Dust in Brown Dwarfs IV. Dust formation and driven turbulence on mesoscopic scales

TL;DR

This paper investigates dust formation in brown dwarf atmospheres through a turbulence model, revealing how stochastic waves induce dust nucleation, leading to patchy, variable dust structures that affect optical properties.

Contribution

It introduces a mesoscopic turbulence model with superimposed waves to simulate dust formation and variability in brown dwarf atmospheres, providing criteria for small-scale closure models.

Findings

Turbulence causes patchy dust formation with lane and curl structures.

Small disturbances significantly influence dust nucleation and optical thickness.

Dust properties vary considerably during formation, affecting observational features.

Abstract

Dust formation in brown dwarf atmospheres is studied by utilising a model for driven turbulence in the mesoscopic scale regime. We apply a pseudo-spectral method where waves are created and superimposed within a limited wavenumber interval. The turbulent kinetic energy distribution follows the Kolmogoroff spectrum which is assumed to be the most likely value. Such superimposed, stochastic waves may occur in a convectively active environment. They cause nucleation fronts and nucleation events and thereby initiate the dust formation process which continues until all condensible material is consumed. Small disturbances are found to have a large impact on the dust forming system. An initially dust-hostile region, which may originally be optically thin, becomes optically thick in a patchy way showing considerable variations in the dust properties during the formation process. The dust appears in lanes and curls as a result of the interaction with waves, i.e. turbulence, which form larger and larger structures with time. Aiming on a physical understanding of the variability of brown dwarfs, related to structure formation in substellar atmospheres, we work out first necessary criteria for small-scale closure models to be applied in macroscopic simulations of dust forming astrophysical systems.