Are These Planets or Brown Dwarfs? Broadly Solar Compositions from High-resolution Atmospheric Retrievals of ∼10–30 M_(Jup) Companions

Creators: Xuan, Jerry W.¹; Hsu, Chih-Chun²; Finnerty, Luke³; Wang, Jason²; Ruffio, Jean-Baptiste⁴; Zhang, Yapeng¹; Knutson, Heather¹; Mawet, Dimitri¹; Mamajek, Eric E.⁵; Inglis, Julie¹; Wallack, Nicole L.⁶; Bryan, Marta L.⁷; Blake, Geoffrey A.¹; Mollière, Paul⁸; Hejazi, Neda⁹; Baker, Ashley¹; Bartos, Randall⁵; Calvin, Benjamin³; Cetre, Sylvain¹⁰; Delorme, Jacques-Robert¹⁰; Doppmann, Greg¹⁰; Echeverri, Daniel¹; Fitzgerald, Michael P.³; Jovanovic, Nemanja¹; Liberman, Joshua^{1, 11}; López, Ronald A.³; Morris, Evan¹²; Pezzato, Jacklyn¹; Sappey, Ben⁴; Schofield, Tobias¹; Skemer, Andrew¹²; Wallace, J. Kent⁵; Wang, Ji¹³; Agrawal, Shubh¹⁴; Horstman, Katelyn¹

1. California Institute of Technology
2. Northwestern University
3. University of California, Los Angeles
4. University of California, San Diego
5. Jet Propulsion Lab
6. Carnegie Institution for Science
7. University of Toronto
8. Max Planck Institute for Astronomy
9. University of Kansas
10. W.M. Keck Observatory
11. University of Arizona
12. University of California, Santa Cruz
13. The Ohio State University
14. University of Pennsylvania

Abstract

Using Keck Planet Imager and Characterizer high-resolution (R ∼ 35,000) spectroscopy from 2.29 to 2.49 μm, we present uniform atmospheric retrievals for eight young substellar companions with masses of ∼10–30 M_Jup, orbital separations spanning ∼50–360 au, and T_eff between ∼1500 and 2600 K. We find that all companions have solar C/O ratios and metallicities to within the 1σ–2σ level, with the measurements clustered around solar composition. Stars in the same stellar associations as our systems have near-solar abundances, so these results indicate that this population of companions is consistent with formation via direct gravitational collapse. Alternatively, core accretion outside the CO snowline would be compatible with our measurements, though the high mass ratios of most systems would require rapid core assembly and gas accretion in massive disks. On a population level, our findings can be contrasted with abundance measurements for directly imaged planets with m < 10 M_Jup, which show tentative atmospheric metal enrichment compared to their host stars. In addition, the atmospheric compositions of our sample of companions are distinct from those of hot Jupiters, which most likely form via core accretion. For two companions with T_eff ∼ 1700–2000 K (κ And b and GSC 6214–210 b), our best-fit models prefer a nongray cloud model with >3σ significance. The cloudy models yield 2σ−3σ lower T_eff for these companions, though the C/O and [C/H] still agree between cloudy and clear models at the 1σ level. Finally, we constrain ¹²CO/¹³CO for three companions with the highest signal-to-noise ratio data (GQ Lup b, HIP 79098b, and DH Tau b) and report

Copyright and License

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

J.X. thanks Melanie Rowland, Yayaati Chachan, Michael Liu, Kazumasa Ohno, Douglas Lin, Ricardo López-Valdivia, Henrique Reggiani, and Jingwen Zhang for helpful discussions. J.X. is supported by the NASA Future Investigators in NASA Earth and Space Science and Technology (FINESST) award No. 80NSSC23K1434. J.X. also acknowledges support from the Keck Visiting Scholars Program (KVSP) to commission KPIC Phase II capabilities. D.E. is supported by NASA FINESST award No. 80NSSC19K1423. D.E. also acknowledges support from the Keck Visiting Scholars Program (KVSP) to install the Phase II upgrades. 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, 2023-4598), the Simons Foundation, and the NSF under grant AST-1611623. The computations presented here were conducted in the Resnick High Performance Center, a facility supported by the Resnick Sustainability Institute at the California Institute of Technology. W. M. Keck Observatory access was supported by Northwestern University and the Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA). 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 work benefited from the 2023 Exoplanet Summer Program in the Other Worlds Laboratory (OWL) at the University of California, Santa Cruz, a program funded by the Heising-Simons Foundation and NASA. The research here acknowledges use of the Hypatia Catalog Database, an online compilation of stellar abundance data as described in Hinkel et al. (2014), which was supported by NASA's Nexus for Exoplanet System Science (NExSS) research coordination network and the Vanderbilt Initiative in Data-Intensive Astrophysics (VIDA).

Facilities

Keck:II (KPIC) - KECK II Telescope.

Software References

petitRADTRANS (Mollière et al. 2019), dynesty (Speagle 2020).

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