arXiv subjects

C. Akın

Publications and source records attributed to C. Akın.

4 recordsLinked to original sources

arXiv2026

Dynamics of Y Dwarf Atmospheres

The global circulation regime of the coolest brown dwarfs, the Y dwarfs, remains largely unexplored. We investigate the interplay between convection, rotation, and cloud thermal feedback using a selected sample of Y dwarf atmospheric models. We explore effective temperatures $400~\mathrm{K} \leq T_{\mathrm{eff}} \leq 600~\mathrm{K}$ and rotation periods $P_{\mathrm{rot}} = 2.5 \text{--} 20\ \mathrm{h}$, where salt and sulfide condensates are expected. We include $\mathrm{KCl,~Na_{2}S}$, and $\mathrm{MnS}$ clouds to assess their atmospheric impact and identify circulation regimes across parameter space. We run twelve general circulation models (GCMs) spanning this grid and develop additional physics modules for the THOR GCM to model brown dwarf atmospheres. The dynamical core is coupled to interior thermal perturbations near the radiative-convective boundary, a mixing-length convection scheme, gray two-stream radiative transfer with Rosseland-mean opacities, and simple cloud tracers including thermal feedback and scattering. All simulations exhibit 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 salt and sulfide cloud layers below the photosphere. Thermal structures equilibrate quickly and cloud radiative feedback remains insignificant, with limited variability. Within the gray radiative transfer framework adopted here, Y dwarf atmospheres in this parameter space are controlled by interior thermal radiation. Rotation sets modest variability, while clouds play a secondary role. Because our single-band approach does not capture spectral windows that could probe deeper cloud layers, our constraints on cloud radiative feedback are likely conservative, and we outline pathways toward more active regimes.

astro-ph.SR↗

arXiv2025

Global flow regimes of hot Jupiters

In hot and ultra-hot Jupiters, stellar irradiation is a primary driver of atmospheric circulation and the wave structures that sustain it. 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 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. We simulate 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 develop a wavelet-based analysis method to decompose simulated wind fields into their underlying wave modes, which we validate on analytical examples. As a preliminary exploration of the flow regime of ultra-hot Jupiters, we perform an additional simulation for WASP-121b, where the mean molecular weight is set to represent an atmosphere dominated by atomic hydrogen. Our results confirm that increasing stellar irradiation diminishes atmospheric heat redistribution efficiency and weakens contributions from smaller-scale modes that are critical to sustain 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, multi-scale 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 ultra-hot Jupiter atmospheres.

astro-ph.EP↗

arXiv2025

Retrieving day- and nightside atmospheric properties of the ultra-hot Jupiter TOI-2109b. Detection of Fe and CO emission lines and evidence for inefficient heat transport

The ultra-hot Jupiter (UHJ) TOI-2109b marks the lower edge of the equilibrium temperature gap between 3500 K 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 structure of TOI-2109b's atmosphere, we obtained high-resolution emission spectra of both the planetary day- and nightsides with CARMENES and CRIRES$^+$. By applying the cross-correlation technique, we identified the emission signatures of Fe I and CO, as well as a thermal inversion layer in the dayside atmosphere; no significant H$_2$O 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 3200 K 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 ($>$ 0.15). The retrieved spectral line broadening is consistent with tidally locked rotation, indicating the absence of strong dynamical processes. An upper temperature limit of 2400 K and a maximum atmospheric temperature gradient of 700 K/log bar could be derived for the nightside. Comparison of the retrieved dayside T-p 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 H$_2$O emission lines, and KELT-9b, where molecular features are largely absent.

astro-ph.EP↗

arXiv2024

The Mantis Network IV: A titanium cold-trap on the ultra-hot Jupiter WASP-121 b

Observations of WASP-121 b have suggested an under-abundance of titanium and titanium-oxide from its terminator region. In this study, we aim to determine whether this depletion is global by investigating the day-side emission spectrum. We analyse 8 epochs of high-resolution spectra obtained with ESPRESSO, targeting orbital phases when the day-side is in view. We use a cross-correlation method to search for various atoms, TiO and VO and compare to models. We constrain the velocities and phase-function of the emission signal using a Bayesian framework. We report significant detections of Ca I, V I, Cr I, Mn I, Fe I, Co I and Ni I, but not T i or TiO. Models containing Ti 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 sub-stellar point. We find that Ti is depleted from regions of the atmosphere where transmission and emission spectroscopy are sensitive. We interpret this as evidence for the night-side condensation of titanium, preventing it from being mixed back into the upper layers of the atmosphere elsewhere on the planet. Species with lower condensation temperatures are unaffected, implying 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 day-side and night-side, and thus condensation chemistry needs to be included in global circulation models. Observed elemental abundances in hot Jupiters are not reliably representative of bulk abundances unless night-side condensation is accounted for or the planet is hot enough to avoid night-side cold-traps entirely. Planetary rotation may significantly lower the apparent orbital velocity of emission signals.

astro-ph.EP↗