PLANETARY POPULATION SYNTHESIS COUPLED WITH ATMOSPHERIC ESCAPE: A STATISTICAL VIEW OF EVAPORATION

Abstract

We apply hydrodynamic evaporation models to different synthetic planet\npopulations that were obtained from a planet formation code based on a\ncore-accretion paradigm. We investigated the evolution of the planet\npopulations using several evaporation models, which are distinguished by the\ndriving force of the escape flow (X-ray or EUV), the heating efficiency in\nenergy-limited evaporation regimes, or both. Although the mass distribution of\nthe planet populations is barely affected by evaporation, the radius\ndistribution clearly shows a break at approximately 2 $R_{\\oplus}$. We find\nthat evaporation can lead to a bimodal distribution of planetary sizes (Owen &\nWu 2013) and to an "evaporation valley" running diagonally downwards in the\norbital distance - planetary radius plane, separating bare cores from low-mass\nplanet that have kept some primordial H/He. Furthermore, this bimodal\ndistribution is related to the initial characteristics of the planetary\npopulations because low-mass planetary cores can only accrete small primordial\nH/He envelopes and their envelope masses are proportional to their core masses.\nWe also find that the population-wide effect of evaporation is not sensitive to\nthe heating efficiency of energy-limited description. However, in two extreme\ncases, namely without evaporation or with a 100\\% heating efficiency in an\nevaporation model, the final size distributions show significant differences;\nthese two scenarios can be ruled out from the size distribution of $Kepler$\ncandidates.\n