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 planet formation, early evolution, and star-planet interaction. However, intense magnetic activity host star makes measuring masses via radial velocity (RV) method very challenging. We report on 1-year, intensive monitoring campaign using 91 observations with HARPS spectrograph, allowing for detailed modelling $\sim 600\, \rm{m\,s^{-1}}$ peak-to-peak activity-induced RV variations. used multidimensional Gaussian Process framework model these planetary signals simultaneously. detect latter semi-amplitudes $\rm{K_b} = 5.8 \pm 2.5\, $\rm{K_c} 8.5 \rm{m\, s^{-1}}$, respectively. resulting mass estimates, $\rm{M_b} 11.7 5.0\, \rm{M_{\rm \oplus}}$ $\rm{M_c} 22.2 6.7\, \oplus}}$, suggest that b might be less dense, c considerably denser than previously thought. These results are in tension current standard core-accretion. They both accreted H/He envelope is smaller expected, trend between planets' fractions opposite what predicted by theory.