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arXiv 1210.7810astro-ph.EP
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开普勒行星的密度和偏心率

Density and Eccentricity of Kepler Planets

Yanqin Wu, Yoram Lithwick

备注：
extended some discussions following referee report; accepted to ApJ
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摘要：

我们分析了22个子类木行星对（17个已发表，5个新的）的开普勒任务获得的过境时间的变化，靠近平均运动共振。我们发现，这些对的TTV相位大部分接近于零，与偏心率分布一致，具有非常低的RMS值e ~ 0.01;但约四分之一的对具有更高的偏心率，高达0.1 - 0.4。对于低偏心率对，我们能够从统计上消除偏心率的影响，从TTV数据获得行星质量。这些质量与径向速度测量的质量一起，得到最佳拟合的质量-半径关系M~3 M_E（R/R_E）。这对应于20 km/s的恒定表面逃逸速度。我们将行星分为两个不同的组，“中等大小”（大于3 R_E）和“紧凑”（较小）。所有中等大小的行星都被发现密度小于水，因此包含广泛的H/He包层，质量可能与它们的核心相当。我们认为，这些行星已显着雕刻的光蒸发。令人惊讶的是，中等大小的行星只在质量大于0.8 M太阳的恒星周围发现。另一方面，致密行星的密度通常比水大。结合我们的密度测量和径向速度研究，我们发现更热的紧凑行星往往密度更大，最热的行星达到岩石密度。此外，较热的行星往往体积较小。这些结果可以解释，如果紧凑的行星是由岩石核心覆盖着少量的氢，< 1%的质量，水对它们的质量或大小贡献很小。在最热的行星中，光蒸发已经暴露出裸露的岩石核心。我们的结论是这些行星可能不是水世界，这与之前的一些研究形成了对比。

英文摘要：

We analyze the transit timing variations obtained by the Kepler mission for 22 sub-jovian planet pairs (17 published, 5 new) that lie close to mean motion resonances. We find that the TTV phases for most of these pairs lie close to zero, consistent with an eccentricity distribution that has a very low RMS value of e ~ 0.01; but about a quarter of the pairs possess much higher eccentricities, up to 0.1 - 0.4. For the low-eccentricity pairs, we are able to statistically remove the effect of eccentricity to obtain planet masses from TTV data. These masses, together with those measured by radial velocity, yield a best fit mass-radius relation M~3 M_E (R/R_E). This corresponds to a constant surface escape velocity of 20km/s. We separate the planets into two distinct groups, "mid-sized" (those greater than 3 R_E), and "compact" (those smaller). All mid-sized planets are found to be less dense than water and therefore contain extensive H/He envelopes, likely comparable in mass to that of their cores. We argue that these planets have been significantly sculpted by photoevaporation. Surprisingly, mid-sized planets are discovered exclusively around stars more massive than 0.8 M_sun. The compact planets, on the other hand, are often denser than water. Combining our density measurements with those from radial velocity studies, we find that hotter compact planets tend to be denser, with the hottest ones reaching rock density. Moreover, hotter planets tend to be smaller in sizes. These results can be explained if the compact planets are made of rocky cores overlaid with a small amount of hydrogen, < 1% in mass, with water contributing little to their masses or sizes. Photoevaporation has exposed bare rocky cores in cases of the hottest planets. Our conclusion that these planets are likely not water-worlds contrasts with some previous studies.

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