N-body simulations of planetary accretion around M dwarf stars

TL;DR

This study uses N-body simulations to explore how planetary accretion and migration around M dwarf stars influence the final planetary system configurations and water-ice delivery, highlighting the importance of migration efficiency.

Contribution

It provides new insights into the impact of type-I migration efficiency on planet formation and water delivery in the habitable zones of M dwarf stars.

Findings

Final planetary systems often consist of 4-6 planets in resonances or non-resonant orbits.

Water-ice delivery is significant unless migration speed is greatly reduced.

Planetary configurations are highly sensitive to the strength of type-I migration.

Abstract

We have investigated planetary accretion from planetesimals in terrestrial planet regions inside the ice line around M dwarf stars through N-body simulations including tidal interactions with disk gas. Because of low luminosity of M dwarfs, habitable zones (HZs) are located in inner regions. In the close-in HZ, type-I migration and the orbital decay induced by eccentricity damping are efficient according to the high disk gas density in the small orbital radii. In the case of full efficiency of type-I migration predicted by the linear theory, we found that protoplanets that migrate to the vicinity of the host star undergo close scatterings and collisions, and 4 to 6 planets eventually remain in mutual mean motion resonances and their orbits have small eccentricities and they are stable both before and after disk gas decays. In the case of slow migration, the resonant capture is so efficient that densely-packed ~ 40 small protoplanets remain in mutual mean motion resonances. In this case, they start orbit crossing, after the disk gas decays and eccentricity damping due to tidal interaction with gas is no more effective. Through merging of the protoplanets, several planets in widely-separated non-resonant orbits with relatively large eccentricities are formed. Thus, the final orbital configurations of the terrestrial planets around M dwarfs sensitively depend on strength of type-I migration. We also found that large amount of water-ice is delivered by type-I migration from outer regions and final planets near the inner disk edge around M dwarfs are generally abundant in water-ice except for the innermost one that is shielded by the outer planets, unless type-I migration speed is reduced by a factor of more than 100 from that predicted by the linear theory.