K2发现一只忙碌的蜜蜂：在蜂巢星团中发现一个不寻常的过境海王星
K2 Discovers a Busy Bee: An Unusual Transiting Neptune Found in the Beehive Cluster
摘要：
疏散星团一直是几个系外行星调查的重点，但迄今为止只发现了少数行星。\emph{Kepler}宇宙飞船揭示了在小型冷恒星周围有大量的小行星，因此，这样的星团成员是系外行星过境搜索的主要目标。开普勒的新任务，K2，目标是几个疏散星团和黄道周围的恒星形成区，以寻找围绕其低质量成分的凌日行星。在这里，我们报告了第一个凌日行星在中期年龄（8亿年）蜂巢集群（Praesepe）的发现。K2-95是一颗来自K2第五次战役的微弱（$\mathrm{Kp = 15.5\,mag}$）$\mathrm{M3.0\pm0.5}$矮星，有效温度为$\mathrm{3471 \pm 124\,K}$，金属丰度接近太阳，半径为$\mathrm{0.402 \pm 0.050 \,R_\odot}$。我们探测到一颗过境行星，半径为$\mathrm{3.47^{+0.78}_{-0.53} \, R_\oplus}$，轨道周期为10.134天。我们结合了测光，中/高分辨率光谱学，自适应光学/斑点成像和档案调查图像，以排除任何假阳性检测方案，验证行星，并进一步表征系统。这颗行星的半径是非常不寻常的，因为M矮星很少有海王星大小的凌日行星。K2- 95 b相对较大的半径与最近发现的其他星团行星K2- 25 b（毕宿二）和K2- 33 b（上天蝎座）一致，表明它们的演化状态或形成存在系统差异。这些来自K2的发现提供了星团环境中行星形成和演化的快照，因此成为测试场星和星团行星种群之间差异的绝佳实验室。
英文摘要：
Open clusters have been the focus of several exoplanet surveys but only a few planets have so far been discovered. The \emph{Kepler} spacecraft revealed an abundance of small planets around small, cool stars, therefore, such cluster members are prime targets for exoplanet transit searches. Kepler's new mission, K2, is targeting several open clusters and star-forming regions around the ecliptic to search for transiting planets around their low-mass constituents. Here, we report the discovery of the first transiting planet in the intermediate-age (800 Myr) Beehive cluster (Praesepe). K2-95 is a faint ($\mathrm{Kp = 15.5\,mag}$) $\mathrm{M3.0\pm0.5}$ dwarf from K2's Campaign 5 with an effective temperature of $\mathrm{3471 \pm 124\,K}$, approximately solar metallicity and a radius of $\mathrm{0.402 \pm 0.050 \,R_\odot}$. We detected a transiting planet with a radius of $\mathrm{3.47^{+0.78}_{-0.53} \, R_\oplus}$ and an orbital period of 10.134 days. We combined photometry, medium/high-resolution spectroscopy, adaptive optics/speckle imaging and archival survey images to rule out any false positive detection scenarios, validate the planet, and further characterize the system. The planet's radius is very unusual as M-dwarf field stars rarely have Neptune-sized transiting planets. The comparatively large radius of K2-95b is consistent with the other recently discovered cluster planets K2-25b (Hyades) and K2-33b (Upper Scorpius), indicating systematic differences in their evolutionary states or formation. These discoveries from K2 provide a snapshot of planet formation and evolution in cluster environments and thus make excellent laboratories to test differences between field-star and cluster planet populations.
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