The role of convection, overshoot, and gravity waves for the transport of dust in M dwarf and brown dwarf atmospheres

Abstract

Context. Observationally, spectra of brown dwarfs indicate the presence of dust in their atmospheres while theoretically it is not clear what prevents the dust from settling and disappearing from the regions of spectrum formation. Consequently, standard models have to rely on ad hoc assumptions about the mechanism that keeps dust grains aloft in the atmosphere. Aims. We apply hydrodynamical simulations to develop an improved physical understanding of the mixing properties of macroscopic flows in M dwarf and brown dwarf atmospheres, in particular of the influence of the underlying convection zone. Methods. We performed two-dimensional radiation hydrodynamics simulations including a description of dust grain formation and transport with the CO5BOLD code. The simulations cover the very top of the convection zone and the photosphere including the dust layers for a sequence of effective temperatures between 900 K and 2800 K, all with log g = 5 assuming solar chemical composition.Results. Convective overshoot occurs in the form of exponentially declining velocities with small scale heights, so that it affects only the region immediately above the almost adiabatic convective layers. From there on, mixing is provided by gravity waves that are strong enough to maintain thin dust clouds in the hotter models. With decreasing effective temperature, the amplitudes of the waves become smaller but the clouds become thicker and develop internal convective flows that are more efficient in transporting and mixing material than gravity waves. The presence of clouds often leads to a highly structured appearance of the stellar surface on short temporal and small spatial scales (presently inaccessible to observations). Conclusions. We identify convectively excited gravity waves as an essential mixing process in M dwarf and brown dwarf atmospheres. Under conditions of strong cloud formation, dust convection is the dominant self-sustaining mixing component.

The ultracool-field dwarf luminosity-function and space density from the Canada-France Brown Dwarf Survey

“…Note that our values disagree for early T-dwarfs. However, the physics at the L/T transition is difficult to model, not only due to the cooling of the atmosphere but also to the clearing of the atmosphere that needs to model properly the hydrodynamics and the clouds formation (Freytag et al 2010), making the timescales uncertain. If confirmed, this low value would back Burgasser et al (2007) suggestion that the L/T transition occurs rapidly.…”

Accretion and diffusion in white dwarfs

“…For the case of DA white dwarfs, this was studied with an extensive comparison of twodimensional radiation-hydrodynamic simulations with 1D structures by Freytag et al (1996). Even the definition of the lower boundary is ambiguous: depending on whether one uses the classical stability criterion, the layers with significant convective flux, or the layers with non-zero velocities, the resulting mass in the "convection zone" can differ by orders of magnitude.…”

On the production of He, C, and N by low- and intermediate-mass stars: a comparison of observed and model-predicted planetary nebula abundances

“…However, some overshooting at convective boundaries does occur as a consequence of the adoption of the numerical algorithm for the determination of the convective boundaries (Lattanzio 1986). On the contrary the FRUITY, ATON and LPCODE models adopt different implementations of an exponentially decaying mixing coefficient (Freytag et al 1996) beyond the formally convective boundaries and with different intensities. While FRUITY models include strong overshooting at the bottom of the CE but no overeshooting at the PDCZ, LPCODE models adopt a moderate overshooting at the base of the PDCZ and no overshooting at the bottom of the CE.…”