Adrien Simonnin

Hot and ultra-hot Jupiters offer a unique laboratory for studying atmospheric chemistry at the population level using ground-based high-resolution spectroscopy. Iron (Fe) and titanium (Ti) are key tracers of thermal and chemical structures, yet they exhibit markedly different observational trends across the population. We present a homogeneous re-analysis of high-resolution transmission spectra for ten hot and ultra-hot Jupiters observed with ESPRESSO on ESO’s Very Large Telescope and MAROON-X on Gemini-North. We searched for neutral Fe and Ti absorption and performed injection–recovery tests using forward models spanning a range of Ti-depletion levels and temperature–pressure profiles. To enable direct comparison across planets observed with different instruments and signal-to-noise ratios, we introduced the relative cross-correlation metric, Δ _Ti-Fe . We detect Fe in seven planets and Ti in four planets at significances above 5 σ . Across the population, Δ _Ti-Fe decreases sharply towards lower equilibrium temperatures. Under the assumption of equal Ti depletion across planets, isothermal models fail to reproduce this trend, instead requiring a temperature-dependent depletion of Ti that increases towards cooler planets, consistent with cold-trapping processes in cooler atmospheres. Models with inverted temperature–pressure profiles naturally reproduce the decline without invoking such temperature-dependent depletion. In these atmospheres, Ti is converted into TiO in deeper, cooler layers and subsequently removed from the gas phase through condensation into Ti-bearing species, leading to a strong suppression of the observable atomic Ti signal while Fe remains largely atomic. Nevertheless, even in these gradient models, an additional overall depletion of Ti relative to Fe is required to match the hottest planets. Our results demonstrate that the observable refractory chemistry is governed by the interplay of molecular partitioning, ionisation, condensation, and cold-trapping processes, as well as the vertical structure of ultra-hot Jupiter atmospheres. Although these results reveal clear population-level trends, additional observations will be necessary to distinguish between temperature-dependent cold-trapping and overall depletion scenarios. Expanding homogeneous high-resolution surveys to larger and more diverse samples, targeting both emission and transmission observations, will refine these constraints and provide critical insights into the chemistry of strongly irradiated giant planets.

Exploration de la diversité physique des exoplanètes et des naines brunes à travers la spectroscopie infrarouge à haute résolution au VLT et à l'ELT: Cette thèse est consacrée au domaine de la recherche sur les exoplanètes, en se concentrant sur la caractérisation atmosphérique à l'aide des spectrographes à haute résolution actuels sur les télescopes de 8m tels que le VLT et Gemini, et en explorant le potentiel des futurs télescopes tels que l'ELT. Je donne d'abord un aperçu des différentes techniques utilisées pour détecter et caractériser les exoplanètes. Une attention particulière est accordée à la spectroscopie de transit et à l'imagerie directe, les deux méthodes principales pour les études atmosphériques. Je discute également des modèles atmosphériques simples jusqu'aux modèles de circulation globale (GCM) plus avancés. Le coeur de la thèse est divisé en deux parties. La première est consacrée aux observations actuelles, en particulier dans le cadre du programme ExoWinds, qui vise à étudier les vents atmosphériques dans les Jupiters ultra-chauds. Sur la base d'un échantillon de cinq cibles, j'ai mené une étude comparative révélant à la fois une diversité et des tendances communes dans les signaux de fer détectés, probablement causées par des variations dans les propriétés planétaires. Ma première publication porte sur une des cibles de ce programme, TOI-1518b. En utilisant deux transits observés avec le spectrographe MAROON-X, j'ai effectué une analyse détaillée et détecté 14 espèces dans l'atmosphère de la planète. La comparaison avec les GCMs a révélé la présence d'un fort "drag", tandis qu'une analyse de "retrieval" a suggéré une abondance de fer cohérente avec les valeurs solaires. Cependant, les limites des GCMs et du modèle de "retrieval" ont également mis en évidence des biais possibles, en particulier pour les espèces ionisées et pour le retrieval multi-espèces. Ce chapitre se termine par mon travail sur deux autres cibles observées avec CRIRES+. Alors que WASP-20b n'a pas donné de résultats concluants en raison d'un faible rapport signal/bruit, l'expérience acquise m'a permis de demander avec succès du temps d'observation sur WASP-76b lors de l'appel à proposition P112 de l'ESO. L'analyse préliminaire montre des indications prometteuses de CO et de H2O, et des recherches supplémentaires sont prévues pour confirmer ces détections. La deuxième partie de la thèse se concentre sur le spectrographe à haute résolution de l'ELT, ANDES. J'étudie les performances de l'instrument en mode haut contraste pour la détection et la caractérisation des exoplanètes. Dans ce contexte, j'ai conçu un coronographe de Lyot capable d'atteindre le contraste requis de 10⁻³ à 25 mas à 1.6 μm. Cette conception a été validée par le groupe SCAO d'ANDES. Le coronographe est désormais développé au Laboratoire de Lagrange, et sera inclus dans le prochain "Preliminary Design Review". Pour simuler des observations réalistes avec ANDES, j'ai développé un simulateur de bout en bout appelé APU. Cet outil reproduit toutes les étapes : la génération d'images à haut contraste, la photométrie et l'estimation du bruit, l'injection de scènes astrophysiques et l'analyse des données. J'ai appliqué APU à un cas scientifique impliquant des jeunes Jupiters en imagerie directe. Les résultats montrent que, dans d'excellentes conditions d'observations, des planètes avec des contrastes allant jusqu'à 10⁻⁷ pourraient être détectées à une séparation angulaire de 30 mas. Ces simulations soulignent l'importance de développer des stratégies optimisées pour combiner le signal planétaire réparti sur plusieurs spaxels. La thèse se termine par une description des perspectives d'avenir pour le simulateur APU comme l'exploration de différents cas scientifiques, y compris la détection de biosignatures dans les exoplanètes semblables à la Terre.

Discovered almost ten years ago, the giant planet 51 Eridani b is one of the least separated (≈0.2″) and faintest ( J ≈ 19.74 mag) directly imaged exoplanets known to date. Its atmospheric properties have been thoroughly investigated through low- and medium-resolution spectroscopic observations, enabling the robust characterization of the planet’s bulk parameters. However, the planet’s intrinsically high contrast renders high-resolution spectroscopic observations difficult, despite their potential to yield key measurements essential for a more comprehensive characterization. This study sought to constrain the planet’s radial velocity, enabling a full 3D orbital solution when integrated with previous measurements. We obtained four high-contrast, high-resolution ( R ≈ 140 000) spectroscopic datasets of the planet, collected over a two-year interval with the HiRISE visitor instrument at the VLT to derive the planet’s radial velocity. Using self-consistent models of atmosphere, we were able to derive the radial velocity of the planet at each of the four epochs. These radial-velocity measurements were then used in combination with all existing relative astrometry in order to constrain the orbit of the planet. Our radial velocity measurements allowed us to break the degeneracy along the line of sight, making the unambiguous interpretation of the phase curve of the companion possible. We further constrained the orbital parameters, and particularly the eccentricity, for which we derive e = 0.55 ^+0.03 _−0.07 . The relatively high eccentricity indicates that the system has experienced dynamical interactions induced by an external perturber. We place constraints on the mass and semimajor axis of a hypothetical, unseen outer planet capable of producing the observed high eccentricities.

Context. Access to medium-resolution spectra (R_λ ~ 1000 − 10 000) at near-infrared wavelengths of young M-L objects allows us to study their atmospheric properties. Specifically, this approach can unveil a rich set of molecular features related to the atmospheric chemistry and physics. Aims. We aim to deepen our understanding of the M-L transition on planetary-mass companions and isolated brown dwarfs, while searching for evidence of possible differences between these two populations of objects. To this end, we present a set of 21 VLT/SINFONI K-band (1.95–2.45 µm) observations from five archival programs at R_λ ~ 4000. We aim to measure the atmospheric properties, such as T_eff, log (ɡ), [M/H], and C/O, and to understand the similarities and differences between objects ranging in spectral type from M5 to L5. Methods. We extracted the spectra of these targets with the TExTRIS code. We modeled them using ForMoSA, a Bayesian forward modeling tool for spectral analysis, and we explored four families of self-consistent atmospheric models: ATMO, BT-Settl, Exo-REM, and Sonora Diamondback. Results. Here, we present the spectra of our targets and the derived parameters from the atmospheric modeling process. We confirm a drop in T_eff as a function of the spectral type of more than 500 K at the M/L transition. In addition, we report C/O measurements for three companions, 2M 0103 AB b, AB Pic b, and CD-35 2722 b, thereby adding to the growing list of exoplanets with measured C/O ratios. Conclusions. The VLT/SINFONI Library highlights two key points. First, there is a critical need to further investigate the discrepancies among grids of spectra generated by self-consistent models, as these models yield varying results and do not uniformly explore the parameter space. Second, we do not observe any obvious discrepancies in the K-band spectra between companions and isolated brown dwarfs, which suggests that these super-Jupiter objects might have formed through a similar process; however, this possibility warrants further investigation.

Context. Wind dynamics play a pivotal role in governing transport processes within planetary atmospheres, influencing atmospheric chemistry, cloud formation, and the overall energy budget. Understanding the strength and patterns of winds is crucial for comprehensive insights into the physics of ultra-hot-Jupiter atmospheres. Current research has proposed different mechanisms that limit wind speeds in these atmospheres. Aims. This study focuses on unraveling the wind dynamics and the chemical composition in the atmosphere of the ultra-hot Jupiter TOI-1518 b. Methods. Two transit observations using the high-resolution (R_λ ∼ 85 000) optical (spectral coverage between 490 and 920 nm) spectrograph MAROON-X were obtained and analyzed to explore the chemical composition and wind dynamics using the cross-correlation techniques, global circulation models (GCMs), and atmospheric retrieval. Results. We report the detection of 14 species in the atmosphere of TOI-1518 b through cross-correlation analysis. VO was detected only with the new HyVO line list, whereas TiO was not detected. Additionally, we measured the time-varying cross-correlation trails for six different species, compared them with predictions from GCMs, and conclude that a strong drag is slowing the winds in TOI-1518 b’s atmosphere (τ_drag ≈ 10³−10⁴ s). We find that the trails are species dependent. Fe+ favors stronger drag than Fe, which we interpret as a sign of magnetic effects being responsible for the observed strong drag. Furthermore, we show that Ca+ probes layers above the Roche lobe, leading to a qualitatively different trail than the other species. Finally, We used a retrieval analysis to further characterize the abundances of the different species detected. Our analysis is refined thanks to the updated planetary mass of 1.83 ± 0.47 M_Jup we derived from new Sophie radial-velocity observations. We measure an abundance of Fe of log₁₀ Fe = −4.88_−0.76^+0.63 corresponding to 0.07 to 1.62 solar enrichment. For the other elements, the retrievals appear to be biased, probably due to the different K_p/V_sys shifts between Fe and the other elements, which we demonstrate for the case of VO.