第一次探测到来自系外行星大气的偏振散射光
First detection of polarized scattered light from an exoplanetary atmosphere
摘要：
我们报告了第一次在可见光偏振光中直接探测到系外行星。凌日行星HD 189733b是非常热的类星体之一，周期最短，因此轨道最小，这使它们成为偏振探测的理想候选者。我们得到的偏振测量HD189733在$B$波段分布良好的轨道周期，并检测到两个偏振最大值附近的行星伸长与峰值振幅为$\sim2\cdot10^{-4}$。假设瑞利散射，我们估计散射大气（朗伯球）的有效尺寸为1.5 $\pm$ 0.2 $R_{\rm J}$，这是30%以上的不透明体的半径先前推断的过境。如果散射物质充满了行星洛希瓣，几何散射的下限可以估计为0.14。偏振的相位依赖性表明行星轨道几乎是南北方向的，其升角$Ω$ =（16 \degr或196 \degr）$\pm$ 8 \degr。我们得到独立的估计轨道倾角$i$ =98 \degr $\pm$ 8 \degr和偏心率$e$ =0.0（与0.05的不确定性），这是在良好的协议与以前确定的值从过境和径向速度。我们的研究结果清楚地表明了偏振测量的力量，并为探索系外行星大气层开辟了一个新的维度，即使是没有凌日的系统。
英文摘要：
We report the first direct detection of an exoplanet in the visible polarized light. The transiting planet HD189733b is one of the very hot Jupiters with shortest periods and, thus, smallest orbits, which makes them ideal candidates for polarimetric detections. We obtained polarimetric measurements of HD189733 in the $B$ band well distributed over the orbital period and detected two polarization maxima near planetary elongations with a peak amplitude of $\sim2\cdot10^{-4}$. Assuming Rayleigh scattering, we estimated the effective size of the scattering atmosphere (Lambert sphere) to be 1.5$\pm$0.2 $R_{\rm J}$, which is 30% larger than the radius of the opaque body previously inferred from transits. If the scattering matter fills the planetary Roche lobe, the lower limit of the geometrical albedo can be estimated as 0.14. The phase dependence of polarization indicates that the planetary orbit is oriented almost in a north-south direction with a longitude of ascending node $Ω$=(16\degr or 196\degr)$\pm$8\degr. We obtain independent estimates of the orbit inclination $i$=98\degr$\pm$8\degr and eccentricity $e$=0.0 (with an uncertainty of 0.05) which are in excellent agreement with values determined previously from transits and radial velocities. Our findings clearly demonstrate the power of polarimetry and open a new dimension in exploring exoplanetary atmospheres even for systems without transits.
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