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

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 (${\lambda }_b=1°\pm 12°$) and finding two possible solutions for AU Mic c: we slightly favor an aligned solution (${\lambda }_c=-10°\pm 16°$) but cannot rule out a polar solution (${\lambda }_c=87°{}_{-29°}^{+36°}$). 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.

arXiv comment22 pages. 9 figures. 3 tables. Published in ApJL

References in corpus (40)

• SciPy 1.0--Fundamental Algorithms for Scientific Computing in Python
• Array Programming with NumPy
• Astropy: A Community Python Package for Astronomy
• The Astropy Project: Building an inclusive, open-science project and status of the v2.0 core package
• The Astropy Project: Sustaining and Growing a Community-oriented Open-source Project and the Latest Major Release (v5.0) of the Core Package
• dynesty: A Dynamic Nested Sampling Package for Estimating Bayesian Posteriors and Evidences
• A terrestrial planet candidate in a temperate orbit around Proxima Centauri
• Las Cumbres Observatory Global Telescope Network
• batman: BAsic Transit Model cAlculatioN in Python
• Fast and scalable Gaussian process modeling with applications to astronomical time series
• The Occurrence and Architecture of Exoplanetary Systems
• An Observational Overview of Solar Flares
• Architecture of Kepler's Multi-transiting Systems: II. New investigations with twice as many candidates
• AstroImageJ: Image Processing and Photometric Extraction for Ultra-Precise Astronomical Light Curves (Expanded Edition)
• AstroImageJ: Image Processing and Photometric Extraction for Ultra-Precise Astronomical Light Curves
• Tidal dissipation in stars and giant planets
• Secular Chaos and the Production of Hot Jupiters
• Breaking the Chains: Hot Super-Earth systems from migration and disruption of compact resonant chains
• Chaotic star formation and the alignment of stellar rotation with disc and planetary orbital axes
• STARRY: Analytic Occultation Light Curves
• The CHEOPS mission
• Real-time processing of the imaging data from the network of Las Cumbres Observatory Telescopes using BANZAI
• A planet within the debris disk around the pre-main-sequence star AU Microscopii
• Evolution of Spin Direction of Accreting Magnetic Protostars and Spin-Orbit Misalignment in Exoplanetary Systems
• Improved Modeling of the Rossiter-McLaughlin Effect for Transiting Exoplanets
• The Einstein Probe Mission
• Science objectives of the Einstein Probe mission
• The Warm Neptune GJ 3470b has a Polar Orbit
• Limits on the Spin-Orbit Angle and Atmospheric Escape for the 22 Myr-old Planet AU Mic b
• Precision radial velocities of 15 M5 - M9 dwarfs
• Activity-induced radial velocity jitter in a flaring M dwarf
• The hot Jupiter of the magnetically-active weak-line T Tauri star V830 Tau
• New Methods for Finding Activity-Sensitive Spectral Lines: Combined Visual Identification and an Automated Pipeline Find a Set of 40 Activity Indicators
• Transmission spectroscopy and Rossiter-McLaughlin measurements of the young Neptune orbiting AU Mic
• Spin-orbit alignment and magnetic activity in the young planetary system AU Mic
• Detection of a high-velocity sodium feature on the ultra-hot Jupiter WASP-121 b
• Star-disc (mis-)alignment in Rho Oph and Upper Sco: insights from spatially resolved disc systems with K2 rotation periods
• Flare activity and photospheric analysis of Proxima Centauri
• One-third of Sun-like stars are born with misaligned planet-forming disks
• A possible misaligned orbit for the young planet AU Mic c