Dust in brown dwarfs and extra-solar planets -3D Monte Carlo versus kinetic approach of TiO2 seed formation

Abstract

<p>Modelling the formation of cloud condensation nuclei is key for predicting cloud properties in and analyzing observational data from exoplanet and brown dwarf atmospheres. Based on kinetic results on cloud formation in exoplanets, we readdress the question about the formation of cloud condensation nuclei through a Monte Carlo approach. We tackle the formation of TiO<sub>2</sub> clusters using a recently developed particle code in 3D. We initiate 1000 TiO<sub>2</sub> molecules in a domain of 1 cm<sup>3</sup> size. We trace individual particles and check after every time step whether particles collide and form larger clusters. We run simulations at temperatures between 500 K and 1500 K, with particle sticking probabilities between 0.1 and 1 and distinguish whether only monomers or all other clusters are allowed to stick to earlier formed clusters. We present the number densities, the size distributions and the formation rate of clusters of different size and compare our results with results from a kinetic approach.<br>Simulating the motion of individual clusters allows us to display the spatial distribution of all particles as well as to determine their mean and maximum size. We calculate the line opacities of (TiO<sub>2</sub>)<sub>N</sub> clusters and discuss their detectability through the James Webb Space Telescope or the upcoming Extremely Large Telescope. Our results present a first step towards a better understanding of the formation of cloud formation nuclei in extrasolar environments by comparing selected results from molecular dynamic simulations with a kinetic approach based on thermodynamic cluster data.</p>