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arXiv 1406.6780astro-ph.EP
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太阳系外双星I:行星-行星散射期间潮汐捕获的形成

Extrasolar Binary Planets I: Formation by tidal capture during planet-planet scattering

H. Ochiai, M. Nagasawa, S. Ida

摘要:

我们已经研究了i)引力约束的气体巨行星对(我们称之为“双行星”)的形成,通过行星-行星动力学潮汐在它们的近距离相遇过程中相互捕获,ii)由于行星-行星和行星-恒星{ \it准静态}潮汐的长期轨道演化。对于阶段i)的初始演化,我们对由三颗木星质量的行星组成的系统进行了N体模拟,并考虑了动力学潮汐。双行星的形成率约为轨道交叉系统的10%,这一比例几乎与行星的初始星心半长轴无关,而喷射和合并率则敏感地依赖于半长轴。由于行星-行星动力学潮汐的循环,典型的双星分离是行星物理半径之和的几倍。轨道圆化后,双星系统的演化受长期准静态潮汐控制。在第二阶段,我们解析计算了准静态潮汐演化。双行星首先通过行星-行星潮汐进入自旋-轨道同步状态。行星-恒星潮汐带走了双星运动的角动量,最终导致行星之间的碰撞。然而,我们发现,如果双行星距离中心恒星超过~0.3 AU,则双行星在太阳型恒星的主序星寿命(~ 10 Gyrs)内仍然存在潮汐衰变。这些结果表明,在0.3AU以上的凌日观测可以探测到双星。

英文摘要:

We have investigated i) the formation of gravitationally bounded pairs of gas-giant planets (which we call "binary planets") from capturing each other through planet-planet dynamical tide during their close encounters and ii) the following long-term orbital evolution due to planet-planet and planet-star {\it quasi-static} tides. For the initial evolution in phase i), we carried out N-body simulations of the systems consisting of three jupiter-mass planets taking into account the dynamical tide. The formation rate of the binary planets is as much as 10% of the systems that undergo orbital crossing and this fraction is almost independent of the initial stellarcentric semi-major axes of the planets, while ejection and merging rates sensitively depend on the semi-major axes. As a result of circularization by the planet-planet dynamical tide, typical binary separations are a few times the sum of the physical radii of the planets. After the orbital circularization, the evolution of the binary system is governed by long-term quasi-static tide. We analytically calculated the quasi-static tidal evolution in later phase ii). The binary planets first enter the spin-orbit synchronous state by the planet-planet tide. The planet-star tide removes angular momentum of the binary motion, eventually resulting in a collision between the planets. However, we found that the binary planets survive the tidal decay for main-sequence life time of solar-type stars (~10Gyrs), if the binary planets are beyond ~0.3 AU from the central stars. These results suggest that the binary planets can be detected by transit observations at >0.3AU.

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