ALBERT

Description: The precipitation of cloud particles in brown dwarf and exoplanet atmospheres establishes an ongoing downward flux of condensable elements. To understand the efficiency of cloud formation, it is therefore crucial to identify and quantify the replenishment mechanism that is able to compensate for these local losses of condensable elements in the upper atmosphere, and to keep the extrasolar weather cycle running. In this paper, we introduce a new cloud formation model by combining the cloud particle moment method we described previously with a diffusive mixing approach, taking into account turbulent mixing and gas-kinetic diffusion for both gas and cloud particles. The equations are of diffusion-reaction type and are solved time-dependently for a prescribed 1D atmospheric structure, until the model has relaxed toward a time-independent solution. In comparison to our previous models, the new hot-Jupiter model results (Teff ≈ 2000 K, log g = 3) show fewer but larger cloud particles that are more concentrated towards the cloud base. The abundances of condensable elements in the gas phase are featured by a steep decline above the cloud base, followed by a shallower, monotonous decrease towards a plateau, the level of which depends on temperature. The chemical composition of the cloud particles also differs significantly from our previous models. Through the condensation of specific condensates such as Mg2SiO4[s] in deeper layers, certain elements, such as Mg, are almost entirely removed early from the gas phase. This leads to unusual (and non-solar) element ratios in higher atmospheric layers, which then favours the formation of SiO[s] and SiO2[s], for example, rather than MgSiO3[s]. These condensates are not expected in phase-equilibrium models that start from solar abundances. Above the main silicate cloud layer, which is enriched with iron and metal oxides, we find a second cloud layer made of Na2S[s] particles in cooler models (Teff ⪅ 1400 K).

Description: We have conducted a re-analysis of publicly available Hubble Space Telescope Wide Field Camera 3 (HST WFC3) transmission data for the hot-Jupiter exoplanet WASP-43b, using the Bayesian retrieval package Tau-REx. We report evidence of AlO in transmission to a high level of statistical significance (〉5σ in comparison to a flat model, and 3.4σ in comparison to a model with H2O only). We find no evidence of the presence of CO, CO2, or CH4 based on the available HST WFC3 data or on Spitzer IRAC data. We demonstrate that AlO is the molecule that fits the data to the highest level of confidence out of all molecules for which high-temperature opacity data currently exists in the infrared region covered by the HST WFC3 instrument, and that the subsequent inclusion of Spitzer IRAC data points in our retrieval further supports the presence of AlO. H2O is the only other molecule we find to be statistically significant in this region. AlO is not expected from the equilibrium chemistry at the temperatures and pressures of the atmospheric layer that is being probed by the observed data. Its presence therefore implies direct evidence of some disequilibrium processes with links to atmospheric dynamics. Implications for future study using instruments such as the James Webb Space Telescope are discussed, along with future opacity needs. Comparisons are made with previous studies into WASP-43b.

Description: Aims. We present ARCiS, a novel code for the analysis of exoplanet transmission and emission spectra. The aim of the modelling framework is to provide a tool able to link observations to physical models of exoplanet atmospheres. Methods. The modelling philosophy chosen in this paper is to use physical and chemical models to constrain certain parameters while leaving certain parts of the model, where our physical understanding remains limited, free to vary. This approach, in between full physical modelling and full parameterisation, allows us to use the processes we understand well and parameterise those less understood. We implemented a Bayesian retrieval framework and applied it to the transit spectra of a set of ten hot Jupiters. The code contains chemistry and cloud formation and has the option for self-consistent temperature structure computations. Results. The code presented is fast and flexible enough to be used for retrieval and for target list simulations for JWST or the ESA Ariel missions for example. We present results for the retrieval of elemental abundance ratios using the physical retrieval framework and compare this to results obtained using a parameterised retrieval setup. Conclusions. We conclude that for most of the targets considered, their elemental abundance ratios cannot be reliably constrained based on the current dataset. We find no significant correlations between different physical parameters. We confirm that planets in our sample with a strong slope in the optical transmission spectrum are those for which we find cloud formation to be most active. Finally, we conclude that with ARCiS we have a computationally efficient tool to analyse exoplanet observations in the context of physical and chemical models.

Description: Clouds, which are common features in Earth's atmosphere, form in atmospheres of planets that orbit other stars than our Sun, in so-called extrasolar planets or exoplanets. Exoplanet atmospheres can be chemically extremely rich. Exoplanet clouds are therefore composed of a mix of materials that changes throughout the atmosphere. They affect atmospheres through element depletion and through absorption and scattering; hence, they have a profound impact on an atmosphere's energy budget. While astronomical observations point us to the presence of extrasolar clouds and make first suggestions on particle size and material composition, we require fundamental and complex modeling work to merge the individual observations into a coherent picture. Part of this work includes developing an understanding of cloud formation in nonterrestrial environments. ▪ Exoplanet atmospheres exhibit a wide chemical diversity that enables the formation of mineral clouds in contrast to the predominant water clouds on Earth. ▪ Clouds consume elements, causing specific atoms and molecules to drop in abundance. Transport processes such as gravitational settling or advection delocalize this process. ▪ Extrasolar planets can have extreme weather conditions where day- and nightside temperatures vary hugely. This affects cloud formation, and hence the cloud coverage and atmosphere's appearance can change dramatically. ▪ Dynamic extrasolar clouds develop intracloud lightning, and electric circuits may occur on more local, smaller scales in giant exoplanets compared to smaller, Earth-like planets with less dramatic hydrodynamics.