Typically, a cool stellar atmosphere has only one convective zone at the bottom of the atmosphere whereas the top of the atmosphere is (and has to be) in radiative equilibrium. The convective zone at the bottom of the atmosphere connects to the convective envelope of the interior of the star. However, our calculations indicate that the convective region at the bottom of the atmosphere can be disrupted by the onset of an isolated radiative zone within specific parameter ranges. These ranges are illustrated in Fig. 3. Each symbol represents a model with multiple convective and radiative zones. The plot shows that a continuous and not an arbitrary parameter range exhibits this behavior.
We investigated the cause of this effect and found it to be due to the relative strengths of H- absorption and H2O absorption. H- absorption is strongest in the inner part of the atmosphere whereas H2O absorption is strongest in the outer part. The maximum of the H2O absorption is in layers of the atmosphere with electron temperatures of roughly 2500 to 3500 K. If in this region the H- absorption is weak enough so that the slope with depth of the overall absorption coefficient is significantly affected by H2O absorption, an inner radiative zone forms. In that case the total absorption coefficient drops fast enough toward the outer boundary to make the atmosphere transparent enough to form a radiative zone. As soon as water forms and the H2O absorption becomes strong enough, the energy is more efficiently carried by convection until the final radiative zone forms at the very outside of the atmosphere.
For the hottest models with the lowest log(g) (i.e. the models left and below the ``multiple zone strip'' in Fig. 3) the single convective zone at the bottom of the atmosphere is substantially different from that of the models right above the ``multiple zone strip''. For the hot models with low log(g), the water absorption will never become strong enough to change the slope of the total absorption and the intermediate radiative zone becomes large enough to remove the intermediate convective zone. In the cool models with high log(g), the water absorption is strong already deep inside the atmosphere and dominates the slope of the total absorption coefficient.
This behavior is demonstrated in Fig. 4 where the most important continuous absorption coefficients have been plotted against optical depth. In the top graph the water absorption changes the steep slope of the total absorption already in the innermost part. In the middle two plots, water forms further out and leaves a steep enough slope in the absorption coefficient to produce an intermediate radiative zone. In the graph at the bottom, the water absorption can no longer change the slope imposed by the H- absorption (they are almost parallel in the outer regions) and the energy is transported by radiation.