Helena Lecoq-Molinos
Universidade de Bern

Abstract
Ultra-hot Jupiters have daysides hot enough ( > 2000 K) to keep refractory species such as TiO and VO in the gas phase. Their much cooler nightsides could condense these species and cold-trap them out of the observable atmosphere. Whether this happens depends on nucleation, the formation of the first stable cluster, which equilibrium condensation models ignore. Nucleation rates depend directly on the thermochemistry of small molecular clusters, and for most cloud species, these data are missing. In this talk I show how density functional theory (DFT) can fill this gap. Using global optimization and DFT calculations, we computed the structures and Gibbs free energies of formation for (VO)N and (V₂O₅)N clusters. Together with existing DFT data for (TiO₂)N clusters, these give a consistent quantum-chemical description of the key Ti- and V-bearing species. From these data, we derived new gas-phase equilibrium constants for the vanadium oxides and a VO surface tension fitted to the cluster energies and, implemented both in the microphysical cloud model CARMA. We find that the very high surface tension of VO strongly suppresses homogeneous nucleation, so vanadium only forms minor mantles on TiO₂ seeds. As a result, vanadium largely escapes nightside cold-trapping, being depleted only mildly and mainly at the morning terminator, in contrast to the strong depletion of Ti and Fe. Limb asymmetries in ultra-hot Jupiters may therefore be species-dependent, set by how efficiently each condensate nucleates.

2026 November 26, 11:00

IA/U.Porto
Centro de Astrofísica da Universidade do Porto (Auditorium)
Rua das Estrelas, 4150-762 Porto