## 6. The convective flux as a function of andBecause the solar model with "overshooting" better fits the observations than that without "overshooting", it has been extrapolated that models with "overshooting" must fit also the observations of stars with temperatures, gravities, and metallicities different from the solar ones. However, if opacity, rather than convection, is the cause of the too high solar computed energy distribution, "overshooting" could make the fit worse in other stars with different absorption lines and different convection than in the Sun. To have an idea of the importance of the convection and of the effect of the "overshooting" as a function of the model parameters, we compared, for different , , and [M/H], the / - log relations for convective fluxes computed with the "overshooting" option switched both on and off. For a few models we added also the relation for the only mixing-length (ML) obtained by dropping both the Lester et al. (1982)(HAO) and the "approximate overshooting" (AO) modifications (Sects. 2.2 and 2.3). Fig. 11 shows the / - and the T- log relations for the Sun. The / -log relations from the SUNK94 model, the SUNNOVERC125 model, and a solar model without the HAO and AO modifications are compared in the upper plot. The T-log relations from the SUNK94 and SUNNOVERC125 models are compared in the lower plot. At log =0, the difference of / from the SUNK94 and the SUNNOVERC125 model is 0.087, namely it is just the value of / in the SUNK94 model, because no convective flux is predicted at log =0 when the "overshooting" option is switched off. We already discussed in the previous sections the effects of the different convection options on the emerging radiation.
Fig. 12 shows, on the left, the changes of the
/ - log
relations as a function of
for =4 and solar
metallicity. The corresponding T-log
relations are plotted on the right. The convective zone increases with
decreasing , but it is gradually shifted towards
larger depths, so that the structure of the superficial layers is less
and less affected by the convection when
decreases, until the convection zone rises again toward the upper
layers for temperatures lower than 4500K, owing to the dissociation of
H
Analogous plots drawn for different gravities, same , and same [M/H] show that the convective flux decreases with decreasing gravity, owing to the decreasing density. At a given effective temperature and gravity, the convective flux increases with decreasing metallicity owing to the increasing gas pressure and decreasing electron pressure which cause a growth of the hydrogen ionization zone. Table 3 shows which models are affected by convection for gravities ranging from =5.0 to =1.0. Furthermore, for the metallicities [M/H]=0 and [M/H]=-3, it lists the models which show the largest difference, at =1, between the / computed with the "overshooting" option switched on and off respectively. The maximum effective temperature of models affected by the different convection options decreases with decreasing .
© European Southern Observatory (ESO) 1997 Online publication: July 3, 1998 |