Homeastro-ph.EParXiv:2605.29946

Spin-Orbit Geometry of AU Mic b and c from Back-to-Back Transits Observed Contemporaneously with Magellan PFS, LCOGT, and CHEOPS

astro-ph.EPastro-ph.SR2026-05v1license

Abstract

Young planets offer a unique window into the early stages of planetary evolution. AU Mic is one of the nearest (9.8 pc) pre-main sequence stars (~20 Myr), hosting two transiting Neptune-sized planets and a debris disk. Previous studies have shown that the rotation of the central star, the debris disk, and the inner planet b are all aligned, suggesting that the system has not undergone violent evolution. Here we report new Rossiter-McLaughlin (RM) measurements for both AU Mic b and c, which happened to transit back-to-back on Aug 24 and 25, 2024, using the Magellan Planet Finder Spectrograph (PFS), accompanioned with contanporaneous photometry from LCOGT and CHEOPS. We confirm the aligned orbit of AU Mic b (λb=1deg±12deg\lambda_b=1{\deg} \pm 12{\deg}) and finding two possible solutions for AU Mic c: we slightly favor an aligned solution (λc=10deg±16deg\lambda_c=-10{\deg} \pm 16{\deg}) but cannot rule out a polar solution (λc=87deg 29deg+36deg\lambda_c=87{\deg}\ ^{+36{\deg}}_{-29{\deg}}). Broader considerations, including dynamical stability and transit possibility, also support the mutually aligned scenario. An unexpected stellar signal during ingress and the poor TTV predictions of AU Mic c prevent a precise constraint on its obliquity, and various attempts using chromatic spectral analyses fail to outperform simple data exclusion in mitigating the stellar contamination. Our observation highlights the importance of understanding stellar activity across multiple timescales and channels when characterizing young, active systems. A robust solution for the AU Mic architecture will require either a better understanding of stellar activity or future observations fortuitously free from strong stellar contamination.

Comments: 22 pages. 9 figures. 3 tables. Accepted for publication in ApJL

Cite

@article{arxiv.2605.29946,
  title  = {Spin-Orbit Geometry of AU Mic b and c from Back-to-Back Transits Observed Contemporaneously with Magellan PFS, LCOGT, and CHEOPS},
  author = {Zitao Lin and Gyula M. Szabó and Krzysztof Sz. Zieliński and Zhen Guo and Zoltán Garai and R. Paul Butler and Alexis Brandeker and Johanna K. Teske and Davide Gandolfi and Haochuan Yu and Nicolas Billot and Suzanne Aigrain and Michael Cretignier and Liang Wang and Xuan Mao and Wei M. Yuan and Hongpeng Lu and Jiayin Li and Yann Alibert and Ádám Boldog and Vincent Bourrier and Giovanni Bruno and Jeffrey D. Crane and Fei Dai and Olivier D. S. Demangeon and Alexis Heitzmann and Zhecheng Hu and Pradip Karkamar and Levente Kriskovics and Monika Lendl and He Y. Liu and Pierre F. L. Maxted and Hugh P. Osborn and Gaetano Scandariato and Stephen A. Shectman and Sérgio G. Sousa and Solène Ulmer-Moll and Mu-Tian Wang and Thomas G. Wilson and Sharon X. Wang},
  journal= {arXiv preprint arXiv:2605.29946},
  year   = {2026}
}