arXiv subjects

Arjun B. Savel

Publications and source records attributed to Arjun B. Savel.

At least 19 recordsLinked to original sources

arXiv2026

Recovering the Morning and Evening Limbs of WASP-94Ab from Its Limb-Averaged JWST Transmission Spectrum

Recent JWST observations have revealed that the morning and evening limbs of the hot Jupiter WASP-94Ab have very different transmission spectra. An analysis of these resolved limb spectra yielded a metallicity more than an order of magnitude below that obtained from the limb-averaged spectrum, which calls into question the accuracy of traditional retrievals for heterogenous planets. Here we analyze the limb-averaged JWST NIRISS and NIRSpec transmission spectra of WASP-94Ab using both a standard one-temperature-profile (1-TP) retrieval with patchy clouds, and a new two-temperature-profile (2-TP) retrieval. The 2-TP retrieval assumes a linear combination of two atmospheric components that share the same bulk elemental composition but have independent thermal structures, chemistries, and aerosol properties. This model is implemented in PLATON 7.0, a JAX-accelerated code that is over 100 times faster than its predecessor, which is key to making the retrieval computationally tractable. The limb-averaged spectrum of WASP-94Ab favors the 2-TP model by a Bayes factor K=1.8x10^{5} and shifts the metallicity downward to Zatm/Zsun = 11^{+9}_{-4} from 170^{+30}_{-24} obtained with the 1-TP model. The 2-TP retrieval predicts two, equally weighted components that reproduce the limb-resolved spectra across both instruments without prior knowledge of them. This work demonstrates that compositional information survives limb averaging for WASP-94Ab, suggesting that 2-TP retrievals can be used to identify limb-asymmetry and extend metallicity studies beyond the small set of planets amenable to limb-resolved spectroscopy.

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arXiv2026

Effects of Super-rotating Jets on Phase-Resolved Transmission Spectra at High Spectral Resolution

Hot Jupiter atmospheric circulation can be shaped by a range of processes and planetary parameters, including rotation rate, irradiation gradients from tidal synchronization, magnetic effects, gravity, and clouds. Observations of exoplanetary spectral Doppler shifts as a function of phase, made possible with high-resolution spectrographs, can now probe the effects of exoplanetary winds at several points throughout transit. However, these measurements are difficult to interpret, given the variety of relevant atmospheric mechanisms. In this work, we generate high-resolution transmission spectra from two sets of circulation models: self-consistent dynamical models and idealized models that isolate circulation components. We identify how the strength of the jet --- or the absence of one --- alters the resultant net Doppler shifts. We find that the jet strength most prominently affects Doppler shifts during ingress and egress. During mid-transit, the strength of the jet linearly influences the Doppler shifts' slope; however, this effect is generally secondary to the overall blueshifting from planetary rotation. The slope induced by equatorial jets contrasts with the effect of pure day--night flows, which tend to add constant offsets to Doppler shifts during transit. Our modeling shows that jets also impact the cross-correlation function width; faster jets increase velocity dispersion across the limbs. We complement our simulations with semi-analytical arguments indicating that first-order changes of the cross-correlation function centroid and width (but not amplitude) probe thermal and velocity asymmetries. This work provides recommendations for interpreting net Doppler shifts in transmission and connecting these shifts with exoplanets' atmospheric circulation.

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arXiv2026

Precise Determination of the Metallicity and C/O of WASP-39~b From a Single JWST Instrument Mode with Phase-Resolved Cross-Correlation Retrievals

Measuring atmospheric metallicities and C/O ratios is a key goal of JWST exoplanet science, given their proposed link to planet formation. Achieving this goal has previously been shown to require broad wavelength coverage ($\sim$1--5 $μ$m), typically demanding multiple instrument modes to complete the molecular inventory. Here, we show that the multi-instrument requirement can be circumvented using phase-resolved cross-correlation retrievals at native pixel resolution -- an approach more typically applied to ground-based high-resolution spectroscopy. By applying this novel analysis technique to an archival single-mode transit of the hot Jupiter WASP-39~b obtained with NIRSpec/G395H, we detect and obtain bounded abundances for all of its major carbon- and oxygen-bearing molecules: H$_2$O ($Δ\ln(Z) = 67$), CO ($Δ\ln(Z) = 25$), CO$_2$ ($Δ\ln(Z) = 475$), and SO$_2$ ($Δ\ln(Z) = 10$). Notably, while standard retrieval methods fail to detect CO in these same data ($Δ\ln(Z) = 0.2$), our approach detects it decisively, confirming it as the dominant carbon carrier in WASP-39 b's atmosphere. From these abundances, we robustly derive a metallicity of $\rm [(C + O) / H] = 1.2 \pm 0.2$ and a C/O ratio of $0.68^{+0.10}_{-0.14} $, generally consistent with previous multi-instrument analyses. In comparison, traditional retrievals performed on the G395H data alone produce biased and inaccurate values of both parameters, driven primarily by the non-detection of CO as well as incomplete water band coverage. Our results establish phase-resolved cross-correlation retrievals as a powerful tool for extracting maximum atmospheric information from existing and future JWST data sets.

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arXiv2026

The Roasting Marshmallows Program with IGRINS on Gemini South V: Atmosphere of MASCARA-1b is Enriched in Refractory Elements

Ultra-hot Jupiters (UHJs; $T_{\rm eq} \gtrsim 2000$ K) enable simultaneous detection of volatile (ice-forming) and refractory (rock-forming) species in planetary atmospheres, providing a powerful diagnostic of planet formation and atmospheric processing. We present a comprehensive high-resolution cross-correlation spectroscopy (HRCCS) analysis of the UHJ MASCARA-1b ($T_{\rm eq} \approx 2600$ K) using the IGRINS and IGRINS-2 spectrographs. We detect robust (SNR$>$4) signals from H$_2$O, CO, OH, Fe I, Mg I, Ca I, and Ti I, marking the most complete atmospheric inventory of MASCARA-1b to date. Using a chemically consistent atmospheric inference framework, we constrain elemental abundances to a typical precision of $\approx$0.2 dex, retrieving a solar atmospheric metallicity ([M/H]$_\odot$ $= 0.07^{+0.17}_{-0.13}$ $\approx 1.2\times$ solar), a C/O ratio (C/O $= 0.65^{+0.08}_{-0.08}$) consistent with solar value (C/O $=$ 0.59), an enhanced refractory abundance ([R/H]$_\odot$ $= 0.40^{+0.23}_{-0.17} \approx 2.5\times$ solar; $\approx 3.8\times$ stellar), and a moderately super-solar refractory-to-volatile ratio ([R/V]$_\odot$ $= 0.36^{+0.11}_{-0.09}$ $\approx 2.3\times$ solar). Comparison with formation models suggests that MASCARA-1b most likely accreted material between the soot-H$_2$O or H$_2$O-CO snowlines (at 68$\%$ confidence). We additionally find stellar values for atmospheric Ti/Mg and Ca/Mg ratios (at 68$\%$ confidence). The Mg/Fe is also found to be consistent with stellar value at 95$\%$ confidence. Therefore, we do not find strong indication of nightside cold trapping in MASCARA-1b. As homogeneous refractory-to-volatile measurements expand across the UHJ population, particularly with upcoming Extremely Large Telescopes, these diagnostics will enable statistically robust tests of emerging trends in giant planet formation and atmospheric evolution.

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arXiv2026

Sub-Neptunes as Soot Factories: Deep Atmosphere Hydrocarbon Formation and Quenching as the Origin of Sub-Neptune Aerosol Trends

Recent population-level studies of sub-Neptune atmospheres have identified a tentative parabolic trend in transmission spectrum amplitude for planets with Teq ~ 500-800 K. While the trend has been commonly attributed to hydrocarbon aerosols, we lack a first-principles explanation of its underlying chemical mechanism. Previous work has focused on the role of methane photolysis and subsequent polymerization, but with limited reaction networks that truncated at C2-species and couldn't reproduce the observed parabolic trend. In this work, enabled by a computer-automated, rate-based chemical network generator, we construct the most comprehensive carbon reaction network for exoplanet atmospheres to date. We explicitly model the formation of polycyclic aromatic hydrocarbons (PAHs), which are well established as soot precursors in combustion chemistry. We calculate the chemical timescales of hydrocarbon species through an eigenvalue timescale method and model their quenched abundances across a range of C/O, metallicities, and Teq. In this framework, the deep atmosphere acts as a "soot factory" analogous to a combustion engine, transporting the ingredients for hydrocarbon aerosol formation to the JWST-observable region of the atmosphere, where it may be further augmented by photochemistry. We find that the predicted abundances of PAHs peak near 600 K, and fall off toward higher and lower Teq, consistent with the observed muted-spectra regime suggested in observational studies by HST and JWST. We also show that PAH abundances are expected to vary with C/O and metallicity, thus providing a natural explanation for observed diversity among planets with similar Teq.

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arXiv2026

Magma ocean interactions can explain JWST observations of the sub-Neptune TOI-270 d

Sub-Neptunes with substantial atmospheres may possess magma oceans in contact with the overlying gas, with chemical interactions between the atmosphere and magma playing an important role in shaping atmospheric composition. Early JWST observations have found high abundances of carbon- and oxygen-bearing molecules in a number of sub-Neptune atmospheres, which may result from processes including accretion of icy material at formation or magma-atmosphere interactions. Previous work examining the effects of magma-atmosphere interactions on sub-Neptunes has mostly been limited to studying conditions at the atmosphere-mantle boundary, without considering implications for the upper atmosphere which is probed by spectroscopic observations. In this work, we present a modeling architecture to determine observable signatures of magma-atmosphere interactions. We combine an equilibrium chemistry code which models reactions between the core, mantle and atmosphere with a radiative-convective model that determines the composition and structure of the observable upper atmosphere. We examine how different conditions at the atmosphere-mantle boundary and different core and mantle compositions impact the upper atmospheric composition. We compare our models to JWST NIRISS+NIRSpec observations of the sub-Neptune TOI-270~d, finding that our models can provide a good fit to the observed transmission spectrum with little fine-tuning. This suggests that magma-atmosphere interactions may be sufficient to explain high abundances of molecules such as H$_2$O, CH$_4$ and CO$_2$ in sub-Neptune atmospheres, without additional accretion of icy material from the protoplanetary disk. Although other processes could lead to similar compositions, our work highlights the need to consider magma-atmosphere interactions when interpreting the observed atmospheric composition of a sub-Neptune.

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arXiv2025

Limb Asymmetries on WASP-39b: A Multi-GCM Comparison of Chemistry, Clouds, and Hazes

With JWST, observing separate spectra of the morning and evening limbs of hot Jupiters has finally become a reality. The first such observation was reported for WASP-39b, where the evening terminator was observed to have a larger transit radius by about 400 ppm and a stronger 4.3 $μ$m CO$_2$ feature than the morning terminator. Multiple factors, including temperature differences, photo/thermochemistry, clouds and hazes, could cause such limb asymmetries. To interpret these new limb asymmetry observations, a detailed understanding of how the relevant processes affect morning and evening spectra grounded in forward models is needed. Focusing on WASP-39b, we compare simulations from five different general circulation models (GCMs), including one simulating disequilibrium thermochemistry and one with cloud radiative feedback, to the recent WASP-39b limb asymmetry observations. We also post-process the temperature structures of all simulations with a 2D photochemical model and one simulation with a cloud microphysics model. Although the temperatures predicted by the different models vary considerably, the models are remarkably consistent in their predicted morning--evening temperature differences. Several equilibrium-chemistry simulations predict strong methane features in the morning spectrum, not seen in the observations. When including disequilibrium processes, horizontal transport homogenizes methane, and these methane features disappear. However, even after including photochemistry and clouds, our models still cannot reproduce the observed ${\sim}2000$ ppm asymmetry in the CO$_2$ feature. A combination of factors, such as varying metallicity and unexplored parameters in cloud models, may explain the discrepancy, emphasizing the need for future models integrating cloud microphysics and feedback across a broader parameter space.

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arXiv2025

The Roasting Marshmallows Program with IGRINS on Gemini South III: Seeing deeper into the metal depleted atmosphere of a gas-giant on the cusp of the hot to ultra-hot Jupiter transition

Ultra-hot Jupiters are a class of gas-giant exoplanets that show a peculiar combination of thermochemical properties in the form of molecular dissociation, atomic ionization, and inverted thermal structures. Atmospheric characterization of gas giants lying in the transitional regime between hot and ultra-hot Jupiters can help in understanding the physical mechanisms that cause the fundamental transition in atmospheres between the two classes of hot gas giants. Using Doppler spectroscopy with IGRINS on Gemini South (1.4 to 2.5 $μ$m), we present the day-side high-resolution spectrum of WASP-122b (T$_{\mathrm{day}}$=2258$ \pm$ 54 K), a gas-giant situated at this transition. We detect the signal from H$_{2}$O, based on which we find that WASP-122b has a significantly metal-depleted atmosphere with metallicity log$_{10}$[Z$_{\mathrm{P}}$/Z$_{\odot}$] = $-$1.48$\pm$0.25 dex (0.033$_{-0.016}^{+0.018}$ $\times$ solar), and solar/sub-solar C/O ratio = 0.36$\pm$0.22 (3$σ$ upper limit 0.82). Drastically low atmospheric metallicity pushes the contribution function to higher pressures, resulting in the planetary spectral lines to originate from a narrow region around 1 bar where the thermal profile is non-inverted. This is inconsistent with solar composition radiative convective equilibrium (RCTE) which predicts an inverted atmosphere with spectral lines in emission. The sub-solar metallicity and solar/sub-solar C/O ratio is inconsistent with expectations from core-accretion. We find the planetary signal to be significantly shifted in K$_{\mathrm{P}}$ and V$_{\mathrm{sys}}$, which is in tension with the predictions from global circulation models and require further investigation. Our results highlight the detailed information content of high-resolution spectroscopy data and their ability to constrain complex atmospheric thermal structures and compositions of exoplanets.

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arXiv2025

TOI-1846b: A super-Earth in the radius valley orbiting a nearby M dwarf

We present the discovery and validation of a super-Earth planet orbiting the M dwarf star TOI-1846 (TIC 198385543). The host star(Kmag = 9.6)is located 47 pc away and has a radius of Rs=0.41+/-0.01R_Sun,a mass of Ms=0.40+/-0.02M_Sun and an effective temperature of Teff=3568+/-44K. Our analyses are based on joint modelling of TESS photometry and ground-based multi-color photometric data. We also use high-resolution imaging and archival images, as well as statistical validation techniques to support the planetary system nature. We find that TOI-1846b is a super-Earth sized planet with radius of Rp=1.79+/-0.07R_Earth and a predicted mass of Mp=4.4+1.6-1.0M_Earth (from the Chen & Kipping relation) on a 3.9 d orbit, with an equilibrium temperature of Teq=589+/-20K (assuming a null Bond Albedo) and an incident flux of Sp=17.6+/-2.0S_Earth. Based on the two RV measurements obtained with the TRES spectrograph and high-resolution imaging, a non-planetary transiting companion is excluded. With a radius of ~1.8R_Earth, TOI-1846b is within the sparsely populated radius range around 2R_Earth known as the radius gap (or radius valley). This discovery can contribute to refining the precise location of the radius valley for small planets orbiting bright M dwarfs, thereby enhancing our understanding of planetary formation and evolution processes.

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arXiv2025

Out on a Limb: The Signatures of East-West Asymmetries in Transmission Spectra from General Circulation Models

In the era of JWST, observations of hot Jupiter atmospheres are becoming increasingly precise. As a result, the signature of limb asymmetries due to temperature or abundance differences and the presence of aerosols can now be directly measured using transmission spectroscopy. Using a grid of general circulation models (GCMs) with varying irradiation temperature (1500 K - 4000 K) and prescriptions of cloud formation, we simulate 3D ingress/egress and morning/evening-limb transmission spectra. We aim to assess the impact that clouds, 3D temperature structure, and non-uniform distribution of gases have on the observed spectra, and how these inhomogeneities can be identified. A second goal is to assess the relative merits of two separate methods (ingress/egress v.s. morning/evening-limb spectroscopy) for isolating atmospheric asymmetries. From our models, it is evident that an east-west temperature difference is the leading order effect for producing ingress/egress or morning/evening-limb spectral differences. We additionally find that clouds contribute strongly to the observed limb asymmetry at moderate irradiation temperatures in our grid ($\sim 2000 \mathrm{K} < T_{\mathrm{irr}} < 3500 \mathrm{K}). At lower temperatures clouds equally dominate the optical depth on both limbs, while at higher temperatures the entire terminator region remains cloud-free. We develop limb asymmetry metrics that can be used to assess the degree of east-west asymmetry for a given planet and predict trends in these metrics with respect to irradiation temperature that are indicative of various physical processes. Our results are useful for predicting and diagnosing the signatures of limb asymmetries in JWST spectra.

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arXiv2025

Clouds and Hazes in GJ 1214b's Metal-Rich Atmosphere

The sub-Neptune GJ 1214b has an infamously flat transmission spectrum, likely due to thick aerosols in its atmosphere. A recent JWST MIRI spectroscopic phase curve of GJ 1214 b added to this picture, suggesting a highly reflective and metal-rich atmosphere. Using a 3D General Circulation Model with both photochemical hazes and condensate clouds, we characterize how different aerosol types affect the atmospheric structure of GJ 1214 b and manifest in its spectroscopic phase curve. Additionally, we reanalyze the original GJ 1214 b JWST phase curve. The reanalysis shows a hotter nightside, similar dayside temperature, and a lower, but still elevated, Bond albedo (0.42 +/- 0.11) than the original results. We find that a scenario with both clouds and hazes is most consistent with the JWST phase curve. Reflective clouds or hazes are needed to explain the large Bond albedo, and hazes or a super-solar metallicity help account for the several hundred Kelvin day-night temperature difference measured by the phase curve.

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arXiv2025

Confirmation of four hot Jupiters detected by TESS using follow-up spectroscopy from MaHPS at Wendelstein together with NEID and TRES

We report the confirmation and characterization of four hot Jupiter-type exoplanets initially detected by TESS: TOI-1295 b, TOI-2580 b, TOI-6016 b, and TOI-6130 b. Using observations with the high-resolution echelle spectrograph MaHPS on the 2.1m telescope at Wendelstein Observatory, together with NEID at Kitt Peak National Observatory and TRES at the Fred Lawrence Whipple Observatory, we confirmed the planetary nature of these four planet candidates. We also performed precise mass measurements. All four planets are found to be hot Jupiters with orbital periods between 2.4 and 4.0 days. The sizes of these planets range from 1.29 to 1.64 Jupiter radii, while their masses range from 0.6 to 1.5 Jupiter masses. Additionally, we investigated whether there are signs of other planets in the systems but have found none. Lastly, we compared the radii of our four objects to the results of an empirical study of radius inflation and see that all four demonstrate a good fit with the current models. These four planets belong to the first array of planets confirmed with MaHPS data, supporting the ability of the spectrograph to detect planets around fainter stars as faint as V=12.

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arXiv2025

Peering into the black box: forward-modeling the uncertainty budget of high-resolution spectroscopy of exoplanet atmospheres

Ground-based high-resolution cross-correlation spectroscopy (HRCCS; R >~ 15,000) is a powerful complement to space-based studies of exoplanet atmospheres. By resolving individual spectral lines, HRCCS can precisely measure chemical abundance ratios, directly constrain atmospheric dynamics, and robustly probe multidimensional physics. But the subtleties of HRCCS datasets -- e.g., the lack of exoplanetary spectra visible by eye and the statistically complex process of telluric removal -- can make interpreting them difficult. In this work, we seek to clarify the uncertainty budget of HRCCS with a forward-modeling approach. We present a HRCCS observation simulator, scope (https://github.com/arjunsavel/scope), that incorporates spectral contributions from the exoplanet, star, tellurics, and instrument. This tool allows us to control the underlying dataset, enabling controlled experimentation with complex HRCCS methods. Simulating a fiducial hot Jupiter dataset (WASP-77Ab emission with IGRINS), we first confirm via multiple tests that the commonly used principal components analysis does not bias the planetary signal when few components are used. Furthermore, we demonstrate that mildly varying tellurics and moderate wavelength solution errors induce only mild decreases in HRCCS detection significance. However, limiting-case, strongly varying tellurics can bias the retrieved velocities and gas abundances. Additionally, in the low-SNR limit, constraints on gas abundances become highly non-Gaussian. Our investigation of the uncertainties and potential biases inherent in HRCCS data analysis enables greater confidence in scientific results from this maturing method.

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arXiv2024

A Measurement of the Water Abundance in the Atmosphere of the Hot Jupiter WASP-43b with High-resolution Cross-correlation Spectroscopy

Measuring the abundances of carbon- and oxygen-bearing molecules has been a primary focus in studying the atmospheres of hot Jupiters, as doing so can help constrain the carbon-to-oxygen (C/O) ratio. The C/O ratio can help reveal the evolution and formation pathways of hot Jupiters and provide a strong understanding of the atmospheric composition. In the last decade, high-resolution spectral analyses have become increasingly useful in measuring precise abundances of several carbon- and oxygen-bearing molecules. This allows for a more precise constraint of the C/O ratio. We present four transits of the hot Jupiter WASP-43b observed between 1.45 $-$ 2.45 $μ$m with the high-resolution Immersion GRating InfraRed Spectrometer (IGRINS) on the Gemini-S telescope. We detected H$_2$O at a signal-to-noise ratio (SNR) of 3.51. We tested for the presence of CH$_4$, CO, and CO$_2$, but we did not detect these carbon-bearing species. We ran a retrieval for all four molecules and obtained a water abundance of $\log_{10}(\text{H}_2\text{O}) = -2.24^{+0.57}_{-0.48}$. We obtained an upper limit on the C/O ratio of C/O $<$ 0.95. These findings are consistent with previous observations from the Hubble Space Telescope and the James Webb Space Telescope.

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arXiv2024

OrCAS: Origins, Compositions, and Atmospheres of Sub-neptunes. I. Survey Definition

Sub-Neptunes - volatile-rich exoplanets smaller than Neptune - are intrinsically the most common type of planet known. However, the formation and nature of these objects, as well as the distinctions between sub-classes (if any), remain unclear. Two powerful tools to tease out the secrets of these worlds are measurements of (i) atmospheric composition and structure revealed by transit and/or eclipse spectroscopy, and (ii) mass, radius, and density revealed by transit photometry and Doppler spectroscopy. Here we present OrCAS, a survey to better elucidate the origins, compositions, and atmospheres of sub-Neptunes. This radial velocity survey uses a repeatable, quantifiable metric to select targets suitable for subsequent transmission spectroscopy and address key science themes about the atmospheric & internal compositions and architectures of these systems. Our survey targets 26 systems with transiting sub-Neptune planet candidates, with the overarching goal of increasing the sample of such planets suitable for subsequent atmospheric characterization. This paper lays out our survey's science goals, defines our target prioritization metric, and performs light-curve fits and statistical validation using existing TESS photometry and ground-based follow-up observations. Our survey serves to continue expanding the sample of small exoplanets with well-measured properties orbiting nearby bright stars, ensuring fruitful studies of these systems for many years to come.

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arXiv2024

Mass determination of two Jupiter-sized planets orbiting slightly evolved stars: TOI-2420 b and TOI-2485 b

Hot and warm Jupiters might have undergone the same formation and evolution path, but the two populations exhibit different distributions of orbital parameters, challenging our understanding on their actual origin. The present work, which is the results of our warm Jupiters survey carried out with the CHIRON spectrograph within the KESPRINT collaboration, aims to address this challenge by studying two planets that could help bridge the gap between the two populations. We report the confirmation and mass determination of a hot Jupiter (orbital period shorter than 10 days), TOI-2420\,b, and a warm Jupiter, TOI-2485\,b. We performed a joint analysis using a wide variety of spectral and photometric data in order to characterize these planetary systems. We found that TOI-2420\,b has an orbital period of P$_{\rm b}$=5.8 days, a mass of M$_{\rm b}$=0.9 M$_{\rm J}$ and a radius of R$_{\rm b}$=1.3 R$_{\rm J}$, with a planetary density of 0.477 \gc; while TOI-2485\,b has an orbital period of P$_{\rm b}$=11.2 days, a mass of M$_{\rm b}$=2.4 M$_{\rm J}$ and a radius of R$_{\rm b}$=1.1 R$_{\rm J}$ with density 2.36 \gc. With current parameters, the migration history for TOI-2420\,b and TOI-2485\,b is unclear: the high-eccentricity migration scenarios cannot be ruled out, and TOI-2485\,b's characteristics may rather support this scenario.

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arXiv2024

cortecs: A Python package for compressing opacities

The absorption and emission of light by exoplanet atmospheres encode details of atmospheric composition, temperature, and dynamics. Fundamentally, simulating these processes requires detailed knowledge of the opacity of gases within an atmosphere. When modeling broad wavelength ranges at high resolution, such opacity data, for even a single gas, can take up multiple gigabytes of system random-access memory (RAM). This aspect can be a limiting factor when considering the number of gases to include in a simulation, the sampling strategy used for inference, or even the architecture of the system used for calculations. Here, we present cortecs, a Python tool for compressing opacity data. cortecs provides flexible methods for fitting the temperature, pressure, and wavelength dependencies of opacity data and for evaluating the opacity with accelerated, GPU-friendly methods. The package is actively developed on GitHub (https://github.com/arjunsavel/cortecs), and it is available for download with pip and conda.

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arXiv2024

Inhomogeneous terminators on the exoplanet WASP-39 b

Transmission spectroscopy has been a workhorse technique over the past two decades to constrain the physical and chemical properties of exoplanet atmospheres. One of its classical key assumptions is that the portion of the atmosphere it probes -- the terminator region -- is homogeneous. Several works in the past decade, however, have put this into question for highly irradiated, hot ($T_{eq}\gtrsim 1000$ K) gas giant exoplanets both empirically and via 3-dimensional modelling. While models predict clear differences between the evening (day-to-night) and morning (night-to-day) terminators, direct morning/evening transmission spectra in a wide wavelength range has not been reported for an exoplanet to date. Under the assumption of precise and accurate orbital parameters on WASP-39 b, here we report the detection of inhomogeneous terminators on the exoplanet WASP-39 b, which allows us to retrieve its morning and evening transmission spectra in the near-infrared ($2-5\ μ$m) using JWST. We observe larger transit depths in the evening which are, on average, $405 \pm 88$ ppm larger than the morning ones, also having qualitatively larger features than the morning spectrum. The spectra are best explained by models in which the evening terminator is hotter than the morning terminator by $177^{+65}_{-57}$ K with both terminators having C/O ratios consistent with solar. General circulation models (GCMs) predict temperature differences broadly consistent with the above value and point towards a cloudy morning terminator and a clearer evening terminator.

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