EMPIRICAL CONSTRAINTS ON THE OBLATENESS OF AN EXOPLANET
EMPIRICAL CONSTRAINTS ON THE OBLATENESS OF AN EXOPLANET
复制标题
对系外行星扁平度的经验限制
DOI：
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发表时间：
2009
期刊：
影响因子：
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通讯作者：
J. Winn
中科院分区：
文献类型：
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作者：
J. Carter;J. Winn
根据斯皮策太空望远镜对七次凌日的光度测量，我们发现气态巨行星HD 189733b比土星更平坦。可观测到的扁率表现可能是在入口和出口阶段的轻微异常，以及由于自旋进动而导致的过境深度变化。我们没有探测到这些效应，这为系外行星的形状提供了第一个经验约束。结果与行星自转周期和轨道周期同步的理论预期一致，在这种情况下，扁率将比我们的上限小一个数量级。相反，如果假设HD 189733b处于同步的零倾角状态，则数据给出了行星四极矩的上限（J2 < 0.068, 95%置信度），该上限太弱，无法约束行星的内部结构。附录描述了计算扁行星凌日光曲线的快速算法，这是我们分析所必需的。
We show that the gas giant exoplanet HD 189733b is less oblate than Saturn, based on Spitzer Space Telescope photometry of seven transits. The observable manifestations of oblateness would have been slight anomalies during the ingress and egress phases, as well as variations in the transit depth due to spin precession. Our nondetection of these effects gives the first empirical constraints on the shape of an exoplanet. The results are consistent with the theoretical expectation that the planetary rotation period and orbital period are synchronized, in which case the oblateness would be an order of magnitude smaller than our upper limits. Conversely, if HD 189733b is assumed to be in a synchronous, zero-obliquity state, then the data give an upper bound on the quadrupole moment of the planet (J2 < 0.068 with 95% confidence) that is too weak to constrain the interior structure of the planet. An Appendix describes a fast algorithm for computing the transit light curve of an oblate planet, which was necessary for our analysis.