Why compositional convection cannot explain substellar objects sharp spectral type transitions

TL;DR

This paper argues that compositional convection cannot explain the sharp spectral transitions in substellar objects because turbulence would increase, not decrease, the thermal gradient, contradicting previous hypotheses.

Contribution

It clarifies that turbulence associated with compositional convection actually transports energy downward, opposing the idea that it causes a more isothermal atmosphere during spectral transitions.

Findings

Turbulent transport increases the thermal gradient in stratified atmospheres.

Turbulence mixes entropy, leading to downward energy transport.

Compositional convection cannot explain sharp spectral features in brown dwarfs.

Abstract

As brown dwarfs and young giant planets cool down, they are known to experience various chemical transitions --- for example from CO rich L-dwarfs to methane rich T-dwarfs. Those chemical transitions are accompanied by spectral transitions whose sharpness cannot be explained by chemistry alone. In a series of articles, Tremblin et al. proposed that some of the yet unexplained features associated to these transitions could be explained by a reduction of the thermal gradient near the photosphere. To explain, in turn, this more isothermal profile, they invoke the presence of an instability analogous to fingering convection -- compositional convection -- triggered by the change in mean molecular weight of the gas due to the chemical transitions mentioned above. In this short note, we use existing arguments to demonstrate that any turbulent transport, if present, would in fact increase the thermal gradient. This misinterpretation comes from the fact that turbulence mixes/homogenizes entropy (potential temperature) instead of temperature. So, while increasing transport, turbulence in an initially stratified atmosphere actually carries energy downward, whether it is due to fingering or any other type of compositional convection. These processes therefore cannot explain the features observed along the aforementioned transitions by reducing the thermal gradient in the atmosphere of substellar objects. Understanding the microphysical and dynamical properties of clouds at these transitions thus probably remains our best way forward.