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

¹ Institute of Astronomy and Astrophysics, Academia Sinica, Taipei 10617, Taiwan
e-mail: ymatsumoto@asiaa.sinica.edu.tw
² National Astronomical Observatory of Japan, 2-21-1, Osawa, Mitaka, 181-8588 Tokyo, Japan
³ Institute for Cosmic Ray Research, University of Tokyo, Hida, Gifu 506-1205, Japan
⁴ Astrobiology Center, NINS, 2-21-1, Osawa, Mitaka, 181-8588 Tokyo, Japan

Received: 4 May 2020
Accepted: 23 July 2020

Abstract

Context. 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.

Aims. We investigate the giant impact stage for the 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.

Methods. We performed an N-body simulation of protoplanets by systematically surveying the parameter space of the stellar mass and surface density of protoplanets.

Results. We find that protoplanets are often ejected after twice or three times the close-scattering around late M dwarfs. The ejection sets the upper limit of the largest planet mass. By adopting the surface density that linearly scales 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.

Conclusions. 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 reproduced well by ejection.