共振与超短周期行星系统位于系外行星年龄分布的两端
Resonant and Ultra-short-period Planet Systems are at Opposite Ends of the Exoplanet Age Distribution
AI总结：
本研究利用亚巨星校准 Kepler 天区年龄—速度弥散关系，推断共振系统较年轻而 USP 行星系统较老，支持共振构型长期扩散与潮汐迁移。
摘要：
系外行星系统被认为会在数十亿年的长期时间尺度上演化。在大多数单个系统中，这种演化不可能在人类时间尺度上被直接观测到。尽管若能对年龄 τ 处于 1 Gyr ≲ τ ≲ 10 Gyr 区间内的单个系外行星宿主恒星获得准确而精确的年龄推断，便可以约束这种演化，但对于像大多数系外行星宿主恒星这样的孤立场矮星而言，获得准确而精确的年龄推断十分困难。薄盘星族的银河系速度弥散随时间单调增长，而在给定银河系位置上，年龄与速度弥散之间的关系可以由一个能够获得准确而精确年龄推断的星族来校准。利用一个具有精确年龄推断的亚巨星样本，我们校准了 Kepler 天区中的年龄—速度弥散关系。将该关系应用于 Kepler 天区的行星群体，我们发现，可能处于二阶平均运动共振中的 Kepler 发现系统具有 1 Gyr ≲ τ ≲ 2 Gyr 的年龄。可能处于一阶平均运动共振中的系统也是如此，但这仅适用于可能受到其最内侧行星内部潮汐耗散影响的系统。这些观测表明，许多行星系统会在长期时间尺度上从最初的共振构型扩散离开。我们校准后的关系还表明，超短周期（USP）行星系统的典型年龄处于 5 Gyr ≲ τ ≲ 6 Gyr 的区间。我们提出，USP 行星在数十亿年以及数万亿次长期偏心率激发和行星内部阻尼循环中，从 1 d ≲ P ≲ 2 d 范围内的初始周期潮汐迁移至 P < 1 d 的观测位置。
英文摘要：
Exoplanet systems are thought to evolve on secular timescales over billions of years. This evolution is impossible to directly observe on human timescales in most individual systems. While the availability of accurate and precise age inferences for individual exoplanet host stars with ages $τ$ in the interval $1~\text{Gyr}\lesssim~τ~\lesssim10~\text{Gyr}$ would constrain this evolution, accurate and precise age inferences are difficult to obtain for isolated field dwarfs like the host stars of most exoplanets. The Galactic velocity dispersion of a thin disk stellar population monotonically grows with time, and the relationship between age and velocity dispersion in a given Galactic location can be calibrated by a stellar population for which accurate and precise age inferences are possible. Using a sample of subgiants with precise age inferences, we calibrate the age--velocity dispersion relation in the Kepler field. Applying this relation to the Kepler field's planet populations, we find that Kepler-discovered systems plausibly in second-order mean-motion resonances have $1~\text{Gyr}\lesssim~τ~\lesssim2~\text{Gyr}$. The same is true for systems plausibly in first-order mean-motion resonances, but only for systems likely affected by tidal dissipation inside their innermost planets. These observations suggest that many planetary systems diffuse away from initially resonant configurations on secular timescales. Our calibrated relation also indicates that ultra-short-period (USP) planet systems have typical ages in the interval $5~\text{Gyr}\lesssim~τ~\lesssim6~\text{Gyr}$. We propose that USP planets tidally migrated from initial periods in the range $1~\text{d}\lesssim~P~\lesssim2~\text{d}$ to their observed locations at $P<1~\text{d}$ over billions of years and trillions of cycles of secular eccentricity excitation and inside-planet damping.
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