日外行星HD189733b的云状大气：B波段偏振的一种可能解释
Cloudy Atmosphere of the Extra-solar Planet HD189733b : A Possible Explanation of the Detected B-band Polarization
摘要：
最近在太阳系外热巨行星HD 189733 b上探测到的线偏振峰值振幅是$10^{-4}$的几倍，比所有理论预测都高出一个数量级以上。虽然$H_2$和$He$的瑞利散射可能会在蓝波段产生$10^{-4}$量级的行星-恒星通量比，但它不能解释高偏振，除非行星有异常扩展的大气层。因此，它建议，高偏振应归因于在上层可见大气中的亚微米尺寸的尘埃颗粒的薄云的存在，这支持了在光学中几乎没有特征的透射光谱的观测发现，没有预期的碱性吸收功能的迹象。据发现，偏振观测允许一个小的偏心率的轨道，是预测从时间延迟的二次日食的行星。升交点的估计经度是$16^o\pm6$，有趣的是，它与观测推断的半球积分亮度峰值位置一致。
英文摘要：
The peak amplitude of linear polarization detected recently from an extrasolar hot giant planet HD 189733b, is a few times of $10^{-4}$, more than an order of magnitude higher than all theoretical predictions. Rayleigh scattering off $H_2$ and $He$ may although give rise to a planet-star flux ratio of the order of $10^{-4}$ in the blue band, it cannot account for the high polarization unless the planet has an unusually extended atmosphere. Therefore, it is suggested that the high polarization should be attributed to the presence of a thin cloud of sub-micron size dust grains in the upper visible atmosphere which supports the observational finding of an almost feature-less transmission spectrum in the optical with no indication of the expected alkaline absorption features. It is found that the polarimetry observation allows for a small eccentricity of the orbit that is predicted from the time delay of the secondary eclipse of the planet. The estimated longitude of the ascending node is $16^o\pm6$ which interestingly coincides with the observationally inferred location of the peak hemisphere-integrated brightness.
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