state is particularly interesting because theMHD-E equation of state includes relativistic effects for the electrons, whereas neither MHD nor OPAL do. This has a significant ef-fect on the sound speed of the computed model, worsening the agreement with the solar sound speed. Using the OPAL equa-tion of state and including the settling and diffusion of helium and heavy elements produces agreement in sound speed with the helioseismic results to within about 0.2%; the inclusion of mixing slightly improves the agreement