Structure of the atmospheres

If Fig. 1 we show the temperature structure for models with varying $\log(g)$ for 3 effective temperatures. For the models with $\hbox{$\,T_{\rm eff}$}=5600\,{\rm K}$, and the smallest gravity, the atmosphere does not become convective within the maximum optical depth of the model, $\ifmmode{\tau_{\rm std}}\else\hbox{$\tau_{\rm std}$}\fi=100$. The apparent flattening of the temperature drop in the outer layers of the atmosphere is a result of the automatic ``compression'' of the optical depth scale toward the outer edge of the model atmosphere. This means that the radii of the different optical depth points are much closer to each other in the outer atmosphere compared to $\ifmmode{\tau_{\rm std}}\else\hbox{$\tau_{\rm std}$}\fi\approx 1$. This is most pronounced in the coolest models shown, whereas the ``scale compression'' is nearly non-existent for the model with $\hbox{$\,T_{\rm eff}$}=5600\,{\rm K}$. We have also compared model atmospheres calculated with the same parameters ($\hbox{$\,T_{\rm eff}$}$, $\log(g)$) but for different stellar masses. For $\hbox{$\,T_{\rm eff}$}=3600\,{\rm K}$, $\log(g)=0.0$ and solar abundances, we find only small temperature differences for stellar masses between 2.5 and $7.5\hbox{$\,$M$_\odot$}$. However, the absolute differences in the synthetic spectra are equivalent to a significant change in $\log(g)$ (see below) whereas the relative differences in the spectra are small. The radial extension of the atmospheres, here defined simply as the ratio of the outer radius $R_{\rm out}$ and the inner radius $R_{\rm in}$of the atmosphere, is typically less than 20% (see Fig. 2). We have ignored all models that become unstable due to large radiative accelerations (low $\log(g)$ models with very low or high temperatures), such atmospheres have to be described by stellar wind models (Aufdenberg et al., in preparation) and typically have much larger radial extensions.