Revisiting Sub-Neptune Population Synthesis: The Role of Core Luminosity in Boil-off, Photoevaporation, and the Radius Distribution

Abstract

The observed bimodal radius distribution of close-in small planets provides important constraints on their formation and evolution. Two atmospheric escape mechanisms, photoevaporation and core-powered mass loss, have been widely invoked to explain this distribution. In this work, we revisit the physical assumptions underlying these models and investigate the role of core luminosity in driving outflows and shaping the radius distribution. We develop a population synthesis framework based on the SEAMIST evolution model, which self-consistently couples the two mass-loss mechanisms while accounting for envelope metallicity. We find that envelope metallicity strongly affects the resulting radius distribution, with 50-100$\times$ solar metallicity providing a good match to the observed population. Boil-off generally dominates atmospheric mass loss, while photoevaporation typically removes at most 3 wt\% of the envelope mass. We find that core luminosity contributes only $\sim$10-20\% of the thermal energy available for envelope re-equilibration during boil-off, producing an absolute mass-loss enhancement of only $\sim$0.4wt\%. Its effect is larger during photoevaporation, where the longer mass-loss timescale allows core heating to accumulate. Nevertheless, core luminosity has little impact on the long-term planetary radius distribution. These results suggest that core-powered mass loss is better understood as a modification to atmospheric escape through its lower-boundary conditions rather than as a distinct mass-loss mechanism. In our best-fit model, the radiative atmosphere contributes a significant $\sim$20\% of the total optical radius. Consequently, we highlight the need for improved radiative-atmosphere modeling in sub-Neptune evolution models to better understand the observed population distribution.