Formation of multiplanetary systems via pebble accretion in externally photoevaporating discs in stellar clusters

ABSTRACT In this paper, we investigate how external photoevaporation influences the formation, dynamical evolution and the resultant planetary architecture of multiplanet systems born in stellar clusters. We use a model of N-body simulations of multiple planet formation via pebble accretion coupled with a 1D viscous disc subject to external photoevaporation. We found that external photoevaporation reduces the planet growth by reducing the pebble mass reservoir in discs containing multiple planetary embryos across a wide range of disc masses, and is particularly effective in suppressing planet growth in less initially massive discs (<0.1 M$_{\odot }$). However, in more initially massive ($\ge$ 0.1 M$_{\oplus }$) discs planets lost due to planet–planet interactions dominate the outcome for final resultant total planet mass, masking the effects of external photoevaporation in curbing the planet mass growth. In terms of the final resulting planetary architectures, the signature of external photoevaporation is visible in less massive (<0.1 M$_{\odot }$) discs, with fewer numbers and lower masses of planets surviving in discs irradiated with stronger external far-ultraviolet (FUV) radiation. External photoevaporation also leaves a signature for the wide orbit (>10 au) terrestrial planets (0.1–1 M$_{\oplus }$), with fewer planets populating this region for stronger FUV field. Finally, the first-order resonant pairs fraction decreases with stronger FUV radiation, although the resonant pairs occur rarely regardless of the FUV radiation environment, due to the small number of planets that survive gravitational encounters.