arXiv subjects

Diana Powell

Publications and source records attributed to Diana Powell.

At least 19 recordsLinked to original sources

arXiv2026

Wet Removal and Cloud Enhancement: The Microphysics of Cloud-Haze Interactions on Sub-Neptunes

Aerosols are a near-ubiquitous feature of sub-Neptune atmospheres, yet their microphysical nature remains poorly understood. Both condensate clouds and photochemical hazes have been proposed to explain observations, but have largely been studied in isolation. Here we present a new bin-scheme microphysical model, adapted from CARMA, that couples cloud and haze formation through heterogeneous nucleation - the dominant mode of cloud formation in the Solar System - in which haze particles act as cloud condensation nuclei (CCN). Applying this model to KCl clouds on GJ 1214 b-like warm sub-Neptunes, we find that the microphysical contact angle $\theta$ between cloud and haze particles governs distinct regimes of aerosol behavior: at moderate contact angles ($25^\circ \lesssim \theta \lesssim 70^\circ$), hazes are efficiently removed from the upper atmosphere through "wet removal" as they seed gravitationally-settling clouds; at small contact angles ($\theta \lesssim 25^\circ$), heterogeneous nucleation instead produces an enhanced population of mixed cloud-haze particles at high altitudes, dramatically increasing aerosol optical depth ("cloud enhancement"). These structural changes produce differences of up to four scale heights in transmission spectra, with strong effects at optical and near-infrared wavelengths relevant to JWST NIRISS/SOSS, while wavelengths beyond about 3 microns remain comparatively unaffected. We map these effects across orders of magnitude in metallicity, haze production rate, and vertical mixing strength, establishing their generality across sub-Neptune parameter space. Because heterogeneous nucleation is a universal phase-change process, this framework extends naturally to other exoplanet atmospheres and potentially any astrophysical environments where condensation onto foreign substrates may occur, including protoplanetary disks and stellar outflows.

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arXiv2026

CARMApy: An Open-Source Python Framework for Simulating Microphysical Clouds in Planetary Atmospheres

CARMApy is a new open-source python code that performs bin-scheme microphysical modeling of clouds in exoplanet atmospheres. It models key cloud properties such as particle size distributions and microphysical rates from first principles. The code is a wrapper of ExoCARMA, a well tested Fortran code with an almost half century long heritage. CARMApy includes the microphysical processes of homogeneous and heterogeneous nucleation, condensational growth, evaporation, coagulation, and vertical transport. CARMApy has 10 built-in default condensates and allows the user to specify additional condensates. In this work we describe CARMApy and the data products that it can generate, along with the history of its code heritage. We additionally compile a complete description of the theory and methods used in CARMA. Lastly we benchmark CARMApy and show that its results are consistent with previous versions of CARMA, while executing the code ~1.9 times faster single threaded ~3.8 times faster multithreaded.

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arXiv2026

Photochemical Production of CS2 in Temperate-to-Warm Gas Giant Exoplanet Atmospheres

Sulfur chemistry has emerged as an important probe of exoplanet atmospheres in the JWST era, although observational constraints have thus far been largely limited to SO2 and H2S in warm and hot exoplanets. Recent JWST observations have revealed CS2 in several cooler gas-giant exoplanets, yielding a new tracer of sulfur chemistry. However, the detailed chemical pathways responsible for the formation of CS2 remain poorly understood. Here, we use TOI-6894 b, a temperate gas giant with evidence for CS2, as a test case for one-dimensional photochemical kinetic-transport modeling and sensitivity analyses of CS2 chemistry. We show that CS2 is produced through coupled thermochemical and photochemical processes involving CH4 and H2S as the primary carbon and sulfur reservoirs, with S2 photolysis driving disequilibrium sulfur chemistry. Our models provide a physically consistent explanation for the observed CS2 feature in TOI-6894 b. Extending our analysis to gas giant exoplanets spanning a wide range of Teq, we find that CS2 abundance peaks in temperate to warm atmospheres (Teq ~ 500 - 700 K), and declines toward both lower and higher temperatures. This temperature dependence provides a unified framework for interpreting current CS2 observations, accounting for reported detections in temperate to warm planets and the lack of detections in colder and hotter giant exoplanets. Our results establish CS2 as a complementary probe of sulfur inventories and atmospheric metallicity in cool gas giant exoplanets

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arXiv2026

Three dimensional temporal evolution of photochemical haze in exoplanet atmospheres I. Description and test application to HD 189733b

The formation and global spatial distribution of photochemically produced haze particles remain a key process in exoplanet atmospheres for understanding their observed properties. We aim to develop a flexible haze particle formation and evolution model suitable for time-dependent exoplanet atmosphere simulations. Inspired by recent 2D photochemical modelling efforts, we include a simple activation timescale mechanism to emulate a delayed formation of solid haze particles. We couple our new microphysical haze formation scheme, mini-haze, to the Exo-FMS general circulation model (GCM) and simulate an idealised HD 189733b case study to examine the 3D spatial distribution and sizes of haze particles. Our results suggest that for our chosen haze formation efficiency, particles do not grow beyond $\sim$30 nm, in line with previous detailed 1D modelling. We find the haze spatial distribution follows the vertical velocity structure of the atmosphere, with equatorial convergence patterns of material deeper in the atmosphere at $\sim$10$^{-2}$ bar. The resulting global distribution leads to enhanced haze opacity at the east and west limbs of the atmosphere. In our test cases, radiative feedback from haze opacity can strongly affect the temperature-pressure structures in the upper atmosphere depending on the production rate. Our synthetic spectra results suggest that longer haze-production timescales give rise to stronger haze opacity effects on the observed transmission spectra compared to short-timescale dayside formation, but the stronger thermal feedback from nightside formation leads to an overall larger dayside emission flux. Our current simulations represent a step towards investigating self-consistent haze formation and evolution with chemical feedback effects in 3D, and can be readily applied to other objects of interest, such as sub-Neptune atmospheres.

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arXiv2026

Magnesium Silicate Clouds in the Atmosphere of HD 209458b from a Rule-Based Tree-Structured Data Reduction

HD 209458b is the canonical hot Jupiter: the first to have its atmosphere measured and the first to hint at the role of aerosols in exoplanet atmospheres through the muting of Na absorption signatures in the optical. Here we present JWST MIRI/LRS transmission observations of HD 209458b from 5-12 microns, directly measuring the absorption signatures of its clouds for the first time. The observations indicate the presence of magnesium silicates, most likely Mg2SiO4 or a mixture of Mg2SiO4 and MgSiO3. We also present a new methodology to reduce observational data, whereby the analysis is formulated as a rule-based model with a tree structure, enabling key decisions to be identified and uncertain decisions to be incorporated into subsequent modeling. With this data reduction, and using a combination of ARCiS free retrievals and PICASO+Virga self consistent forward models, we are able to show that amorphous Mg2SiO4 clouds explain the LRS data to high significance over either a clear (Delta ln(Z)=16.63) or gray cloud atmosphere (Delta ln(Z)=22.26). By combining the LRS dataset with archival JWST NIRCam and HST optical and near-infrared observations, we are able to more robustly constrain the properties of the magnesium silicate condensates, finding particle sizes of approximately 0.1 microns and atmospheric pressures of the clouds of roughly 1-10 millibar. Our results add to the growing detections of silicate clouds as a dominant atmospheric component of hot Jupiters, with the exact silicate species contextualizing the atmospheric chemistry and potentially formation conditions of these planets.

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arXiv2026

Cloudy With a Chance of Meatballs

Speculative fiction has long served an inspiration for genuine scientific inquiry. One notable work that has almost acted in this manner is the the seminal comedic speculative fiction work Cloudy with a Chance of Meatballs. While exoplaneteers reference this work frequently, we have never engaged with the central prediction of this work... until now! We perform detailed microphysical modeling of meatball clouds, both bare and coated with marinara sauce, and find that while meatball condensation is possible in temperate atmospheres, the meatballs do not quite grow to the sizes predicted by Cloudy. We do find, however, that such meatball condensation, across a large enough planet, would be able to sustain humanity calorically.

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arXiv2026

Analytic Modeling of Tidally Locked Rocky Planet Atmospheres Across Dynamical Regimes

We present a new first-principles analytic approach to interpreting eclipses and phase curves of rocky planets. Observations with JWST have reported nondetections of atmospheres around the majority of hot rocky planets orbiting M dwarfs. However, most of these "bare rock" inferences are based on models that are ill-suited to many currently observable planets, as they were developed for use on cooler, slower-rotating bodies. In particular, these models rely on the weak temperature gradient assumption, in which rotation is neglected and temperature gradients can be simply related to wind speeds. We find that this assumption may not be valid for over 40% of terrestrials observed with JWST, including TRAPPIST-1b, GJ 367b, and TOI-2445b. Our simple new four-box model does not rely on this assumption, and instead allows the heat transport efficiency to be specified or follow scalings derived herein. This method is fast, interpretable, physically motivated, reproduces previous general circulation model results, and can be used as a starting point for more detailed modeling. We observe that the longitudinal temperature structure of tidally locked terrestrials depends strongly on the atmospheric circulation. Considering the applicable range of atmospheric dynamical regimes, we find that a given planet's nightside temperature can plausibly vary by 100s of Kelvin (from detectable to undetectable). Furthermore, a planet's dayside energy balance can display complex behavior, with degeneracies between surface pressure and dayside temperature. Illustrating an application to observations, we find that assumptions about atmospheric dynamics and longitudinal temperature structure can bias atmospheric constraints at the order-of-magnitude level.

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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 $\mu$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

Mapping the SO2 Shoreline in Gas Giant Exoplanets

JWST has revealed sulfur chemistry in giant exoplanet atmospheres, where molecules such as SO2 trace photochemistry, metallicity, and formation and migration. To ascertain the conditions that determine whether (or how much) SO2, H2S, and other sulfur-bearing species are present in exoplanet atmospheres, we present a grid of planetary atmospheres covering metallicities from 0.3-1000x Solar and temperatures from 250-2050 K. These models map out the 'SO2 shoreline,' the region of metallicity and irradiation for which SO2 may be sufficiently abundant to be detectable. SO2 is a sensitive indicator of metallicity; expected SO2 abundances also depend strongly on overall temperature and C/O ratio; the SO2 abundance depends surprisingly weakly on XUV irradiation, also weakly on Kzz (for Teq > 600 K), and is essentially independent of internal temperature. Despite its detection in a growing number of giant planets, SO2 is never the dominant sulfur-bearing molecule: depending on temperature and metallicity, H2S, S2, NS, SO, SH, and even S8 or atomic S are frequently as common (or more so) as SO2. Nonetheless SO2 remains the most easily detectable sulfur-bearing species, followed by H2S, though perhaps SO and SH could be detectable in some gas giants. Aside from a pressing need for additional observational constraints on sulfur, we also identify the need for future work to account for the effects of clouds and hazes, fully self-consistent atmospheric models, 2D and 3D models, a wider range of planetary masses and radii, and studies to measure and refine reaction rates and molecular opacities of sulfur-bearing species

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arXiv2025

Volcanic Satellites Tidally Venting Na, K, SO2 in Optical & Infrared Light

Recent infrared spectroscopy from the James Webb Space Telescope (JWST) has spurred analyses of common volcanic gases such as carbon dioxide (CO2), sulfur dioxide (SO2), alongside alkali metals sodium (Na I) and potassium (K I) surrounding the hot Saturn WASP-39 b. We report more than an order-of-magnitude of variability in the density of neutral Na, K, and SO2 between ground-based measurements and JWST, at distinct epochs, hinting at exogenic physical processes similar to those sourcing Io's extended atmosphere and torus. Tidally-heated volcanic satellite simulations sputtering gas into a cloud or toroid orbiting the planet, are able to reproduce the probed line-of-sight column density variations. The estimated SO2 flux is consistent with tidal gravitation predictions, with a Na/SO2 ratio far smaller than Io's. Although stable satellite orbits at this system are known to be < 15.3 hours, several high-resolution alkali Doppler shift observations are required to constrain a putative orbit. Due to the Roche limit interior to the planetary photosphere at ~ 8 hours, atmosphere-exosphere interactions are expected to be especially important at this system.

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arXiv2025

BOWIE-ALIGN: Sub-stellar metallicity and carbon depletion in the aligned TrES-4b with JWST NIRSpec transmission spectroscopy

The formation and migration history of a planet is expected to be imprinted in its atmosphere, in particular its carbon-to-oxygen (C/O) ratio and metallicity. The BOWIE-ALIGN programme is performing a comparative study of JWST spectra of four aligned and four misaligned hot Jupiters, with the aim of characterising their atmospheres and corroborating the link between the observables and the formation history. In this work, we present the $2.8-5.2$ micron transmission spectrum of TrES-4b, a hot Jupiter with an orbit aligned with the rotation axis of its F-type host star. Using free chemistry atmospheric retrievals, we report a confident detection of H$_2$O at an abundance of $\log X_\mathrm{H_2O}=-2.98^{+0.68}_{-0.73}$ at a significance of $8.4\sigma$. We also find evidence for CO and small amounts of CO$_2$, retrieving abundances $\log X_\mathrm{CO}= -3.76^{+0.89}_{-1.01}$ and $\log X_\mathrm{CO_2}= -6.86^{+0.62}_{-0.65}$ ($3.1\sigma$ and $4.0\sigma$ respectively). The observations are consistent with the the atmosphere being in chemical equilibrium; our retrievals yield $\mathrm{C/O}$ between $0.30-0.42$ and constrain the atmospheric metallicity to the range $0.4-0.7\times$ solar. The inferred sub-stellar properties (C/O and metallicity) challenge traditional models, and could have arisen from an oxygen-rich gas accretion scenario, or a combination of low-metallicity gas and carbon-poor solid accretion.

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arXiv2024

BOWIE-ALIGN: JWST reveals hints of planetesimal accretion and complex sulphur chemistry in the atmosphere of the misaligned hot Jupiter WASP-15b

We present a transmission spectrum of the misaligned hot Jupiter WASP-15b from 2.8-5.2 microns observed with JWST's NIRSpec/G395H grating. Our high signal to noise data, which has negligible red noise, reveals significant absorption by H$_2$O ($4.2\sigma$) and CO$_2$ ($8.9\sigma$). From independent data reduction and atmospheric retrieval approaches, we infer that WASP-15b's atmospheric metallicity is super-solar ($\gtrsim 15\times$ solar) and its carbon-to-oxygen ratio is consistent with solar, that together imply planetesimal accretion. Our general circulation model simulations for WASP-15b suggest that the carbon-to-oxygen we measure at the limb is likely representative of the entire photosphere due to the mostly uniform spatial distribution of H$_2$O, CO$_2$ and CO. We additionally see evidence for absorption by SO$_2$ and absorption at 4.9$\mu$m, for which the current leading candidate is OCS, albeit with several caveats. If confirmed, this would be the first detection of OCS in an exoplanet atmosphere and point towards complex photochemistry of sulphur-bearing species in the upper atmosphere. These are the first observations from the BOWIE-ALIGN survey which is using JWST's NIRSpec/G395H instrument to compare the atmospheric compositions of aligned/low-obliquity and misaligned/high-obliquity hot Jupiters around F stars above the Kraft break. The goal of our survey is to determine whether the atmospheric composition differs across two populations of planets that have likely undergone different migration histories (disc versus disc-free) as evidenced by their obliquities (aligned versus misaligned).

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arXiv2024

Nightside Clouds on Tidally-locked Terrestrial Planets Mimic Atmosphere-Free Scenarios

We investigate the impact of nightside cloud formation on the observable night-day contrast of tidally-locked terrestrial planet atmospheres. We demonstrate that, in the case where the planetary dayside is only 10s of Kelvin hotter than the planetary nightside, the presence of optically thick nightside clouds can lead to observations that mimic a planet without an atmosphere, despite the planet actually hosting a significant (10 bar) atmosphere. The scenario presented in this work requires a level of intrinsic atmospheric day/night temperature contrast such that the nightside can form clouds while the dayside is too hot for cloud formation to occur. This scenario is most likely for hotter terrestrials and terrestrials with low volatile inventories. We thus note that a substantial dayside/nightside temperature difference alone does not robustly indicate that a planet does not host an atmosphere and additional observations and modeling are essential for characterization. We further discuss several avenues for future study to improve our understanding of the terrestrial planets and how best to characterize them with JWST.

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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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arXiv2024

Sulphur dioxide in the mid-infrared transmission spectrum of WASP-39b

The recent inference of sulphur dioxide (SO$_2$) in the atmosphere of the hot ($\sim$1100 K), Saturn-mass exoplanet WASP-39b from near-infrared JWST observations suggests that photochemistry is a key process in high temperature exoplanet atmospheres. This is due to the low ($<$1 ppb) abundance of SO$_2$ under thermochemical equilibrium, compared to that produced from the photochemistry of H$_2$O and H$_2$S (1-10 ppm). However, the SO$_2$ inference was made from a single, small molecular feature in the transmission spectrum of WASP-39b at 4.05 $μ$m, and therefore the detection of other SO$_2$ absorption bands at different wavelengths is needed to better constrain the SO$_2$ abundance. Here we report the detection of SO$_2$ spectral features at 7.7 and 8.5 $μ$m in the 5-12 $μ$m transmission spectrum of WASP-39b measured by the JWST Mid-Infrared Instrument (MIRI) Low Resolution Spectrometer (LRS). Our observations suggest an abundance of SO$_2$ of 0.5-25 ppm (1$σ$ range), consistent with previous findings. In addition to SO$_2$, we find broad water vapour absorption features, as well as an unexplained decrease in the transit depth at wavelengths longer than 10 $μ$m. Fitting the spectrum with a grid of atmospheric forward models, we derive an atmospheric heavy element content (metallicity) for WASP-39b of $\sim$7.1-8.0 $\times$ solar and demonstrate that photochemistry shapes the spectra of WASP-39b across a broad wavelength range.

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arXiv2024

BOWIE-ALIGN: How formation and migration histories of giant planets impact atmospheric compositions

Hot Jupiters present a unique opportunity for measuring how planet formation history shapes present-day atmospheric composition. However, due to the myriad pathways influencing composition, a well-constructed sample of planets is needed to determine whether formation history can be accurately traced back from atmospheric composition. To this end, the BOWIE-ALIGN survey will compare the compositions of 8 hot Jupiters around F stars, 4 with orbits aligned with the stellar rotation axis and 4 misaligned. Using the alignment as an indicator for planets that underwent disc migration or high-eccentricity migration, one can determine whether migration history produces notable differences in composition between the two samples of planets. This paper describes the planet formation model that motivates our observing programme. Our model traces the accretion of chemical components from the gas and dust in the disc over a broad parameter space to create a full, unbiased model sample from which we can estimate the range of final atmospheric compositions. For high metallicity atmospheres (O/H > 10 times solar), the C/O ratios of aligned and misaligned planets diverge, with aligned planets having lower C/O (< 0.25) due to the accretion of oxygen-rich silicates from the inner disc. However, silicates may rain out instead of releasing their oxygen into the atmosphere. This would significantly increase the C/O of aligned planets (C/O > 0.6), inverting the trend between the aligned and misaligned planets. Nevertheless, by comparing statistically significant samples of aligned and misaligned planets, we expect atmospheric composition to constrain how planets form.

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arXiv2024

BOWIE-ALIGN: A JWST comparative survey of aligned vs misaligned hot Jupiters to test the dependence of atmospheric composition on migration history

A primary objective of exoplanet atmosphere characterisation is to learn about planet formation and evolution, however, this is challenged by degeneracies. To determine whether differences in atmospheric composition can be reliably traced to differences in evolution, we are undertaking a transmission spectroscopy survey with JWST to compare the compositions of a sample of hot Jupiters that have different orbital alignments around F stars above the Kraft break. Under the assumption that aligned planets migrate through the inner disc, while misaligned planets migrate after disc dispersal, the act of migrating through the inner disc should cause a measurable difference in the C/O between aligned and misaligned planets. We expect the amplitude and sign of this difference to depend on the amount of planetesimal accretion and whether silicates accreted from the inner disc release their oxygen. Here, we identify all known exoplanets that are suitable for testing this hypothesis, describe our JWST survey, and use noise simulations and atmospheric retrievals to estimate our survey's sensitivity. With the selected sample of four aligned and four misaligned hot Jupiters, we will be sensitive to the predicted differences in C/O between aligned and misaligned hot Jupiters for a wide range of model scenarios.

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arXiv2024

Debris Disks can Contaminate Mid-Infrared Exoplanet Spectra: Evidence for a Circumstellar Debris Disk around Exoplanet Host WASP-39

The signal from a transiting planet can be diluted by astrophysical contamination. In the case of circumstellar debris disks, this contamination could start in the mid-infrared and vary as a function of wavelength, which would then change the observed transmission spectrum for any planet in the system. The MIRI/LRS WASP-39b transmission spectrum shows an unexplained dip starting at $\sim$10 $μ$m that could be caused by astrophysical contamination. The spectral energy distribution displays excess flux at similar levels to that which are needed to create the dip in the transmission spectrum. In this article, we show that this dip is consistent with the presence of a bright circumstellar debris disk, at a distance of $>$2 au. We discuss how a circumstellar debris disk like that could affect the atmosphere of WASP-39b. We also show that even faint debris disks can be a source of contamination in MIRI exoplanet spectra.

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