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Measuring Tracers of Planet Formation in the Atmosphere of WASP-77A b: Sub-stellar O/H and C/H ratios, with a stellar C/O ratio and a potentially Super-stellar Ti/H ratio

By Keith Cowing

Status Report

astro-ph.EP

February 29, 2024

Filed under astro-ph.EP, astrochemistry, atmosphere, exoplanet, hot jupiter, Hubble, JWST, metallicity, Spectroscopy, WASP-77A b

Measuring Tracers of Planet Formation in the Atmosphere of WASP-77A b: Sub-stellar O/H and C/H ratios, with a stellar C/O ratio and a potentially Super-stellar Ti/H ratio — Best-fit free retrieval of the JWST NIRSpec and HST WFC3 data for WASP-77A b. The coloured regions show the individual contribution of three species (H2O, CO, and TiO) and Collision Induced Absorption (CIA), while the inset shows the 1, 2, and 3σ confidence intervals for the retrieved temperature-pressure (T − p) profile. These three species are detected in the HST and JWST data, allowing us to place constraints on elemental ratios for this atmosphere. The best-fit T − p profile has a positive lapse rate (i.e, no evidence for a thermal inversion) in the pressure region probed by our observations (p ∈ [105 , 100] Pa). — astro-ph.EP

We present a comprehensive atmospheric retrieval study of the hot Jupiter WASP-77A b using eclipse observations from the Hubble Space Telescope (HST) and JWST.

Using atmospheric retrievals, the spectral features of H₂O, CO, and TiO are identified, with volume mixing ratios estimated at log₁₀(VMR) = -4.40^+0.14−0.11, -4.44^+0.34−0.28, and -6.40^+0.22−0.23, respectively. We derive the atmospheric carbon-to-oxygen ratio — a key planetary formation tracer — to be C/O = 0.54±0.12, which is consistent with both the stellar host value and previous studies of the planet’s atmosphere, suggesting a relatively close-in formation.

Computing other elemental ratios (i.e., C/H, O/H, and Ti/H), we conclude that the general enrichment of the atmosphere (i.e., metallicity) is sub-stellar, is depleted in C and O, but that Ti appears slightly super-stellar. A low C and O content could be obtained, in combination with a stellar C/O ratio, if the planet formed outside of the CO₂ snow line before migrating inwards.

Meanwhile, a super-stellar Ti/H could be obtained by late contamination from refractory rich planetesimals. While broadly in agreement with previous works, we do find some differences and discuss these while also highlighting the need for homogeneous analyses when comparative exoplanetology is conducted.

Billy Edwards, Quentin Changeat

Comments: Accepted for publication in ApJL (December 2023)
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2402.18574 [astro-ph.EP] (or arXiv:2402.18574v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2402.18574
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Journal reference: ApJL 962 L30 (2024)
Related DOI:
https://doi.org/10.3847/2041-8213/ad2000
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Submission history
From: Billy Edwards
[v1] Wed, 28 Feb 2024 18:59:34 UTC (4,073 KB)
https://arxiv.org/abs/2402.18574
Astrobiology

Keith Cowing

Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻

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