One year of AU Mic with HARPS: I-measuring the masses of the two transiting planets

Journal article

The system of two transiting Neptune-sized planets around the bright, young M-dwarf AU Mic provides a unique opportunity to test models of planet formation, early evolution, and star-planet interaction. However, the intense magnetic activity of the host star makes measuring the masses of the planets via the radial velocity (RV) method very challenging. We report on a 1-yr, intensive monitoring campaign of the system using 91 observations with the HARPS spectrograph, allowing for detailed modelling of the ∼600 m s-1 peak-to-peak activity-induced RV variations. We used a multidimensional Gaussian Process framework to model these and the planetary signals simultaneously. We detect the latter with semiamplitudes of Kb = 5.8 ± 2.5 m s-1 and Kc = 8.5 ± 2.5 m s-1, respectively. The resulting mass estimates, Mb = 11.7 ± 5.0 M⊕ and Mc = 22.2 ± 6.7 M⊕, suggest that planet b might be less dense, and planet c considerably denser than previously thought. These results are in tension with the current standard models of core-accretion. They suggest that both planets accreted a H/He envelope that is smaller than expected, and the trend between the two planets' envelope fractions is the opposite of what is predicted by theory.

Authors: Zicher, N; Barragan, O; Klein, B; Aigrain, S; Owen, JE

• Publication status: Published
• Peer review status: Peer reviewed
• Publisher: Oxford University Press
• Journal: Monthly Notices of the Royal Astronomical Society
• Volume: 512
• Issue: 2
• Pages: 3060-3078
• Publication date: 2022-03-12
• Acceptance date: 2022-03-02
• DOI: 10.1093/mnras/stac614
• EISSN: 1365-2966
• ISSN: 0035-8711
• Language: English
• Keywords: techniques: spectroscopic; stars: activity; FFR; techniques: radial velocities; planets and satellites: fundamental parameters; stars: starspots; stars: individual: AU Microscopii