The effect of thermal non-equilibrium on aerosol formation in astrophysical environments

From our Solar System, we know that gas giant planets are covered in clouds and we expect the same to hold true for many exoplanet gas giants. Knowing the composition and size of cloud particles and where they form within exoplanet atmospheres is crucial to understand exoplanet observations. The first step in cloud formation is the nucleation of aerosols. These nano-sized particles function as seeds for condensation processes through which cloud particles grow. The nucleation and condensation processes are highly temperature dependent. The day side of ultra-hot Jupiters are expected to be cloudless whereas their night side and colder gas giants are expected to be covered in clouds. Recent observations and studies of stellar outflows have shown that thermal non-equilibrium can be present in low density environments and that thermal non-equilibrium can have a significant impact on the formation of larger clusters.

 

In this study, we investigate the effect of thermal non-equilibrium on the formation of aerosols. We derive a kinetic homogeneous cluster nucleation model for non-uniform cluster temperatures. We use this model to study the nucleation of titania (TiO2) which is considered to be an important condensate in exoplanet atmospheres. We analyse the impact of thermal non-equilibrium on the number densities of (TiO2)N, N=1-10, clusters. We find that small temperature offsets between different cluster sizes can have a significant impact on the formation of aerosols. Therefore, studies of low density environments should consider the effects of thermal non-equilibrium on nucleation. In collision dominated regimes, clusters are efficiently driven towards thermal equilibrium and thermal non-equilibria are unlikely to occur.