黄道带系外行星的时间（ZEIT）IV：七个过境行星在Praesepe集群
Zodiacal Exoplanets in Time (ZEIT) IV: seven transiting planets in the Praesepe cluster
摘要：
疏散星团和年轻的星协是寻找行星和检验行星形成、迁移和演化理论的有吸引力的地点。我们提出了我们的搜索，和表征，凌日行星在~800万年前的Praesepe（蜂巢，M44）集群从K2光变曲线。我们确定了七个行星候选者，其中六个我们统计验证是真正的行星，最后一个需要更多的数据。对于每个宿主星，我们获得高分辨率的近红外光谱，以测量其预计的旋转增宽和径向速度，其中后者，我们用来确认集群成员。我们将低分辨率光谱与已知的星团距离和金属丰度相结合，以提供宿主恒星的精确温度，质量，半径和光度。结合我们的测量旋转加宽，旋转周期，和我们推导出的恒星半径，我们表明，所有的行星轨道是一致的对准他们的主恒星的旋转。我们拟合了K2光变曲线，包括恒星密度的先验，以限制行星的偏心率，所有这些都与零一致。在Praesepe和Hyades中发现的行星数量（8颗行星）与Pleiades的类似数据集（0颗行星，约1.25亿年）之间的差异表明了年龄的趋势，但可能是由于目前对年轻，快速旋转恒星的搜索管道不完整。我们看到越来越多的证据表明，一些行星在超过8亿年后继续失去大气，因为现在有两颗行星在这个年龄的半径明显大于开普勒的老行星。
英文摘要：
Open clusters and young stellar associations are attractive sites to search for planets and to test theories of planet formation, migration, and evolution. We present our search for, and characterization of, transiting planets in the ~800 Myr old Praesepe (Beehive, M44) Cluster from K2 light curves. We identify seven planet candidates, six of which we statistically validate to be real planets, the last of which requires more data. For each host star we obtain high-resolution NIR spectra to measure its projected rotational broadening and radial velocity, the latter of which we use to confirm cluster membership. We combine low-resolution spectra with the known cluster distance and metallicity to provide precise temperatures, masses, radii, and luminosities for the host stars. Combining our measurements of rotational broadening, rotation periods, and our derived stellar radii, we show that all planetary orbits are consistent with alignment to their host star's rotation. We fit the K2 light curves, including priors on stellar density to put constraints on the planetary eccentricities, all of which are consistent with zero. The difference between the number of planets found in Praesepe and Hyades (8 planets) and a similar dataset for Pleiades (0 planets, ~125 Myr) suggests a trend with age, but may be due to incompleteness of current search pipelines for younger, faster-rotating stars. We see increasing evidence that some planets continue to lose atmosphere past 800 Myr, as now two planets at this age have radii significantly larger than their older counterparts from Kepler.
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