Strong NUV Refractory Absorption and Dissociated Water in the Hubble Transmission Spectrum of the Ultra Hot Jupiter KELT-20 b
Creators
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1.
University of California, Santa Cruz
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2.
Space Telescope Science Institute
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3.
California Institute of Technology
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4.
University of Maryland, College Park
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5.
University of Montreal
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6.
Johns Hopkins University
Abstract
Ultra-hot Jupiters (UHJs) present a promising pathway for drawing a link between a planet’s composition and formation history. They retain both refractory and volatiles species in gas phase in their atmospheres, which allows us to place unique constraints on their building blocks. Here, we present the 0.2–1.7 μm transmission spectrum of KELT-20 b/MASCARA-2 b taken with the Hubble Space Telescope (HST). Unlike other UHJs around early-type stars, KELT-20 b’s orbit is well aligned with its host star’s spin axis, and we test whether its distinct dynamical configuration is reflected in its composition. We observe a tremendous rise (>10 scale heights) in the planet’s transit depth at the near-UV (NUV) wavelengths, akin to that observed for WASP-178 b and WASP-121 b, and a muted water absorption feature in the near-IR. Our retrievals indicate that the large NUV depth is driven by Fe II and/or SiO and that the water is mostly thermally dissociated. Assuming equilibrium chemistry, we obtain constraints on Z/H and O/H that indicate accretion of volatile-rich solids and/or gas. Both our low-resolution spectrum and the refractory elemental ratios from S. Gandhi et al. suggest that nightside condensation and rainout are limited to only the most refractory species in the planet’s atmosphere. Within the precision limits of the HST spectra, no strong evidence for limb asymmetry is detected. We contextualize this lack of asymmetry by comparing to predictions from general circulation models with and without the effects of kinematic magnetohydrodynamics. Lastly, we find no major differences in the HST transmission spectra of KELT-20 b, WASP-178, and WASP-121 b despite their different dynamical configurations.
Copyright and License
© 2025. The Author(s). Published by the American Astronomical Society. Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.
Acknowledgement
We are grateful to the referee for the helpful suggestions. We thank Sid Gandhi for sharing the posteriors from his fit to the high-resolution data of KELT-20 b and thank Luca Fossati for sharing the LTE and NLTE models from their work. This work is based on observations from the Hubble Space Telescope, operated by AURA, Inc. on behalf of NASA/ESA. Support for this work was provided by NASA through Space Telescope Science Institute grant GO-17082. The authors gratefully acknowledge the computing time granted by the Resource Allocation Board and provided on the supercomputer Emmy at NHR-Nord@Göttingen as part of the NHR infrastructure. The calculations for this research were conducted with computing resources under the project hhp00051. The Hubble data presented in this article were obtained from the Mikulski Archive for Space Telescopes (MAST) at the Space Telescope Science Institute. The specific observations analyzed can be accessed via doi: 10.17909/85yb-4k35.
Facilities
HST - Hubble Space Telescope satellite (WFC3).
Software References
astropy (Astropy Collaboration et al. 2013, 2018), batman (L. Kreidberg 2015), catwoman (K. Jones & N. Espinoza 2020), LDTk (H. Parviainen & S. Aigrain 2015), Matplotlib (J. D. Hunter 2007), NumPy (T. E. Oliphant 2006; S. van der Walt et al. 2011), petitRADTRANS (P. Molliére et al. 2019), PETRA (J. D. Lothringer & T. S. Barman 2020)
Files
Chachan_2025_AJ_170_234.pdf
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Additional details
Related works
- Is new version of
- Discussion Paper: arXiv:2508.10092 (arXiv)
- Is supplemented by
- Dataset: 10.17909/85yb-4k35 (DOI)
Funding
- Space Telescope Science Institute
- GO-17082
Dates
- Accepted
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2025-08-13
- Available
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2025-09-23Published online