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

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 similar to 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 semi-amplitudes of K-b = 5.8 +/- 2.5 m s(-1) and K-c = 8.5 +/- 2.5 m s(-1), respectively. The resulting mass estimates, M-b = 11.7 +/- 5.0 M-circle plus and M-c = 22.2 +/- 6.7 M-circle plus, 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.