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One year of AU Mic with HARPS - I. Measuring the masses of the two transiting planets

Abstract : 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 ${\rm m\, s^{-1}}$ 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 semi-amplitudes of Kb = 5.8 ± 2.5 ${\rm m\, s^{-1}}$ and Kc = 8.5 ± 2.5 ${\rm m\, s^{-1}}$, 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.
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Submitted on : Monday, June 27, 2022 - 12:26:10 PM
Last modification on : Tuesday, July 5, 2022 - 3:06:12 AM

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Norbert Zicher, Oscar Barragán, Baptiste Klein, Suzanne Aigrain, James E. Owen, et al.. One year of AU Mic with HARPS - I. Measuring the masses of the two transiting planets. Monthly Notices of the Royal Astronomical Society, 2022, 512, pp.3060-3078. ⟨10.1093/mnras/stac614⟩. ⟨insu-03705346⟩

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