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| Format: | Preprint |
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2026
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| Subjects: | |
| Online Access: | https://arxiv.org/abs/2605.29946 |
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| _version_ | 1866911728424452096 |
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| author | Lin, Zitao Szabó, Gyula M. Zieliński, Krzysztof Sz. Guo, Zhen Garai, Zoltán Butler, R. Paul Brandeker, Alexis Teske, Johanna K. Gandolfi, Davide Yu, Haochuan Billot, Nicolas Aigrain, Suzanne Cretignier, Michael Wang, Liang Mao, Xuan Yuan, Wei M. Lu, Hongpeng Li, Jiayin Alibert, Yann Boldog, Ádám Bourrier, Vincent Bruno, Giovanni Crane, Jeffrey D. Dai, Fei Demangeon, Olivier D. S. Heitzmann, Alexis Hu, Zhecheng Karkamar, Pradip Kriskovics, Levente Lendl, Monika Liu, He Y. Maxted, Pierre F. L. Osborn, Hugh P. Scandariato, Gaetano Shectman, Stephen A. Sousa, Sérgio G. Ulmer-Moll, Solène Wang, Mu-Tian Wilson, Thomas G. Wang, Sharon X. |
| author_facet | Lin, Zitao Szabó, Gyula M. Zieliński, Krzysztof Sz. Guo, Zhen Garai, Zoltán Butler, R. Paul Brandeker, Alexis Teske, Johanna K. Gandolfi, Davide Yu, Haochuan Billot, Nicolas Aigrain, Suzanne Cretignier, Michael Wang, Liang Mao, Xuan Yuan, Wei M. Lu, Hongpeng Li, Jiayin Alibert, Yann Boldog, Ádám Bourrier, Vincent Bruno, Giovanni Crane, Jeffrey D. Dai, Fei Demangeon, Olivier D. S. Heitzmann, Alexis Hu, Zhecheng Karkamar, Pradip Kriskovics, Levente Lendl, Monika Liu, He Y. Maxted, Pierre F. L. Osborn, Hugh P. Scandariato, Gaetano Shectman, Stephen A. Sousa, Sérgio G. Ulmer-Moll, Solène Wang, Mu-Tian Wilson, Thomas G. Wang, Sharon X. |
| contents | 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 ($λ_b=1° \pm 12°$) and finding two possible solutions for AU Mic c: we slightly favor an aligned solution ($λ_c=-10° \pm 16°$) but cannot rule out a polar solution ($λ_c=87°\ ^{+36°}_{-29°}$). 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. |
| format | Preprint |
| id |
arxiv_https___arxiv_org_abs_2605_29946 |
| institution | arXiv |
| publishDate | 2026 |
| record_format | arxiv |
| spellingShingle | Spin-Orbit Geometry of AU Mic b and c from Back-to-Back Transits Observed Contemporaneously with Magellan PFS, LCOGT, and CHEOPS Lin, Zitao Szabó, Gyula M. Zieliński, Krzysztof Sz. Guo, Zhen Garai, Zoltán Butler, R. Paul Brandeker, Alexis Teske, Johanna K. Gandolfi, Davide Yu, Haochuan Billot, Nicolas Aigrain, Suzanne Cretignier, Michael Wang, Liang Mao, Xuan Yuan, Wei M. Lu, Hongpeng Li, Jiayin Alibert, Yann Boldog, Ádám Bourrier, Vincent Bruno, Giovanni Crane, Jeffrey D. Dai, Fei Demangeon, Olivier D. S. Heitzmann, Alexis Hu, Zhecheng Karkamar, Pradip Kriskovics, Levente Lendl, Monika Liu, He Y. Maxted, Pierre F. L. Osborn, Hugh P. Scandariato, Gaetano Shectman, Stephen A. Sousa, Sérgio G. Ulmer-Moll, Solène Wang, Mu-Tian Wilson, Thomas G. Wang, Sharon X. Earth and Planetary Astrophysics Solar and Stellar Astrophysics 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 ($λ_b=1° \pm 12°$) and finding two possible solutions for AU Mic c: we slightly favor an aligned solution ($λ_c=-10° \pm 16°$) but cannot rule out a polar solution ($λ_c=87°\ ^{+36°}_{-29°}$). 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. |
| title | Spin-Orbit Geometry of AU Mic b and c from Back-to-Back Transits Observed Contemporaneously with Magellan PFS, LCOGT, and CHEOPS |
| topic | Earth and Planetary Astrophysics Solar and Stellar Astrophysics |
| url | https://arxiv.org/abs/2605.29946 |