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In the stars and the planets, the condensation of chemical species, such as H$_2$O or Fe, inevitably leads to the presence of compositional gradients. Since this process occurs on a relatively short scale height, it may inhibit convection.
We make use of the Ledoux instability criterion (i.e. we compare the superadiabatic gradient to the compositional gradient) to determine whether the convection is affected by condensation or not. We show that this criterion is not satisfied at the condensation level of both water and iron in Jupiter and Saturn, due to the fact that convective regions are almost adiabatic in these planets. The luminosity of brown dwarfs being much larger, we expect this phenomenon to be more marginal for these objects.
The medium can be convective even if the Ledoux criterion is not satisfied. Thus, we estimate the importance of various processes which oppose to the stabilizing effect of condensation: the removal of condensates by sedimentation, moist convection, overshooting convection and overstability which stems from the larger thermal diffusion than microscopic diffusion.
This research is supported by an ESA fellowship.
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