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arXiv 1111.0031astro-ph.EPastro-ph.SR
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日冕X射线年龄关系及其对系外行星蒸发的意义

The coronal X-ray - age relation and its implications for the evaporation of exoplanets

Alan P. Jackson, Timothy A. Davis, Peter J. Wheatley

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20 pages, 14 figures, accepted by MNRAS. Full versions of Tables 3, 4 and 5 available with the online journal version
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摘要：

研究了晚型恒星的日冕X射线发射与恒星年龄的关系，以及这种关系随光谱型的变化。我们从13个疏散星团中选取了717颗恒星，发现在日冕辐射的饱和阶段，X射线与辐射热光度的比值从晚期K型矮星的10^-3.1降低到早期F型恒星的10^-4.3（在0.29<（B-V）_0<1.41的范围内）。我们确定的饱和时间尺度在10^7.6和10^8.3年之间变化，尽管在整个FGK范围内没有明确的趋势。我们使用简单的能量限制蒸发模型并考虑罗氏瓣效应和不同的加热/蒸发效率，将我们的X射线发射-年龄关系应用于121颗已知凌日系外行星的蒸发历史的研究。我们确认，行星在M^2/R^3平面和^-2平面上的线性截止分布是蒸发导致的种群变化的预期结果，已知的凌日系外行星也显示出这样的截止分布。我们发现，对于25%的蒸发效率，我们预计大约十分之一的已知凌日系外行星自形成以来已经失去了超过5%的质量。此外，我们提供的最低形成质量的估计，一个行星可以预期生存4个陀螺的恒星和行星参数的范围。我们强调的重要性，一个行星的生命的最早期，其蒸发的历史与75%预计将发生在第一个Gyr。这就提出了利用蒸发历史来区分不同迁移情景的可能性。对于有自旋轨道角的行星，没有发现轨道偏离的行星和轨道对准的行星的分布有什么区别。这表明在行星系的生命早期就发生了错位，尽管还需要更多的数据。

英文摘要：

We study the relationship between coronal X-ray emission and stellar age for late-type stars, and the variation of this relationship with spectral type. We select 717 stars from 13 open clusters and find that the ratio of X-ray to bolometric luminosity during the saturated phase of coronal emission decreases from 10^-3.1 for late K-dwarfs to 10^-4.3 for early F-type stars (across the range 0.29<(B-V)_0<1.41). Our determined saturation timescales vary between 10^7.6 and 10^8.3 years, though with no clear trend across the whole FGK range. We apply our X-ray emission - age relations to the investigation of the evaporation history of 121 known transiting exoplanets using a simple energy-limited model of evaporation and taking into consideration Roche lobe effects and different heating/evaporation efficiencies. We confirm that a linear cut-off of the planet distribution in the M^2/R^3 versus a^-2 plane is an expected result of population modification by evaporation and that the known transiting exoplanets display such a cut-off. We find that for an evaporation efficiency of 25% we expect around 1 in 10 of the known transiting exoplanets to have lost > 5% of their mass since formation. In addition we provide estimates of the minimum formation mass for which a planet could be expected to survive for 4 Gyrs for a range of stellar and planetary parameters. We emphasise the importance of the earliest periods of a planet's life for its evaporation history with 75% expected to occur within the first Gyr. This raises the possibility of using evaporation histories to distinguish between different migration scenarios. For planets with available spin-orbit angles no difference is found between the distributions of planets with misaligned orbits and those with aligned orbits. This suggests that misalignment occurs early in the life of the planetary system, though more data is needed.

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