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arXiv 1205.0010astro-ph.EP
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热演化和质量损失如何塑造超级地球和次海王星的种群：对开普勒-11系统及更远的应用

How Thermal Evolution and Mass Loss Sculpt Populations of Super-Earths and Sub-Neptunes: Application to the Kepler-11 System and Beyond

Eric D. Lopez, Jonathan J. Fortney, Neil K. Miller

备注：
17 pages, 7 figures. Accepted for publication in ApJ. Compared to the submitted version we made modest modifications to our mass loss model including reducing our default mass loss efficiency. However, the central conclusions of the paper are unchanged
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摘要：

我们使用热演化和XUV驱动的质量损失模型来探索低质量低密度凌日行星的组成和历史。我们详细研究了开普勒-11系统，并对当前和过去的行星组成进行了估计。我们发现Kepler-11b上的H/He大气非常容易受到质量损失的影响。通过比较形成模型，我们表明，在原位形成的系统是不可能的。相反，我们认为这是一个富含水的亚海王星系统，从雪线以外迁移而来。对于更广泛的人口观察到的行星，我们表明，有一个阈值，在散装行星密度和入射通量以上，没有低质量过境行星已被观察到。我们认为，这个阈值是由于H/He大气的不稳定性，XUV驱动的质量损失。重要的是，我们发现，这个通量密度阈值很好地再现了我们的热演化/收缩模型，其中包括一个标准的质量损失处方。处理行星的收缩历史是必不可少的，因为行星在高XUV通量的早期具有明显更大的半径。随着时间的推移，具有H/He包层的低质量行星可以转变为具有蒸汽大气层或岩石超级地球的水主导世界。最后，我们使用这个阈值提供可能的最小质量和径向速度振幅的开普勒候选人的一般人口。同样，我们使用这个阈值来限制径向速度调查发现的低质量行星的最大半径。

英文摘要：

We use models of thermal evolution and XUV-driven mass loss to explore the composition and history of low-mass low-density transiting planets. We investigate the Kepler-11 system in detail and provide estimates of both the current and past planetary compositions. We find that a H/He atmosphere on Kepler-11b is highly vulnerable to mass loss. By comparing to formation models, we show that in situ formation of the system is unlikely. Instead we propose that it is a water-rich system of sub-Neptunes that migrated from beyond the snow line. For the broader population of observed planets, we show that there is a threshold in bulk planet density and incident flux above which no low-mass transiting planets have been observed. We suggest that this threshold is due to the instability of H/He atmospheres to XUV-driven mass loss. Importantly, we find that this flux-density threshold is well reproduced by our thermal evolution/contraction models that incorporate a standard mass loss prescription. Treating the planets' contraction history is essential because the planets have significantly larger radii during the early era of high XUV fluxes. Over time low mass planets with H/He envelopes can be transformed into water-dominated worlds with steam atmospheres or rocky super-Earths. Finally, we use this threshold to provide likely minimum masses and radial velocity amplitudes for the general population of Kepler candidates. Likewise, we use this threshold to provide constraints on the maximum radii of low-mass planets found by radial velocity surveys.

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