The origin of the high metallicity of close-in giant exoplanets. Combined effects of resonant and aerodynamic shepherding

Context. Recent studies suggest that in comparison to their host star, many giant exoplanets are highly enriched with heavy elements and can contain several tens of Earth masses of heavy elements or more. Such enrichment is considered to have been delivered by the accretion of planetesimals in late formation stages. Previous dynamical simulations, however, have shown that planets cannot accrete such high masses of heavy elements through "in situ" planetesimal accretion.
Aims: We investigate whether a giant planet migrating inward can capture planetesimals efficiently enough to significantly increase its metallicity.
Methods: We performed orbital integrations of a migrating giant planet and planetesimals in a protoplanetary gas disc to infer the planetesimal mass that is accreted by the planet.
Results: We find that the two shepherding processes of mean motion resonance trapping and aerodynamic gas drag inhibit the planetesimal capture of a migrating planet. However, the amplified libration allows the highly-excited planetesimals in the resonances to escape from the resonance trap and to be accreted by the planet. Consequently, we show that a migrating giant planet captures planetesimals with total mass of several tens of Earth masses if the planet forms at a few tens of AU in a relatively massive disc. We also find that planetesimal capture occurs efficiently in a limited range of semi-major axis and that the total captured planetesimal mass increases with increasing migration distances. Our results have important implications for understanding the relation between giant planet metallicity and mass, as we suggest that it reflects the formation location of the planet - or more precisely, the location where runaway gas accretion occurred. We also suggest the observed metal-rich close-in Jupiters migrated to their present locations from afar, where they had initially formed.