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Astron. Astrophys. 331, 1011-1021 (1998) 5. Thermodynamics and convection theoryBefore entering the discussion of the possible explanations for
both the presently observed solar First we will discuss the effect of the thermodynamic treatment on Li-depletion in pre-MS, mainly through the value of the adiabatic gradient which, in the end, determines the thickness of the convective envelope. In Fig. 1 we show the standard track compared to a track (MIHEOS) computed assuming, as the only difference, the Mihalas et al. (1988) equation of state in place of the OPAL one. Remember that in DM94 the Mihalas et al. EOS was adopted. The MIHEOS track shows lower Li-depletion than the STD one by a
factor 4. In order to understand the reason for this apparently large
difference between results arising from two apparently equally updated
thermodynamic treatments, we computed several tracks by increasing
from 5000 K (as in the standard track) to larger and larger values of
T the point at which we switch from the Mihalas et al. to the OPAL
EOS. The final Li-abundances were the same as for the STD track as
long as the Mihalas et al. EOS was substituted only up to relatively
low T's (that is: below H and He-ionization, The difference is then in the treatment of the partial ionization regime, namely in the value of the adiabatic gradient in those regions which largely influence the convective envelope thickness. It is presently hard to say which of the two EOS' is more physically sound: we chose the OPAL one both for consistency with the opacities and because extensive tables are provided for several H-abundances at a given Z. However, a warning immediately arises: since pre-MS Li-depletion is largely affected by the value of the adiabatic gradient in partial ionization regime, no interpolation between pure-H and pure-He compositions can be adopted in these computations, since pure element compositions do not include the reciprocal influence of ionization, which is instead dominant for the effect under scrutiny (Saumon et al. 1995). An indirect proof of the importance of having a correct treatment of the adiabatic gradient will result at the end of Sect. 8, where we discuss the computations including a variable magnetic field. Next test deals with the influence of the convection model on
Li-depletion. In Fig. 1 we show the difference between the
standard track and an MLT one (MLT track) computed with the value of
the free parameter fitting the present sun, that is:
![]() ![]() ![]() ![]() © European Southern Observatory (ESO) 1998 Online publication: March 3, 1998 ![]() |