Mineral snowflakes on exoplanets and brown dwarfs

Coagulation and fragmentation of cloud particles with HyLandS

¹ Space Research Institute, Austrian Academy of Sciences, Schmiedlstrasse 6, 8042 Graz, Austria
e-mail: Dominic.Samra@oeaw.ac.at
² Centre for Exoplanet Science, University of St Andrews, North Haugh, St Andrews KY16 9SS, UK
³ SUPA, School of Physics & Astronomy, University of St Andrews, North Haugh, St Andrews KY16 9SS, UK
⁴ TU Graz, Fakultät für Mathematik, Physik und Geodäsie, Petersgasse 16 8010 Graz, Austria
⁵ Universitäts-Sternwarte, Ludwig-Maximilians-Universität München, Scheinerstr. 1, 81679 München, Germany
⁶ Exzellenzcluster ORIGINS, Boltzmannstr. 2, 85748 Garching, Germany

Received: 12 November 2021
Accepted: 9 March 2022

Abstract

Context. Brown dwarfs and exoplanets provide unique atmospheric regimes that hold information about their formation routes and evolutionary states. Cloud particles form through nucleation, condensation, evaporation, and collisions, which affect the distribution of cloud particles in size and throughout these atmospheres. Cloud modelling plays a decisive role in understanding these regimes.

Aims. Modelling mineral cloud particle formation in the atmospheres of brown dwarfs and exoplanets is a key element in preparing for missions and instruments like CRIRES+, JWST, and ARIEL, as well as possible polarimetry missions like PolStar. The aim is to support the increasingly detailed observations that demand greater understanding of the microphysical cloud processes.

Methods. 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 Hybrid moments (Ls) and Size (HyLandS) is then applied to a grid of Drift-Phoenix (T_gas, p_gas) profiles. Effective medium theory and Mie theory are used to investigate the optical properties.

Results. Turbulence proves to be the main driving process of particle-particle collisions, with collisions becoming the dominant process in the lower atmosphere (p > 10⁻⁴ bar) at the cloud base. Particle-particle collisions produce one of three outcomes for brown dwarf and gas-giant atmospheres: fragmenting atmospheres (log₁₀(g[cms⁻²])=3.0) coagulating atmospheres (log₁₀(g)=5.0), T_eff ≤1800K) or condensational growth dominated atmospheres (log₁₀(g) = 5.0, T_eff > 1800 K). Cloud particle opacity slope at optical wavelengths (Hubble) is increased with fragmentation, as are the silicate features at JWST NIRSpec, JWST MIRI, and ARIEL AIRS wavelengths.

Conclusions. The hybrid moment-bin method HyLandS demonstrates the feasibility of combining a moment and a bin method for cloud modelling, whilst assuring element conservation. It provides a powerful and fast tool for capturing general trends of particle collisions, consistently with other microphysical growth processes. Collisions are an important process in exoplanet and brown dwarf atmospheres, but cannot be assumed to be hit-and-stick only. The spectral effects of cloud particle collisions in both optical and mid-infrared wavelengths complicate inferences of cloud particle size and material composition from observational data.