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Astron. Astrophys. 326, 187-194 (1997)
1. Introduction
Recent improvements concerning radiative opacities and equation of state removed significant discrepancies between observations and theory of stellar evolution. Updated solar models (Turck-Chièze et al. 1993; Charbonnel & Lebreton 1993; Gabriel 1994a; Basu & Thompson 1996) are in good agreement with the solar helioseismic data.
In this work models are constructed for the Sun starting when model
becomes stable against gravitational contraction, with different EOS
and opacities, by using Ezer's stellar evolutionary code (EC) (Ezer
& Cameron 1967;
Y
ldz&
Kzloglu
1995). EOS of MHD (Mihalas et al. 1990; and references therein) which
uses an approach known as chemical picture is based on minimization of
the free energy and is applicable for stellar modeling. We have chosen
opacity tables of OPAL (Iglesias et al. 1992). Since those tables do
not extend to low temperatures, Alexander & Ferguson opacity
(1994) is used for outer layers of the stars. The models computed for
comparison are obtained using Cox & Stewart (1970) (CS) opacity
and EC EOS which takes into account degenerate electrons and pressure
ionization in an artificial way. Since OPAL opacity, near the bottom
of the convective zone, is larger than the CS opacity by a factor of
about two, it strongly affects the structure of the convective zone.
In performing the solar calibration, the lower luminosity due to
enhancement of the opacity also in radiative regions is balanced by
decreasing H (or increasing He) abundance. Thus, the abundance of H by
mass may also be significantly altered.
It was shown that the MHD EOS removes significantly the
discrepancies between the observational and the theoretical
frequencies of the solar oscillations (Christensen-Dalsgaard et al.
1988). However we do not mention about the solar oscillations. The
incorporation of the Coulomb interaction in EOS reduces both of the
pressure and the energy. To compensate this reduction in pressure, the
mean molecular weight per free particle, that is He abundance, becomes
smaller, in contrast to the effect of OPAL opacity. The application of
MHD EOS requires the calculation of ionization and internal energy of
each chemical species. Avoiding the time consuming calculation of the
Saha equation for the heavy elements, we use two different methods,
namely, Henyey (Gabriel 1994b) method and Gabriel &
Yldz
method (1995).
In Sect. 2 we present the basic features of the code. The EOS with its computational method and opacity used in the construction of the solar models are given in Sect 3. Sect. 4 is devoted to the influence of MHD EOS including the results of the two methods for the ionization and internal energy of heavy elements. In Sect. 5 we give the results of the solar models with different EOS and opacity.
© European Southern Observatory (ESO) 1997
Online publication: April 20, 1998
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