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Funding:D.G. and L.D. acknowledge funding by the ERC consolidator grant number 646758. J.-P. S. and H. L.-M. acknowledge the European Union's Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No 860470. ; Stellar dust grains are predominantly composed of mineralic, anorganic material forming in the circumstellar envelopes of oxygen-rich AGB stars. However, the initial stage of the dust synthesis, or its nucleation, is not well understood. In particular, the chemical nature of the nucleating species, represented by molecular clusters, is uncertain. We investigate the vertical and adiabatic ionization energies of four different metal-oxide clusters by means of density functional theory. They include clusters of magnesia (MgO)n, silicon monoxide (SiO)n, alumina (Al2O3)n, and titania (TiO2)n with stoichiometric sizes of n=1−8. The magnesia, alumina and titania clusters show relatively little variation in their ionization energies with respect to the cluster size n, ranging from 7.1−8.2 eV for (MgO)n, 8.9−10.0 eV for (Al2O3)n, and 9.3−10.5 eV for (TiO2)n. In contrast, the (SiO)n ionization energies decrease with size n, starting from 11.5 eV for n=1, and decreasing to 6.6 eV for n=8. Therefore, we set constraints on the stability limit for neutral metal-oxide clusters to persist ionization through radiation or high temperatures and for the nucleation to proceed via neutral-neutral reactions ; Publisher PDF ; Peer reviewed

51 pags., 35 figs., 1 tab., 5 apps. ; The atmospheres of synchronously rotating exoplanets are intrinsically 3D, and fast vertical and horizontal winds are expected to mix the atmosphere, driving the chemical composition out of equilibrium. Due to the longer computation times associated with multidimensional forward models, horizontal mixing has only been investigated for a few case studies. In this paper, we aim to generalize the impact of horizontal and vertical mixing on the chemistry of exoplanet atmospheres over a large parameter space. We do this by applying a sequence of post-processed forward models for a large grid of synchronously rotating gaseous exoplanets, where we vary the effective temperature (between 400 and 2600 K), surface gravity, and rotation rate. We find that there is a dichotomy in the horizontal homogeneity of the chemical abundances. Planets with effective temperatures below 1400 K tend to have horizontally homogeneous, vertically quenched chemical compositions, while planets hotter than 1400 K exhibit large compositional day-night differences for molecules such as CH4. Furthermore, we find that the planet's rotation rate impacts the planetary climate, and thus also the molecular abundances and transmission spectrum. By employing a hierarchical modelling approach, we assess the relative importance of disequilibrium chemistry on the exoplanet transmission spectrum, and conclude that the temperature has the most profound impact. Temperature differences are also the main cause of limb asymmetries, which we estimate could be observable with the James Webb Space Telescope. This work highlights the value of applying a consistent modelling setup to a broad parameter space in exploratory theoretical research. ; RB is a PhD fellow of the Research Foundation – Flanders (FWO). LD acknowledges support from the FWO research grant G086217N. LC acknowledges support by the DFG grant CA 1795/3. OV thanks the CNRS/INSU Programme National de Planetologie ´ (PNP) and CNES for funding support. MA acknowledges support from Spanish MICIU through grants RyC-2014-16277, AYA2016- 75066-C2-1-P, PID2019-106110GB-I00, and PID2019-107115GBC21. PM acknowledges support from the European Research Council under the European Union's Horizon 2020 research and innovation program under grant agreement no. 832428. The computational resources and services used in this work were provided by the VSC (Flemish Supercomputer Center), funded by the Research Foundation Flanders (FWO) and the Flemish Government – department EWI.