Can Akıin

Context . The global circulation regime of the coolest class of brown dwarfs, known as the Y dwarfs, remains largely unexplored. Aims . We aim to investigate the interplay between convection, rotation, and cloud thermal feedback through a selected sample of Y dwarf atmospheric models. We explore a range of effective temperatures 400 K ≤ T _eff ≤ 600 K and rotation rates P _rot = 2.5–20 h. In this temperature range, salt and sulfide condensates are expected to form. We include KCl, Na ₂ S, and MnS clouds in our simulations to study their effect on the atmosphere. Our goal is to identify circulation regimes and emergent trends across this space, providing insights into the dynamical processes governing Y dwarf atmospheres. Methods . We ran a suite of twelve general circulation models (GCMs) across the outlined parameter grid. For this purpose, we developed additional physics modules for the THOR GCM to model brown dwarf atmospheres. The THOR dynamical core was coupled to modules for interior thermal perturbations near the radiative–convective boundary, a mixing-length convection scheme, a gray two-stream radiative transfer module using Rosseland-mean opacities, and simple cloud tracers with thermal feedback and scattering. Results . Across all simulations, the circulation resides in a radiative-forcing-dominated regime with weak winds, minimal horizontal temperature contrasts, and no persistent jets. Convection controls vertical mixing and sets the extent of the salt and sulfide cloud layers that form below the photosphere. Thermal structures equilibrate quickly and cloud radiative feedback remains insignificant, with limited variability. Conclusions . Y dwarf atmospheres in this parameter range, within the gray radiative transfer framework adopted in this work, remain controlled by thermal radiation from the interior, with small variability primarily set by rotation and clouds playing a secondary role. As our single-band approach does not capture spectral windows that could probe deeper into the cloud-rich layers, our constraints on cloud radiative feedback are likely conservative, and we outline possible causes and pathways toward more active regimes.

Recent advances in space-based and ground-based facilities now allow the atmospheric characterization of a selected sample of rocky exoplanets. These atmospheres offer key insights into planetary formation and evolution, but their interpretation requires models that couple atmospheric processes with both the planetary interior and the surrounding space environment. This work focuses on the Earth-size planet LP 791-18 d, which is estimated to receive continuous tidal heating due to the orbital configuration of the system; thus, it is expected to exhibit volcanic activity. Using a 1D radiative-convective model coupled with chemical kinetics and an outgassing scheme at the lower boundary, we simulated the planet’s atmospheric composition across a range of oxygen fugacities, surface pressures, and graphite activities. We estimated the mantle temperature of ≈1680–1880 K, balancing the competing contributions of interior tidal heating and convective cooling. Our results show that the atmospheric mean molecular weight gradient is controlled by oxygen fugacity rather than bulk metallicity. Furthermore, we used the atmospheric steady-state solutions produced from the interior redox state versus surface pressure parameter space, and explored their atmospheric stability. We find that stability is achieved only in highly oxidized scenarios, fO₂ − IW ≳ 2, while reduced interior states fall into the hydrodynamic escape regime with mass loss rates on the order of ≈10⁵−10⁸ kg/s. We argue that scenarios with reduced interior states are likely to have exhausted their volatile budget during the planet’s lifetime. Furthermore, we predict the atmospheric footprint of the planet’s interior based on its oxidation state and assess its detectability using current or forthcoming tools to constrain the internal and atmospheric composition. We show that the degeneracy between bare rock surfaces and thick atmospheres can be resolved by using three photometric bands to construct a color-color diagram that accounts for potential effects from photochemical hazes and clouds. For JWST/MIRI, this discrimination is possible only in the case of highly oxidized atmospheres. The case of LP 791-18 d enables the investigation of secondary atmosphere formation through outgassing, with implications for similar rocky exoplanets. Our modeling approach connects interior and atmospheric processes, providing a basis for exploring volatile evolution and potential habitability.

Context. The atmospheric dynamics of hot and ultrahot Jupiters are influenced by the stellar irradiation they receive, which shapes their atmospheric circulation and the underlying wave structures. Aims. We aim to investigate how variations in radiative and dynamical timescales influence global flow regimes, atmospheric circulation efficiency, and the interplay of wave structures across a curated sample of hot Jupiters. In particular, we explore a previously predicted transition in the global flow regime, where enhanced stellar irradiation suppresses the smaller-scale wave and eddy features that feed into superrotating jets and ultimately leads to simpler, day-to-night dominated flows. Methods. We simulated a suite of eight well-studied hot Jupiters with the THOR general circulation model, spanning equilibrium temperatures from about 1100 K to 2400 K. We developed a wavelet-based analysis method to decompose simulated wind fields into their underlying wave modes, which we validated on analytical examples. As a preliminary exploration of the flow regime of ultrahot Jupiters, we performed an additional simulation for WASP-121b, where the mean molecular weight was set to represent an atmosphere dominated by atomic hydrogen. Results. Our results confirm that increasing stellar irradiation diminishes the efficiency of atmospheric heat redistribution and weakens the contribution of smaller-scale eddy modes critical for sustaining superrotation. As equilibrium temperatures rise, large-scale modes dominate the atmospheric circulation, driving a transition from jet-dominated flows toward day-to-night circulation. Additionally, by artificially lowering the mean molecular weight, we partially restore circulation efficiency and reintroduce a more complex, multiscale flow pattern. These findings refine our understanding of how atmospheric circulation evolves with increasing irradiation and composition changes, offering a more nuanced framework for interpreting hot and ultrahot Jupiter atmospheres.

The ultra-hot Jupiter (UHJ) TOI-2109b marks the lower edge of the equilibrium temperature gap between 3500 and 4500 K, an unexplored thermal regime that separates KELT-9b, the hottest planet yet discovered, from all other currently known gas giants. To study the thermochemical structure of TOI-2109b’s atmosphere, we obtained high-resolution emission spectra of both the planetary day- and nightsides with CAHA/CARMENES and VLT/CRIRES⁺. By applying the cross-correlation technique to the high-resolution spectra, we identified the emission signatures of Fe I (S/N = 4.3) and CO (S/N = 6.3), as well as a thermal inversion layer in the dayside atmo-sphere; no significant H₂O signal was detected from the dayside. None of the analyzed species were detectable from the nightside atmosphere. We applied a Bayesian retrieval framework that combines high-resolution spectroscopy with photometric measurements to constrain the dayside atmospheric parameters and derive upper limits for the nightside hemisphere. The dayside thermal inversion extends from approximately 3200 to 4600 K, with an atmospheric metallicity consistent with that of the host star (0.36 dex). Only weak constraints could be placed on the C/O ratio, with a lower limit of 0.15. The retrieved spectral line broadening is consistent with tidally locked rotation, indicating the absence of strong dynamical processes in the atmosphere. An upper temperature limit of approximately 2400 K and a maximum atmospheric temperature gradient of about 700 K/log bar could be derived for the planetary nightside. Comparison of the retrieved dayside temperature-pressure profile with theoretical models, the absence of strong atmospheric dynamics, and significant differences in the thermal constraints between the day- and nightside hemispheres suggest a limited heat transport efficiency across the planetary atmosphere. Overall, our results place TOI-2109b in a transitional regime between the UHJs below the thermal gap, which show both CO and H₂O emission lines, and KELT-9b, where molecular features are largely absent.

Photochemistry studies the interactions between light and molecules. Ultraviolet radiation interacts with the atmosphere, and due to its energy, it can dissociate, excite, or ionize its constituents, which initiate other processes. A good knowledge of the interaction between photons of different energies with molecules and atoms is crucial for accurately modeling the atmospheric physics and for climate predictions. Despite its importance, photo-fragment dynamics lacks data because the experimental setup is difficult. We used the upper Martian atmosphere as a natural laboratory to measure the quantum yield O(¹S) from CO₂ + hv as a function of wavelength. We analyzed 4 years of continuous remote-sensing observations from the NASA MAVEN/IUVS spectrograph within a Bayesian framework analysis tool. We retrieved the quantum yield for the first time through its entire production spectral range, ≈80–126 nm, and achieved uncertainty from 10% to 20% on average. While at Lyman-α (121.6 nm), we achieved a precision of 2% by taking advantage of the properties of the upper Martian atmosphere.

Context. Using emission lines from metals, we investigate the three-dimensional distribution of temperature and chemistry in ultra-hot Jupiters. Aims. Existing observations of WASP-121 b have suggested an underabundance of titanium and titanium oxide in its terminator region. In this study, we aim to determine whether this depletion is global by investigating the dayside emission spectrum. Methods. We analyzed eight epochs of high-resolution spectra obtained with the ESPRESSO spectrograph, targeting orbital phases when the dayside is in view. We used a cross-correlation method to search for various atoms, TiO, and VO, and compare the results to models. We constrained the velocities and phase function of the emission signal using a Bayesian framework. Results. We report significant detections of Ca I, V I, Cr I, Mn I, Fe I, Co I, and Ni I, but not Ti or TiO. Models containing titanium are unable to reproduce the data. The detected signals are consistent with the known orbital and systemic velocities and with peak emission originating from the substellar point. Conclusions. We find that titanium is depleted from regions of the atmosphere where transmission and emission spectroscopy are sensitive. Supported by recent HST observations of the nightside, we interpret this as evidence for the nightside condensation of titanium, which prevents it from being mixed back into the upper layers of the atmosphere elsewhere on the planet. Species with lower condensation temperatures are unaffected, implying that sharp chemical transitions exist between ultra-hot Jupiters that have slight differences in temperature or dynamical properties. As TiO can act as a strong source of stratospheric heating, cold-trapping creates a coupling between the thermal structures on the dayside and nightside, and thus condensation chemistry needs to be included in global circulation models. Observed elemental abundances in hot Jupiters will not reliably be representative of bulk abundances unless nightside condensation is robustly accounted for or the planet is hot enough to avoid nightside cold traps entirely. Secondary eclipse observations by JWST/NIRISS have the potential to confirm an absence of TiO bands at red-optical wavelengths. We also find that the abundance ratios of metal oxides to their atomic metals (e.g., TiO/Ti) depend strongly on the atmospheric C/O ratio, and that planetary rotation may significantly lower the apparent orbital velocity of the emission signal.