arXiv subjects

Charlotte Fairman

Publications and source records attributed to Charlotte Fairman.

8 recordsLinked to original sources

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.

astro-ph.EP↗

arXiv2026

BOWIE-ALIGN: Exploring degeneracies in the muted transmission spectrum of the aligned hot Jupiter NGTS-2b with NIRSpec/G395H

We present the first atmospheric observation and characterisation of the aligned, 1468 K hot Jupiter, NGTS-2b, with one JWST NIRSpec/G395H transit. These observations complete the GO 3838 observing campaign of the BOWIE-ALIGN program, which aims to investigate the link between hot Jupiter atmospheric composition and formation history through the atmospheric analysis of planets orbiting F stars that are aligned and misaligned with the host stellar spin axis. The 2.84-5.18 micron spectrum shows weak absorption features attributed to H$_2$O and CO$_2$ absorption, which our free chemistry retrievals fit with posteriors that converge on high mean molecular weight solutions attained through significant H$_2$O mixing ratios. By comparing our results to interior modelling, we show that some of these solutions exceed the 43.5x solar upper limit we obtained from our interior structure models. Such solutions are likely due to cloud-metallicity degeneracies and insufficient wavelength coverage to resolve them. We show that, in the case of our observations, the likelihood distribution of H$_2$O abundances is flat and uninformative, such that our retrievals are biased by the prior. Additionally, our statistically favoured atmospheric solution contains absorption from SO. The chemical abundances retrieved with this model are likely not astrophysically feasible and we demonstrate that the presence of SO is driven by only two data points. Our equilibrium chemistry retrievals hint at a subsolar C/O ratio and supersolar metallicity; however, we find wide posterior distributions that extend to solar values.

astro-ph.EP↗

arXiv2026

BOWIE-ALIGN: Sub-solar C/O ratio and metallicity atmosphere of the misaligned hot Jupiter HAT-P-30b

We present the JWST NIRSpec/G395H transmission spectrum of the misaligned hot Jupiter HAT-P-30b from 2.8--5.2 $\mu$m as part of the BOWIE-ALIGN survey, a comparative survey designed to probe the link between planet formation and atmospheric composition in samples of misaligned and aligned hot Jupiters orbiting F-type stars. Through independent data reductions and retrieval analyses, we find evidence for absorption features of H$_2$O and CO$_2$ in the atmosphere of HAT-P-30b. Our retrieved abundances are consistent with equilibrium chemistry, from which we infer a sub-solar C/O ratio (0.16--0.45), and sub-solar and sub-stellar metallicity (0.2--0.8$\times$solar, compared to a stellar metallicity of 1.1--1.6$\times$solar), with muted spectral features. This composition challenges formation models of continuous migration and accretion within a steady disc of stellar metallicity, and could be the result of low C/O ratio gas accretion within the water ice line, low metallicity accretion due to the trapping of volatiles further out in the disc, or the combined accretion of low metallicity gas and carbon-poor solids.

astro-ph.EP↗

arXiv2025

Modelling the effect of stellar metallicity on the XUV evolution of low-mass stars and its impact on exoplanet atmospheres/habitability

Understanding how exoplanet atmospheres evolve is a key question in the context of habitability. One key process governing this evolution is atmospheric evaporation by stellar X-ray and EUV emission (collectively, XUV). As such, the evolution of exoplanet atmospheres is closely tied to the evolution of the host star's magnetic activity. Many studies have modelled the combined evolution of exoplanet atmospheres and their host stars. However, to date, the impact of the host star's metallicity on stellar activity/exoplanet atmosphere evolution has not been explored. In this work, we investigate how stellar metallicity affects the rotation and activity evolution of solar-like stars as well as the corresponding exoplanet atmospheric evolution. We reconfirm previous results that metal-rich stars spin down more rapidly than metal-poor stars. We also find that the XUV flux that an exoplanet in the habitable zone of its host star receives is larger when the host star is more metal-rich. As such, the atmospheres of exoplanets in the habitable zones of metal-rich stars are evaporated more rapidly than exoplanets in the habitable zones of metal-poor stars. Lastly, we find that the atmospheric evolution is most sensitive to the host star metallicity when the host star has a higher mass. In the highest mass solar-stars, the metallicity can have a larger influence on the atmospheric evolution than the initial rotation period of the star.

astro-ph.EP↗

arXiv2025

BOWIE-ALIGN: Weak spectral features in KELT-7b's JWST NIRSpec/G395H transmission spectrum imply a high cloud deck or a low-metallicity atmosphere

Hot Jupiters and their atmospheres are prime targets for transmission spectroscopy due to their extended atmospheres and the corresponding large signal-to-noise, providing the best possible constraints for the atmospheric carbon-to-oxygen (C/O) ratio and metallicity of exoplanets. Within BOWIE-ALIGN, we aim to compare JWST spectra of a sample of orbitally aligned and misaligned hot Jupiters orbiting F-type stars to probe the link between hot Jupiter atmospheres and planet formation history. Here, we present a near-infrared transmission spectrum of the aligned planet KELT-7b using one transit observed with JWST NIRSpec/G395H. We find weak features, only tentative evidence for H$_2$O and CO$_2$ in the atmosphere of KELT-7b. This poses a challenge to constrain the atmospheric properties of KELT-7b and two possible scenarios emerge from equilibrium chemistry and free chemistry retrievals: a high-altitude cloud deck muting all features or an extremely low metallicity atmosphere, respectively. The retrieved C/O ratios from our data reductions range from $0.43 - 0.74$, while the atmospheric metallicity is suggested to be solar to super-solar ($1-16 \times$ solar). Although these wide constraints prevent detailed conclusions about KELT-7b's formation history, a solar-to-super-solar metallicity would imply the accretion of solid material during its formation, which is valuable information for the survey's wider goals of understanding the relative importance of gaseous to solid accretion.

astro-ph.EP↗

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 $8.4σ$. 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 $\log X_\mathrm{CO_2}= -6.86^{+0.62}_{-0.65}$ ($3.1σ$ and $4.0σ$ 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.

astro-ph.EP↗

arXiv2025

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 ($4.2σ$) and CO$_2$ ($8.9σ$). 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$μ$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).

astro-ph.EP↗

arXiv2024

The Importance of Optical Wavelength Data on Atmospheric Retrievals of Exoplanet Transmission Spectra

Exoplanet transmission spectra provide rich information about the chemical composition, clouds and temperature structure of exoplanet atmospheres. Most exoplanet transmission spectra only span infrared wavelengths ($\gtrsim$ 1 $\rm{μm}$), which can preclude crucial atmospheric information from shorter wavelengths. Here, we explore how retrieved atmospheric parameters from exoplanet transmission spectra change with the addition of optical data. From a sample of 14 giant planets with transit spectra from 0.3-4.5 $\rm{μm}$, primarily from the Hubble and Spitzer space telescopes, we apply a free chemistry retrieval to planetary spectra for wavelength ranges of 0.3-4.5 $\rm{μm}$, 0.6-4.5 $\rm{μm}$, and 1.1-4.5 $\rm{μm}$. We analyse the posterior distributions of these retrievals and perform an information content analysis, finding wavelengths below 0.6 $\rm{μm}$ are necessary to constrain cloud scattering slope parameters ($\log{a}$ and $γ$) and alkali species Na and K. There is limited improvement in the constraints on the remaining atmospheric parameters. Across the population, we find limb temperatures are retrieved colder than planetary equilibrium temperatures but have an overall good agreement with Global Circulation Models. As JWST extends to a minimum wavelength of 0.6 $\rm{μm}$, we demonstrate that exploration into complementing JWST observations with optical HST data is important to further our understanding of aerosol properties and alkali abundances in exoplanet atmospheres.

astro-ph.EP↗