Dust in brown dwarfs and extra-solar planets: VI. Assessing seed formation across the brown dwarf and exoplanet regimes

Context. The cloud formation process starts with the formation of seed particles, after which, surface chemical reactions grow or erode the cloud particles. If seed particles do not form, or are not available by another means, an atmosphere is unable to form a cloud complex and will remain cloud free.

Aims. We aim to investigate which materials may form cloud condensation seeds in the gas temperature and pressure regimes (T_gas = 100-2000 K, p_gas = 10^{− 8} -100 bar) expected to occur in planetary and brown dwarf atmospheres.

Methods. We have applied modified classical nucleation theory which requires surface tensions and vapour pressure data for each solid species, which are taken from the literature. Input gas phase number densities are calculated assuming chemical equilibrium at solar metallicity.

Results. We calculated the seed formation rates of TiO₂[s] and SiO[s] and find that they efficiently nucleate at high temperatures of T_gas = 1000-1750 K. Cr[s], KCl[s] and NaCl[s] are found to efficiently nucleate across an intermediate temperature range of T_gas = 500-1000 K. We find CsCl[s] may serve as the seed particle for the water cloud layers in cool sub-stellar atmospheres. The nucleation rates of four low temperature ice species (T_gas = 100-250 K), H₂O[s/l], NH₃[s], H₂S[s/l], and CH₄[s], are also investigated for the coolest sub-stellar and planetary atmospheres.

Conclusions. Our results suggest a possibly (T_gas , p_gas ) distributed hierarchy of seed particle formation regimes throughout the sub- stellar and planetary atmospheric temperature-pressure space. With TiO₂[s] providing seed particles for the most refractory cloud formation species (e.g. Al₂O₃[s], Fe[s], MgSiO₃[s], Mg₂SiO₄[s]), Cr[s] providing the seed particles for MnS[s], Na₂S[s], and ZnS[s] sulfides, and K/Na/Rb/Cs/NH₄-Cl binding solid species providing the seed particles for H₂O[s / l] and NH₄-H₂PO₄/SH[s] clouds.

A detached, high-altitude aerosol layer may form in some sub-stellar atmospheres from the nucleation process, dependent on the upper atmosphere temperature, pressure and availability of volatile elements. In order to improve the accuracy of the nucleation rate calculation, further research into the small cluster thermochemical data for each cloud species is warranted. The validity of these seed particle scenarios will be tested by applying it to more complete cloud models in the future.