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Published January 2024 | Published
Journal Article Open

Atmospheric Metallicity and C/O of HD 189733 b from High-resolution Spectroscopy


We present high-resolution K-band emission spectra of the quintessential hot Jupiter HD 189733 b from the Keck Planet Imager and Characterizer. Using a Bayesian retrieval framework, we fit the dayside pressure–temperature profile, orbital kinematics, mass-mixing ratios of H2O, CO, CH4, NH3, HCN, and H2S, and the 13CO/12CO ratio. We measure mass fractions of logH₂O = −2.0_(−0.4)^(+0.4) and log CO= −2.2_(−0.5)^(+0.5), and place upper limits on the remaining species. Notably, we find logCH4 < −4.5 at 99% confidence, despite its anticipated presence at the equilibrium temperature of HD 189733 b assuming local thermal equilibrium. We make a tentative (∼3σ) detection of 13CO, and the retrieved posteriors suggest a 12C/13C ratio similar to or substantially less than the local interstellar value. The possible 13C enrichment would be consistent with accretion of fractionated material in ices or in the protoplanetary disk midplane. The retrieved abundances correspond to a substantially substellar atmospheric C/O = 0.3 ± 0.1, while the carbon and oxygen abundances are stellar to slightly superstellar, consistent with core-accretion models which predict an inverse correlation between C/O and metallicity. The specific combination of low C/O and high metallicity suggests significant accretion of solid material may have occurred late in the formation process of HD 189733 b.

Copyright and License

© 2024. 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.


We thank the anonymous referee whose detailed and insightful comments improved the quality of this paper. L.F. is a member of UAW local 2865. L.F. acknowledges the support of the W. M. Keck Foundation, which also supports development of the KPIC facility data-reduction pipeline. The contributed Hoffman2 computing node used for this work was supported by the Heising-Simons Foundation grant No. 2020-1821. Funding for KPIC has been provided by the California Institute of Technology, the Jet Propulsion Laboratory, the Heising-Simons Foundation (grant Nos. 2015-129, 2017-318, 2019-1312, 2023-4597, 2023-4598), the Simons Foundation (through the Caltech Center for Comparative Planetary Evolution), and the NSF under grant AST-1611623. D.E. is supported by a NASA Future Investigators in NASA Earth and Space Science and Technology (FINESST) fellowship under award No. 80NSSC19K1423. D.E. also acknowledges support from the Keck Visiting Scholars Program (KVSP) to install the Phase II upgrades required for KPIC VFN. J.X. acknowledges support from the NASA Future Investigators in NASA Earth and Space Science and Technology (FINESST) award \#80NSSC23K1434.

This work used computational and storage services associated with the Hoffman2 Shared Cluster provided by UCLA Institute for Digital Research and Educations Research Technology Group. L.F. thanks Briley Lewis for her helpful guide to using Hoffman2, and Paul Mollière for his assistance in adding additional opacities to petitRADTRANS.

The data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Maunakea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain.

This research has made use of the NASA Exoplanet Archive, which is operated by the California Institute of Technology, under contract with the National Aeronautics and Space Administration under the Exoplanet Exploration Program.



Software References

astropy (Astropy Collaboration et al. 20132018), dynesty (Speagle 2020), corner (Foreman-Mackey 2016), VULCAN (Tsai et al. 2021) petitRADTRANS (Mollière et al. 20192020)


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Additional details

February 7, 2024
February 7, 2024