Mineral Snowflakes on Exoplanets and Brown Dwarfs: Coagulation and Fragmentation of Cloud Particles with {\sc HyLandS}

Abstract

Brown dwarfs and exoplanets provide unique atmospheric regimes that hold information about their formation routes and evolutionary states. Modelling mineral cloud particle formation is key to prepare for missions and instruments like CRIRES+, JWST and ARIEL as well as possible polarimetry missions like {\sc PolStar}. The aim is to support more detailed observations that demand greater understanding of microphysical cloud processes. We extend our kinetic cloud formation model that treats nucleation, condensation, evaporation and settling of mixed material cloud particles to consistently model cloud particle-particle collisions. The new hybrid code, {\sc HyLandS}, is applied to a grid of {\sc Drift-Phoenix} (T, p)-profiles. Effective medium theory and Mie theory are used to investigate the optical properties. Turbulence is the main driving process of collisions, with collisions becoming the dominant process at the cloud base ($p>10^{-4}\,{\rm bar}$). Collisions produce one of three outcomes: fragmenting atmospheres ($\log_{10}(g)=3$), coagulating atmospheres ($\log_{10}(g)=5$, $T_{\rm eff} \leq 1800\, {\rm K}$) and condensational growth dominated atmospheres ($\log_{10}(g\,)=5$, $T_{\rm eff} > 1800\, {\rm K}$). Cloud particle opacity slope at optical wavelengths (HST) is increased with fragmentation, as are the silicate features at mid-infrared wavelengths. The hybrid moment-bin method {\sc HyLandS} demonstrates the feasibility of combining a moment and a bin method whilst assuring element conservation. It provides a powerful and fast tool for capturing general trends of particle collisions, consistently with other microphysical processes. Collisions are important in exoplanet and brown dwarf atmospheres but cannot be assumed to be hit-and-stick only. The spectral effects of collisions complicates inferences of cloud particle size and material composition from observational data.