The origin of the large heavy element contents of warm Jupiters

Abstract

Planet interior models have revealed that warm Jupiters should harbor large heavy element masses (up to 30-40% of the planetary mass). Within the pebble drift and evaporation scenario, inward drifting pebbles can evaporate and enrich the inner disc with vapor to super-stellar values, accreteable by giant planets. As H$_2$O ice evaporates in the inner disc, while CO$_2$, CH$_4$ and CO evaporate in the outer disc, the enrichment of the disc with vapor decreases with increasing orbital distance. We use the chemcomp code to model the migration and growth of planets via pebble and gas accretion in discs around stars with different masses and discs with different sizes. We show (i) that pebble drift and evaporation can explain the wide range of derived heavy element contents of giant planets. (ii) Giant planets forming around stars with larger disc sizes can accrete larger quantities of heavy elements, because these discs can maintain a high pebble flux for a longer time. (iii) Planets forming or migrating interior to the water ice line can accrete larger amounts of heavy elements via the accretion of vapor enriched gas compared to planets forming in the outer disc regions. (iv) Metal rich stars can allow the formation of planets that are more enriched in heavy elements, if they form in the inner disc regions. Planets forming in the outer disc regions will be - independently of the disc metallicity - much less enriched compared. (v) Planets forming in discs with the same size and total mass, but around stars with higher mass, are only marginally more enriched compared to their counterparts forming around lower mass star. Our study suggests that the trend of increasing total heavy element mass with increasing planetary mass is caused by the accretion of vapor enriched gas in discs with different sizes in the combination with different origins of warm giant planets from the inner and outer disc.