Eccentricity Evolution Through Accretion of Protoplanets

Abstract

Most of super-Earths detected by the radial velocity (RV) method have significantly smaller eccentricities than the eccentricities corresponding to velocity dispersion equal to their surface escape velocity ("escape eccentricities"). If orbital instability followed by giant impacts among protoplanets that have migrated from outer region is considered, it is usually considered that eccentricities of the merged bodies become comparable to those of orbital crossing bodies, which are excited up to their escape eccentricities by close scattering. However, the eccentricity evolution in the {\it in situ} accretion model has not been studied in detail. Here, we investigate the eccentricity evolution through {\it N}-body simulations. We have found that the merged planets tend to have much smaller eccentricities than the escape eccentricities due to very efficient collision damping. If the protoplanet orbits are initially well separated and their eccentricities are securely increased, an inner protoplanet collides at its apocenter with an outer protoplanet at its pericenter. The eccentricity of the merged body is the smallest for such configuration. Orbital inclinations are also damped by this mechanism and planets tend to share a same orbital plane, which is consistent with {\it Kepler} data. Such efficient collision damping is not found when we start calculations from densely packed orbits of the protoplanets. If the protoplanets are initially in the mean-motion resonances, which corresponds to well separated orbits, the {\it in situ} accretion model well reproduces the features of eccentricities and inclinations of multiple super-Earths/Earth systems discovered by RV and {\it Kepler} surveys.

Accepted for publication in ApJ

References in corpus (6)

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Cited by in corpus (7)

• The orbital eccentricity of small planet systems
• Correlations between compositions and orbits established by the giant impact era of planet formation
• Dynamically hot Super-Earths from outer giant planet scattering
• Formation of Close-in Super-Earths by Giant Impacts: Effects of Initial Eccentricities and Inclinations of Protoplanets
• The origins of nearly coplanar, non-resonant systems of close-in super-Earths
• Ejection of close-in super-Earths around low-mass stars in the giant impact stage
• Size evolution of close-in super-Earths through giant impacts and photoevaporation