Stellar Obliquity of the Ultra-short-period Planet System HD 93963

Creators: Teng 滕, Huan-Yu 环宇^{1, 2, 3}; Dai 戴, Fei 飞²; Howard, Andrew W.⁴; Halverson, Samuel⁵; Isaacson, Howard^{6, 7}; Kokubo 小久保, Eiichiro 英一郎⁸; Rubenzahl, Ryan A.⁹; Fulton, Benjamin¹⁰; Householder, Aaron¹¹; Lubin, Jack¹²; Giacalone, Steven⁴; Handley, Luke⁴; Van Zandt, Judah¹²; Petigura, Erik A.¹²; Ong 王, J. M. Joel 加冕²; Premnath, Pranav¹³; Yu 于, Haochuan 皓川¹⁴; Gibson, Steven R.⁴; Rider, Kodi⁶; Roy, Arpita¹⁵; Baker, Ashley⁴; Edelstein, Jerry⁶; Smith, Chris⁶; Walawender, Josh¹⁶; Lee, Byeong-Cheol¹; Liu 刘, Yu-Juan 玉娟³; Winn, Joshua N.¹⁷

1. Korea Astronomy and Space Science Institute
2. University of Hawaii at Manoa
3. National Astronomical Observatories
4. California Institute of Technology
5. Jet Propulsion Lab
6. University of California, Berkeley
7. University of Southern Queensland
8. National Astronomical Observatory of Japan
9. Center for Computational Astrophysics, Flatiron Institute, 162 Fifth Avenue, New York, NY 10010, USA
10. NASA Exoplanet Science Institute
11. Massachusetts Institute of Technology
12. University of California, Los Angeles
13. University of California, Irvine
14. University of Oxford
15. Astrophysics & Space Institute, Schmidt Sciences, New York, NY 10011, USA
16. W.M. Keck Observatory
17. Princeton University

Abstract

We report an observation of the Rossiter–McLaughlin (RM) effect of the transiting planet HD 93963 Ac, a mini-Neptune planet orbiting a G0-type star with an orbital period of P_c = 3.65 days, accompanied by an inner super-Earth planet with P_b = 1.04 days. We observed a full transit of planet c on 2024 May 3 UT with the Keck/Keck Planet Finder. The observed RM effect has an amplitude of ∼1 m s⁻¹ and implies a sky-projected obliquity of λ = 14⁺¹⁷₋₁₉ degrees for HD 93963 Ac. Our dynamical analysis suggests that the two inner planets are likely well aligned with the stellar spin, to within a few degrees, thus allowing both to transit. Along with WASP-47, 55 Cnc, and HD 3167, HD 93963 is the fourth planetary system with an ultrashort-period planet and obliquity measurement(s) of any planet(s) in the system. HD 93963, WASP-47, and 55 Cnc favor largely coplanar orbital architectures, whereas HD 3167 has been reported to have a large mutual inclination (∼100°) between its transiting planets b and c. In this configuration, the probability that both planets transit is low. Moreover, one planet would quickly evolve to be nontransiting due to nodal precession. Future missions such as ESO/PLATO should detect the resulting transit duration variations. We encourage additional obliquity measurements of the HD 3167 system to better constrain its orbital architecture.

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

This work is supported by the Korea Astronomy and Space Science Institute under the R&D program (project No. 2025-1-830-05) supervised by the Ministry of Science and ICT, and National Key R&D Program of China, No. 2024YFA1611802. H.Y.T. appreciates the support by the EACOA/EAO Fellowship Program under the umbrella of the East Asia Core Observatories Association. E.K. is supported by JSPS KAKENHI grants Nos. 24K00698 and 24H00017. J.M.J.O. acknowledges support from NASA through the NASA Hubble Fellowship grant HST-HF2-51517.001-A, awarded by STScI. STScI is operated by the Association of Universities for Research in Astronomy, Incorporated, under NASA contract NAS5-26555. H.Y. acknowledges funding from the European Research Council under the European Union’s Horizon 2020 research and innovation program (grant agreement No. 865624, GPRV). 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. We thank the time assignment committees of the University of California, the California Institute of Technology, NASA, and the University of Hawai’i for supporting the TESS-Keck Survey with observing time at the W. M. Keck Observatory. We gratefully acknowledge the efforts and dedication of the Keck Observatory staff for support of HIRES and remote observing. We recognize and acknowledge the cultural role and reverence that the summit of Manua Kea has within the indigenous Hawaiian community. We are deeply grateful to have the opportunity to conduct observations from this mountain. We thank Songhu Wang for helpful suggestions and comments that improved the paper.

Facilities

Keck:I - KECK I Telescope (KPF and HIRES), TESS - , Exoplanet Archive - .

Software References

numpy (C. R. Harris et al. 2020), scipy (P. Virtanen et al. 2020), batman (L. Kreidberg 2015), emcee (D. Foreman-Mackey et al. 2013), EXOFAST (J. Eastman et al. 2013), isoclassify (D. Huber et al. 2017), gyro-interp (L. G. Bouma et al. 2023), SpecMatch (E. A. Petigura 2015; S. W. Yee et al. 2017).

Files

Teng_2025_AJ_170_51.pdf

Files (8.0 MB)

Name	Size	Download all
Teng_2025_AJ_170_51.pdf md5:94e77bff0880cc7157189c0dc48f88f5	8.0 MB	Preview Download

	All versions	This version
Views	0	0
Downloads	0	0
Data volume	0 Bytes	0 Bytes