低质量Kepler行星的潮汐演化研究

Tidal Evolution of the Kepler Planets with Radii Less than 4 R_⊕

Kepler空间计划发现了大量半径小于4 R_⊕(R_⊕为地球半径)的近轨道行星,成为Kepler探测的特色之一,它们对当前的行星形成模型提出了新的挑战.行星与其中心恒星之间的潮汐效应对重塑这类行星的轨道构型具有重要影响.基于各种初始的轨道分布数值模拟了近轨道、低质量行星的潮汐演化,定性地给出了行星最后的轨道分布特征,轨道半长轴和峰值均随着初始的半长轴和偏心率增大而变大.对于初始的平均半长轴在0.1au以内,平均偏心率大于0.25时,数值模拟结果与观测比较接近.潮汐耗散系数、恒星和行星的质量等相关参数对潮汐演化后的半长轴分布影响都比较小.基于数值模拟结果,尝试了揭示低质量行星的形成机制:它们很可能形成于原行星盘的较远处,具有中等的轨道偏心率,后来在原行星盘中经历了Ⅰ类迁移到达目前的轨道,但是这不能排除行星的当地形成机制.

The planets with a radius 〈 4 Re observed by the Kepler mission exhibit a unique feature, and propose a challenge for current planetary formation models. The tidal effects between the planet and the host star play an essential role in reconfiguring the final orbits of the short-period planets. In this work, based on various initial Rayleigh distributions of the orbital elements, the final semi-major axis distributions of the planets with a radius 〈 4 Re after suffering tidal evolution are investigated. Our simulations qualitatively reveal various statistical properties： the peaks of semimajor axes increase with the mean semi-major axis, and the peak amplitude of the final semi-major axis distribution increases with the mean eccentricity for all kinds of semi-major axis distributions. For the case that the mean semi-major axis is about 0.1 au and the mean eccentricity is larger than 0.25, the amplitude peaks of the final semi-major axis are approximately consistent with observations. In addition, the roles of other parameters such as the tidal dissipation factor, stellar, and planetary mass etc, are explored in this study by performing numerical simulations, and it is found that they have little effect on the amplitudes of the peaks. Our simulation results provide some clues of planetary formation for such low-mass planets. We speculate that these low-mass planets are possible to form in a farther place of the proto-planetary disk with a moderate eccentricity via type I migration, and it is possible to form in situ too.