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 H₂O at an abundance of log_XH2O=−2.98+0.68−0.73 at a significance of 8.4σ. We also find evidence for CO and small amounts of CO₂, retrieving abundances log_XCO=−3.76+0.89−1.01 and log_XCO2=−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 C/O between 0.30−0.42 and constrain the atmospheric metallicity to the range 0.4−0.7× 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.

Annabella Meech, Alastair B. Claringbold, Eva-Maria Ahrer, James Kirk, Mercedes López-Morales, Jake Taylor, Richard A. Booth, Anna B.T. Penzlin, Lili Alderson, Duncan A. Christie, Emma Esparza-Borges, Charlotte Fairman, Nathan J. Mayne, Mason McCormack, James E. Owen, Vatsal Panwar, Diana Powell, Denis E. Sergeev, Daniel Valentine, Hannah R. Wakeford, Peter J. Wheatley, Maria Zamyatina

Comments: 23 pages, 20 figures, 7 tables. Accepted to MNRAS on 26 March 2025
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2503.24280 [astro-ph.EP] (or arXiv:2503.24280v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2503.24280
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Submission history
From: Annabella Meech
[v1] Mon, 31 Mar 2025 16:25:50 UTC (7,039 KB)
https://arxiv.org/abs/2503.24280
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