Dust in brown dwarfs and extra-solar planets

Abstract

Context. Substellar objects have extremely long life spans. The cosmological consequence for older objects are low abundances for heavy elements, which in turn results in a wide distribution of objects over metallicity, hence over age. Within their cool atmosphere, dust clouds become a dominant feature, affecting the opacity and the remaining gas phase abundance of heavy elements. Aims. We investigate the influence of the stellar metallicity on the dust formation in substellar atmospheres and on the dust cloud structure and its feedback on the atmosphere. This work has implications for the general questions of star formation and of dust formation in the early universe. Methods. We utilise numerical simulations to solve a set of moment equations to determine the quasi-static dust cloud structure (Drift). These equations model the nucleation, the kinetic growth of composite particles, their evaporation, and the gravitational settling as a stationary dust formation process. Element conservation equations augment this system of equations by including the element replenishment by convective overshooting. The integration with an atmosphere code (Phoenix) allows determination of a consistent (T, p, v conv )-structure (T -local temperature, p -local pressure, v conv -convective velocity), hence, to calculate synthetic spectra. Results. A grid of Drift-Phoenix model atmospheres was calculated for a wide range of metallicity, [M/H] ∈ [+0.5, −0.0, −0.5, ..., −6.0], to allow for systematic study of atmospheric cloud structures throughout the evolution of the universe. We find dust clouds in even the most metal-poor ([M/H] = −6.0) atmosphere of brown dwarfs. Only the most massive among the youngest brown dwarfs and giant gas planets can resist dust formation. For very low heavy element abundances, a temperature inversion develops that has a drastic impact on the dust cloud structure. Conclusions. The combination of metal depletion by dust formation and the uncertainty of interior element abundances makes the modelling of substellar atmospheres an intricate problem in particular for old substellar objects. We furthermore show that the dust-togas ratio does not scale linearly with the object's [M/H] for a given effective temperature. The mean grain sizes and the composition of the grains change depending on [M/H], which influences the dust opacity that determines radiative heating and cooling, as well as the spectral appearance.

Dust in brown dwarfs and extra-solar planets

“…Witte et al (2009) gave a more general analysis of dust and atmosphere properties over the parameter space. The current Drift-Phoenix model atmospheres grid covers the following model atmosphere parameters:…”

“…We use the Drift-Phoenix model atmosphere code (Dehn 2007;Helling et al 2008b) in the most recent version presented by Witte et al (2009). The general-purpose model atmosphere code Phoenix (Hauschildt & Baron 1999;Baron et al 2003) solves the gas-phase equation of state, provides the atmosphere structure to Drift, determines the gas opacities and solves the radiative transfer.…”

“…The model code Drift-Phoenix has been discussed in more detail by Witte et al (2009). Since then, the most significant improvement of the code has been the implementation of the EOS module ACES (Barman, in prep.…”

“…Moreover, the forming dust clouds provide an enormous opacity (Jones & Tsuji 1997). As a result, the spectral appearance of the object is considerably changed, while the local gas at the cloud can by heated by up to several hundred Kelvins compared to a dustfree atmosphere (e.g., Witte et al 2009). Another aspect of the problem is the element replenishment of the upper atmosphere, which is driven by the weak gas motion above the convection zone (Ludwig et al 2002).…”

Detecting planets around very cool dwarfs at near infrared wavelengths with the radial velocity technique

“…For T eff below 3000 K, del Burgo et al (2009b) used the Drift-PHOENIX code to produce theoretical spectra of late M-and T-dwarfs, which satisfactorily reproduced the observed spectra. That code is a merger of the PHOENIX code and the dust model Drift (Helling et al 2008a;Witte et al 2009). The dust grains are composites and yield improved opacities in contrast to the grains in earlier models, and the use of a kinematic, phase-nonequilibrium dust-formation model avoids an overestimated condensation/evaporation (Helling et al 2008b).…”