[论文解读] FORMATION AND EVOLUTION OF CLOSE-IN PLANETS

Soko Matsumura, S. J. Peale+1 more|arXiv (Cornell University)|Jan 1, 2010

Stellar, planetary, and galactic studies参考文献 3被引用 2

一句话总结

本文研究了轨道偏心率较高的热木星的潮汐演化，表明潮汐耗散主导了其轨道衰减与圆化过程，且演化路径取决于恒星与行星的潮汐耗散效率，存在两种截然不同的演化路径。修正了先前研究中的数值错误后，本文确认偏心率迅速衰减，支持行星-行星散射后通过潮汐圆化形成许多具有非零偏心率的热木星的演化路径。

ABSTRACT

Therst observations of close-in planets indicated that mo st had circular and low-inclination orbits, with measured eccentricities consistent with zero and very small misalignment angles between the stellar spin and orbital angular momentum. However, this simple, and perhaps expected, picture has begun to change with the recent discoveries of several transiting planets with clearly non-zero eccentricities, and some large inclinations. Two major scenarios to form such close-in planets are planet migration in a disk, and planetplanet interactions combined with tidal dissipation. The former scenario can naturally produce a circular and low-inclination orbit, while the latter implicitly assumes an initially highly eccentric and possibly high-inclination orbit, which then becomes more circular and aligned with the stellar equator through tidal dissipation. Motivated by these observations, we investigate the tidal evolution of transiting planets on eccentric orbits. We show that the current and future orbital evolution of these systems is likely dominated by tidal dissipation, and unlikely to be strongly aected by a more distant companion. As shown in previous studies, wend that most of these close-in planets experience orbital decay all the way to the Roche limit. Wend that there are two characteristic evolut ion paths for these systems, depending on the relative eciency of tidal dissipation inside the star a nd inside the planet, and we point out that the current observations may be consistent with one of them. Our results suggest that at least some of the close-in planets with non-zero orbital eccentricity are likely to have been formed via planet� planet scattering followed by tidal circularization. We alsond that even when the stellar spin-orbit misalignment is observed to be small at present, some systems could have had a highly misaligned orbit in the past. Finally, we also re-examine the recent claim by Levrard et. al., who found that all orbital and spin parameters, including eccentricity and stellar obliquity, evolve on a similar timescale to orbital decay. In particular, this would imply that eccentricity cannot be damped much during the lifetime of the planet, even when the estimated circularization timescale is shorter than the orbital decay timescale. This counter-intuitive result turns out to have been caused by an error in the numerical code used by Levrard et al. (2009). Correcting this error, wend that the eccentricity behaves as expected, with orbits usually circularizing rapidly compared to the orbital decay rate. Subject headings: planetary systems

研究动机与目标

• 理解主导具有非零偏心率的热木星轨道与自转演化的主要物理过程。
• 解决先前主张中的不一致之处，特别是Levrard等人（2009年）提出的观点：尽管圆化 timescale 很短，但偏心率与轨道倾角的演化却很缓慢。
• 确定恒星或行星中的潮汐耗散在决定这些系统演化路径中起主导作用。
• 评估当前观测到的低恒星自转-轨道倾角是否可能掩盖了过去高度倾斜的演化状态。

提出的方法

• 对近距离系外行星系统的潮汐演化进行数值建模，重点研究由潮汐耗散驱动的轨道衰减与圆化过程。
• 比较宿主恒星与行星的潮汐耗散效率，以识别两种截然不同的演化路径。
• 对Levrard等人（2009年）的模型进行重新分析，使用修正后的数值代码，重新评估偏心率衰减与自转-轨道倾角演化的 timescale。
• 利用潮汐圆化与衰减 timescale 评估轨道衰减抵达洛希半径的可能性。
• 引入观测约束条件，如测得的偏心率与自转-轨道倾角，以检验模型的一致性。
• 分析偏心率衰减、轨道衰减与自转-轨道倾角演化之间的相对 timescale。

实验结果

研究问题

• RQ1 对于具有非零偏心率的热木星，潮汐耗散过程是否主导其轨道演化，还是遥远伴星的影响显著？
• RQ2 能否将某些系统中观测到的低自转-轨道倾角与行星-行星散射导致的过去高倾角状态相协调？
• RQ3 为何Levrard等人（2009年）报告称，尽管圆化 timescale 很短，但偏心率与自转-轨道倾角参数却演化缓慢？
• RQ4 在由恒星与行星中潮汐耗散效率相对大小决定的两种可能演化路径中，哪一种与当前观测一致？
• RQ5 对于轨道偏心率较高的热木星，轨道衰减至洛希半径是否是大多数系统的可能结局？

主要发现

• 恒星与行星中的潮汐耗散主导了热木星的轨道演化，遥远伴星的影响可忽略不计。
• 大多数热木星因潮汐力作用，其轨道衰减会持续至洛希半径。
• 根据恒星与行星中潮汐耗散效率的相对大小，演化路径分为两种截然不同的类型，当前观测与其中一种路径一致。
• Levrard等人（2009年）得出的反直觉结论——即偏心率演化缓慢——源于其代码中的数值错误。
• 修正错误后，模型确认偏心率的衰减速度远快于轨道衰减，这与物理预期一致。
• 即使当前自转-轨道倾角较小的系统，过去也可能经历过高度倾斜的阶段，与行星-行星散射的演化历史一致。