从transit光变曲线推断系外行星上的不对称边缘云
Inferring asymmetric limb cloudiness on exoplanets from transit light curves
摘要：
云已经被证明存在于许多系外行星的大气中。云形成模型预测相当大的云量不均匀性，与光学相位曲线观测一致。然而，光学相位曲线不能解决云的位置和云的光学性质之间的一些现有的简并。我们提出了一个概念上简单的技术来检测非均匀云覆盖的系外行星。这种不均匀的云层覆盖造成了行星在主星前方的不对称主凌日。与标准对称模型相比，非对称凌日产生特征残差。此外，平分线跨度可以用于确定在过渡光变曲线中的不对称性。我们应用一个模型的不对称凌日的光变曲线的HAT-P-7 b，开普勒-7b和HD209458 b和搜索可能的云签名。几乎不间断的开普勒测光特别适合这种方法，因为它允许非常高的时间分辨率。我们在所考虑的行星的数据中没有发现任何统计上合理的云特征。对于HAT-P-7 b，发现了一个不对称的云层覆盖的试探性检测，与光学相位曲线的分析一致。基于贝叶斯概率参数，在过境星历中具有偏移的对称模型仍然是最可行的模型。尽管如此，这项工作表明，对于合适的目标，即明亮恒星周围的低重力行星，该方法可用于限制云层覆盖特性，因此是研究系外行星大气的有用工具。
英文摘要：
Clouds have been shown to be present in many exoplanetary atmospheres. Cloud formation modeling predicts considerable inhomogeneities of cloud cover, consistent with optical phase curve observations. However, optical phase curves cannot resolve some existing degeneracies between cloud location and cloud optical properties. We present a conceptually simple technique to detect inhomogeneous cloud cover on exoplanets. Such an inhomogeneous cloud cover produces an asymmetric primary transit of the planet in front of the host star. Asymmetric transits produce characteristic residuals compared to a standard symmetric model. Furthermore, bisector spans can be used to determine asymmetries in the transit light curve. We apply a model of asymmetric transits to the light curves of HAT-P-7b, Kepler-7b and HD209458b and search for possible cloud signatures. The nearly uninterrupted Kepler photometry is particularly well-suited for this method since it allows for a very high time resolution. We do not find any statistically sound cloud signature in the data of the considered planets. For HAT-P-7b, a tentative detection of an asymmetric cloud cover is found, consistent with analysis of the optical phase curve. Based on Bayesian probability arguments, a symmetric model with an offset in the transit ephemeris remains, however, the most viable model. Still, this work demonstrates that for suitable targets, namely low-gravity planets around bright stars, the method can be used to constrain cloud cover characteristics and is thus a helpful additional tool to study exoplanetary atmospheres.
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