[Paper Review] AU Mic b is the Youngest Planet to have a Spin-Orbit Alignment Measurement

Brett Addison, Jonathan Horner+21 more|arXiv (Cornell University)|Jun 24, 2020

Stellar, planetary, and galactic studiesPhysics and Astronomy100 references30 citations

TL;DR

This study presents the first measurement of the sky-projected spin-orbit angle for AU Mic b, a Neptune-sized planet around a 20 Myr-old M dwarf star, using radial velocity observations from the Minerva-Australis telescope array. Despite challenges from stellar activity and incomplete transit coverage, the analysis yields λ = 47⁺²⁶₋⁵⁴°, consistent with both aligned and misaligned orbits, and supports disk-migration as the likely formation mechanism for this young exoplanet system.

ABSTRACT

We report measurements of the sky-projected spin-orbit angle for AU\,Mic\,b, a Neptune-size planet orbiting a very young ($\sim20$\,Myr) nearby pre-main sequence M dwarf star which also hosts a bright, edge-on, debris disk. The planet was recently discovered from preliminary analysis of radial velocity observations and confirmed to be transiting its host star from photometric data from the NASA's extit{TESS} mission. We obtained radial velocity measurements of AU\,Mic over the course of two partially observable transits and one full transit of planet b from high-resolution spectroscopic observations made with the { extsc{Minerva}}-Australis telescope array. Only a marginal detection of the Rossiter--McLaughlin effect signal was obtained from the radial velocities, in part due to AU Mic being an extremely active star and the lack of full transit coverage plus sufficient out-of-transit baseline. As such, a precise determination of the obliquity for AU\,Mic\,b is not possible in this study and we find a sky-projected spin-orbit angle of $\lambda = 47{^{+26}_{-54}}^{\circ}$. This result is consistent with both the planet's orbit being aligned or highly misaligned with the spin-axis of its host star. Our measurement independently agrees with, but is far less precise than observations carried out on other instruments around the same time that measure a low obliquity orbit for the planet. AU\,Mic is the youngest exoplanetary system for which the projected spin-orbit angle has been measured, making it a key data point in the study of the formation and migration of exoplanets -- particularly given that the system is also host to a bright debris disk.

Motivation & Objective

• To measure the sky-projected spin-orbit angle (λ) for AU Mic b, the youngest known transiting exoplanet with such a measurement.
• To investigate the dynamical history of AU Mic b by determining whether its orbit is aligned with the spin axis of its host star.
• To assess the role of disk migration in shaping the architecture of young planetary systems, particularly in systems hosting debris disks.
• To contribute to the growing dataset of spin-orbit measurements in young exoplanetary systems to understand early planetary system evolution.

Proposed method

• Acquired high-resolution spectroscopic radial velocity data during three transits of AU Mic b using the Minerva-Australis telescope array.
• Applied Rossiter-McLaughlin effect modeling to detect anomalies in radial velocity caused by the planet transiting the rotating stellar disk.
• Used a hybrid linear and polynomial fitting method to remove stellar activity signals and improve signal-to-noise for the Rossiter-McLaughlin effect.
• Conducted Bayesian Markov Chain Monte Carlo (MCMC) analysis with informative priors on stellar parameters and orbital elements.
• Simulated stellar spot effects to test their potential impact on the Rossiter-McLaughlin signal and assess false positive risks.
• Compared results with contemporaneous observations from other instruments to validate the spin-orbit measurement.

Experimental results

Research questions

• RQ1 What is the sky-projected spin-orbit angle (λ) of AU Mic b, and what does it imply about the alignment of its orbit with the star's spin axis?
• RQ2 How does the obliquity of AU Mic b compare with other known exoplanets, particularly in young systems?
• RQ3 Can the observed spin-orbit angle be explained by disk migration or alternative migration mechanisms such as planet-planet scattering?
• RQ4 To what extent do stellar activity and spot crossings affect the reliability of spin-orbit measurements in active young stars like AU Mic?
• RQ5 How does the alignment of AU Mic b’s orbit compare with the orientation of its host star’s debris disk, and what does this imply for system co-evolution?

Key findings

• The sky-projected spin-orbit angle for AU Mic b is measured as λ = 47⁺²⁶₋⁵⁴°, with a wide uncertainty range due to low signal-to-noise and stellar activity.
• The measurement is consistent with both aligned and highly misaligned orbits, indicating no strong evidence for significant orbital misalignment.
• The result aligns with independent, less precise measurements from other instruments that also suggest a low obliquity for AU Mic b.
• The system’s low obliquity, combined with the co-planar and edge-on debris disk, supports a scenario of quiescent disk migration rather than high-eccentricity migration.
• Stellar activity and incomplete transit coverage limited the precision of the spin-orbit measurement, highlighting challenges in studying young, active stars.
• The alignment of the planet’s orbit with the debris disk and stellar equator suggests that the system likely formed with co-planar angular momentum, favoring disk-driven migration mechanisms.