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Astron. Astrophys. 335, 959-968 (1998)

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Appendix A: input physics

The calculations are performed with the Toulouse-Geneva stellar evolution code, in which the equations described in Sect. 3.1 have been implemented. Transport processes are engaged on the arrival on the zero age main sequence. The stellar models were obtained with the following input micro-physics:

  • Microscopic diffusion: The tables of collision integrals by Paquette et al. (1986) are used to calculate the diffusion coefficients.

  • Opacities: The radiative opacities for the interior are taken from Iglesias & Rogers (1996). The low-temperature opacities are from Alexander & Ferguson (1994) which account for a wide variety of atomic and molecular species.

  • Nuclear cross-sections: All the thermonuclear reaction rates are due to Caughlan & Fowler (1988), with the exception of the 17O(p,[FORMULA]F and 17O(p,[FORMULA]N for which we adopt the values from Landré et al. (1990). Screening factors for the reaction rates are taken into account according to the analytical prescription by Graboske et al. (1973).

  • Abundances: We use the proto-solar lithium abundance derived from carbonaceous chondrites by Anders & Grevesse (1989), log N(Li)=3.31[FORMULA]0.04, as cosmic value. This Li abundance is consistent with the photospheric Li abundances in F stars of young open clusters like [FORMULA]Per (Balachandran et al. 1996) and the Pleiades (Soderblom et al. 1993).

    We use [Fe/H]=+0.12 (Cayrel de Strobel et al. 1997) for the logarithm of the number abundances of iron to hydrogen relative to the solar values. The initial helium content is determined by Y[FORMULA]Y[FORMULA]Z)Z, where the value for [FORMULA]Y[FORMULA]Z we use (2.524) was obtained by calibration of the solar models with the micro-physics described above and a solar metal abundance Z/X=0.0244 (Grevesse & Noels 1993). The relative ratios for the heavy elements correspond to the mixture by Grevesse & Noels (1993). We take the same isotopic ratios than Maeder (1983).

  • Convection: Turbulent convection is described by the classical mixing-length theory of Böhm-Vitense (1958). Calibration of the solar model gave us a value of 1.6 for the free parameter [FORMULA], ratio of the mixing-length to the pressure scale height. Neither overshooting, nor convective penetration have been considered in this work.

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© European Southern Observatory (ESO) 1998

Online publication: June 26, 1998