Dust in brown dwarfs and extra-solar planets

Abstract

Context. Clouds form in atmospheres of brown dwarfs and planets. The cloud particle formation processes, seed formation and growth/evaporation are very similar to the dust formation process studied in circumstellar shells of AGB stars and in supernovae. Cloud formation modelling in substellar objects requires gravitational settling and element replenishment in addition to element depletion. All processes depend on the local conditions, and a simultaneous treatment is required. Aims. We apply new material data in order to assess our cloud formation model results regarding the treatment of the formation of condensation seeds. We look again at the question of the primary nucleation species in view of new (TiO 2 ) N -cluster data and new SiO vapour pressure data. Methods. We applied the density functional theory (B3LYP, 6-311G(d)) using the computational chemistry package G 09 to derive updated thermodynamical data for (TiO 2 ) N clusters as input for our TiO 2 seed formation model. We tested different nucleation treatments and their effect on the overall cloud structure by solving a system of dust moment equations and element conservation for a prescribed D-P atmosphere structure. Results. Updated Gibbs free energies for the (TiO 2 ) N clusters are presented, as well as a slightly temperature dependent surface tension for T = 500 . . . 2000 K with an average value of σ ∞ = 480.6 erg cm −2 . The TiO 2 seed formation rate changes only slightly with the updated cluster data. A considerably larger effect on the rate of seed formation, and hence on grain size and dust number density, results from a switch to SiO nucleation. The question about the most efficient nucleation species can only be answered if all dust/cloud formation processes and their feedback are taken into account. Despite the higher abundance of SiO over TiO 2 in the gas phase, TiO 2 remains considerably more efficient at forming condensation seeds by homogeneous nucleation. The paper discusses the effect on the cloud structure in more detail.

Dust formation in the oxygen-rich AGB star IK Tauri

“…For deposition, the sticking coefficient S of an atom or molecule on the surface of grains depends on the chemical composition of the accreting species and the grain surface, and the surface and gas temperature. The quantity may vary over orders of magnitude depending on the grain material, with value range of ∼10 −4 -1 (e.g., Leitch-Devlin & Williams 1985;Brune et al 1993;Nagahara & Ozawa 1996). Existing studies on dust A6, page 6 of 15 formation in circumstellar environments often assume a growth process through surface deposition only, and an S value equal to unity or in the range 0.1-1 (e.g., Gail & Sedlmayr 1999;Bladh et al 2015).…”

Information Content Analysis for Selection of Optimal JWST Observing Modes for Transiting Exoplanet Atmospheres

“…Additionally, more complex cloud model parameterizations are not suitably motivated by the data and our generally poor understanding of the very complex coupled, 3D-dynamical-radiative-microphysics in non-Earth-like planets (e.g. Lee et al 2015). A 1 mbar pressure level was chosen arbitrarily so that the absorption features as viewed in transmission were muted, but not completely masked, as demonstrated in (Iyer et al 2016).…”

The mineral clouds on HD 209458b and HD 189733b

“…6). Here, the concept of homogeneous nucleation is applied to model the formation of TiO2 seed particles (Helling & Fomins 2013;Lee et al 2015b) which allows us to calculate the nucleation rate, J * [cm −3 s −1 ], which determines the number of cloud particles, n d [cm −3 ], and therefore influences the total cloud surface. The cloud particle size, a [cm], is determined by the efficiency of surface growth and evaporation reactions.…”