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-year, intensive monitoring campaign of the system using 91 observations with the HARPS spectrograph, allowing for detailed modelling of the $\sim 600\mathrm{m}{\mathrm{s}}^{-1}$ 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 ${\mathrm{K}}_{\mathrm{b}}=5.8\pm 2.5\mathrm{m}{\mathrm{s}}^{-1}$ and ${\mathrm{K}}_{\mathrm{c}}=8.5\pm 2.5\mathrm{m}{\mathrm{s}}^{-1}$, respectively. The resulting mass estimates, ${\mathrm{M}}_{\mathrm{b}}=11.7\pm 5.0{\mathrm{M}}_{\oplus }$ and ${\mathrm{M}}_{\mathrm{c}}=22.2\pm 6.7{\mathrm{M}}_{\oplus }$, 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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