Ejection of close-in super-Earths around low-mass stars in the giant impact stage

TL;DR

This study uses N-body simulations to explore how giant impacts and scattering around low-mass stars influence the formation, ejection, and orbital characteristics of close-in super-Earths, revealing ejection as a key factor.

Contribution

It demonstrates that strong scattering and ejection during giant impacts around low-mass stars shape the mass and orbital distributions of super-Earths, aligning with observations.

Findings

Protoplanets are often ejected after multiple close-scattering events.

Ejection limits the maximum mass of planets around late M dwarfs.

The mass distribution of observed super-Earths matches the ejection-based model.

Abstract

Earth-sized planets were observed in close-in orbits around M dwarfs. While more and more planets are expected to be uncovered around M dwarfs, theories of their formation and dynamical evolution are still in their infancy. We investigate the giant impact growth of protoplanets, which includes strong scattering around low-mass stars. The aim is to clarify whether strong scattering around low-mass stars affects the orbital and mass distributions of the planets. We perform $N$-body simulation of protoplanets by systematically surveying the parameter space of the stellar mass and surface density of protoplanets. We find that protoplanets are often ejected after twice or three times close-scattering around late M dwarfs. The ejection sets the upper limit of the largest planet mass. Adopting the surface density scaling linearly with the stellar mass, we find that as the stellar mass decreases less massive planets are formed in orbits with higher eccentricities and inclinations. Under this scaling, we also find that a few close-in protoplanets are generally ejected. The ejection of protoplanets plays an important role in the mass distribution of super-Earths around late M dwarfs. The mass relation of observed close-in super-Earths and their central star mass is well reproduced by ejection.