KPF Confirms a Polar Orbit for KELT-18 b

Creators: Rubenzahl, Ryan A.¹; Dai, Fei^{1, 2}; Halverson, Samuel³; Howard, Andrew W.¹; Householder, Aaron⁴; Fulton, Benjamin⁵; Behmard, Aida⁶; Gibson, Steven R.¹; Roy, Arpita⁷; Shaum, Abby P.¹; Isaacson, Howard⁸; Brodheim, Max⁹; Deich, William¹⁰; Hill, Grant M.⁹; Holden, Bradford¹⁰; Laher, Russ R.⁵; Lanclos, Kyle⁹; Payne, Joel N.⁹; Petigura, Erik A.¹¹; Schwab, Christian¹²; Smith, Chris⁸; Stefánsson, Guðmundur¹³; Walawender, Josh⁹; Wang, Sharon X.¹⁴; Weiss, Lauren M.¹⁵; Winn, Joshua N.¹⁶; Wishnow, Edward⁸

1. California Institute of Technology
2. University of Hawaii System
3. Jet Propulsion Lab
4. Massachusetts Institute of Technology
5. NASA Exoplanet Science Institute
6. American Museum of Natural History
7. Schmidt Sciences
8. University of California, Berkeley
9. W.M. Keck Observatory
10. University of California, Santa Cruz
11. University of California, Los Angeles
12. Macquarie University
13. University of Amsterdam
14. Tsinghua University
15. University of Notre Dame
16. Princeton University

Abstract

We present the first spectroscopic transit results from the newly commissioned Keck Planet Finder on the Keck-I telescope at W. M. Keck Observatory. We observed a transit of KELT-18 b, an inflated ultrahot Jupiter orbiting a hot star (T_eff = 6670 K) with a binary stellar companion. By modeling the perturbation to the measured cross-correlation functions using the Reloaded Rossiter–McLaughlin technique, we derived a sky-projected obliquity of λ = − 948 ± 07 (ψ=93.8_(−1.8)_^(+1.6)∘ for isotropic i_⋆). The data are consistent with an extreme stellar differential rotation (α = 0.9), though a more likely explanation is moderate center-to-limb variations of the emergent stellar spectrum. We see additional evidence for the latter from line widths increasing toward the limb. Using loose constraints on the stellar rotation period from observed variability in the available TESS photometry, we were able to constrain the stellar inclination and thus the true 3D stellar obliquity to ψ=91.7_(−1.8)^(+2.2)∘. KELT-18 b could have obtained its polar orbit through high-eccentricity migration initiated by Kozai–Lidov oscillations induced by the binary stellar companion KELT-18 B, as the two likely have a large mutual inclination as evidenced by Gaia astrometry. KELT-18 b adds another data point to the growing population of close-in polar planets, particularly around hot stars.

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

We are grateful to Heather Cegla and Michael Palumbo for illuminating discussions on center-to-limb variations. Some of the data presented herein were obtained at Keck Observatory, which is a private 501(c)3 nonprofit organization 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. Keck Observatory occupies the summit of Maunakea, a place of significant ecological, cultural, and spiritual importance within the indigenous Hawaiian community. We understand and embrace our accountability to Maunakea and the indigenous Hawaiian community, and commit to our role in long-term mutual stewardship. We are most fortunate to have the opportunity to conduct observations from Maunakea.

R.A.R. acknowledges support from the National Science Foundation through the Graduate Research Fellowship Program (DGE 1745301). A.W.H. acknowledges funding support from NASA award 80NSSC24K0161 and the JPL President's and Director's Research and Development Fund. This paper made use of data collected by the TESS mission and is publicly available from the Mikulski Archive for Space Telescopes (MAST) operated by the Space Telescope Science Institute (STScI). All the TESS data used in this paper can be found in MAST (MAST 2021). This research was carried out, in part, at the Jet Propulsion Laboratory and the California Institute of Technology under a contract with the National Aeronautics and Space Administration and funded through the President's and Director's Research & Development Fund Program.

Facilities

Keck:I - KECK I Telescope (KPF; S. R. Gibson et al. 2020), TESS - (G. R. Ricker et al. 2015).

Software References

astropy (Astropy Collaboration et al. 2013, 2018, 2022), corner (D. Foreman-Mackey 2016), emcee (D. Foreman-Mackey et al. 2013), lightkurve (Lightkurve Collaboration et al. 2018), matplotlib (J. D. Hunter 2007), numpy (C. R. Harris et al. 2020), pandas (pandas development team, T., 2020), radvel (B. J. Fulton et al. 2018), scipy (P. Virtanen et al. 2020), SpecMatch-Emp (S. W. Yee et al. 2017), SpecMatch-Synth (E. A. Petigura 2015).

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