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Published February 10, 2024 | Published
Journal Article Open

Validation of Elemental and Isotopic Abundances in Late-M Spectral Types with the Benchmark HIP 55507 AB System


M dwarfs are common host stars to exoplanets but often lack atmospheric abundance measurements. Late-M dwarfs are also good analogs to the youngest substellar companions, which share similar Teff ∼ 2300–2800 K. We present atmospheric analyses for the M7.5 companion HIP 55507 B and its K6V primary star with Keck/KPIC high-resolution (R ∼ 35,000) K-band spectroscopy. First, by including KPIC relative radial velocities between the primary and secondary in the orbit fit, we improve the dynamical mass precision by 60% and find M_B = 88.0_(−3.2)^(+3.4) M_Jup, putting HIP 55507 B above the stellar–substellar boundary. We also find that HIP 55507 B orbits its K6V primary star with a = 38₋₃⁺⁴ au and e = 0.40 ± 0.04. From atmospheric retrievals of HIP 55507 B, we measure [C/H] = 0.24 ± 0.13, [O/H] = 0.15 ± 0.13, and C/O = 0.67 ± 0.04. Moreover, we strongly detect 13CO (7.8σ significance) and tentatively detect H218O (3.7σ significance) in the companion's atmosphere and measure 12C/13C = 98₋₂₂⁺²⁸ and H₂16O/H₂18O = 240₋₈₀⁺¹⁴⁵ after accounting for systematic errors. From a simplified retrieval analysis of HIP 55507 A, we measure 12C/13C = 79₋₁₆⁺²¹ and C16O/C18O = 288₋₇₀⁺¹²⁵ for the primary star. These results demonstrate that HIP 55507 A and B have consistent 12C/13C and 16O/18O to the <1σ level, as expected for a chemically homogeneous binary system. Given the similar flux ratios and separations between HIP 55507 AB and systems with young substellar companions, our results open the door to systematically measuring 13CO and H₂18O abundances in the atmospheres of substellar or even planetary-mass companions with similar spectral types.

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.


J.X. thanks Paul Mollière for help with generating new opacities in petitRADTRANS and using easyCHEM. 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, and 2023-4598), the Simons Foundation, and the NSF under grant AST-1611623. The computations presented here were conducted at 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). This research has made use of the Keck Observatory Archive (KOA), which is operated by the W. M. Keck Observatory and the NASA Exoplanet Science Institute (NExScI), under contract with the National Aeronautics and Space Administration. 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.


Keck:II (KPIC) -

Software References

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


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Additional details

February 7, 2024
February 7, 2024