Development of a differential stellar coronograph for direct search of exoplanets

为了直接探测系外行星并观测其光谱，我们进行了几次光学模拟实验。我们的方法是基于偏振差分恒星日冕，已提出的巴巴和村上。星光通常被认为是非偏振的。另一方面，来自它的行星的光预计是部分偏振的，因为行星光是被母星星光散射和反射的光。当我们用相互正交的偏振光观察一个恒星-行星系统，并取这两个图像的差时，非偏振分量被抵消，部分偏振分量保留下来。这就是极化微分法的原理，然而，简单的极化微分法并不总是能得到好的结果。因此，我们建议进行偏振度的分析，为图像和目标光谱。这里，偏振度是指通过该点处的强度归一化的两个正交线性偏振之间的差的值。在我们的光学模拟中，我们将行星模型的偏振度设置为50%。星星模型和其行星模型之间的角距离被设置为4.8λ/D，其中λ是用于观测的中心波长，D是望远镜的直径。行星模型与星星模型的强度比为1.1 × 10 ~ 4<-5>。在偏振差分图像中，我们不能区分散斑噪声和行星图像，但可以从偏振度的表示中清楚地识别出行星图像。我们的实验结果表明，偏振度的分析也是非常有用的，以区分行星光谱噪声恒星光谱。

We conducted several optical simulation experiments for detecting directly exoplanets and observing their spectra. Our method is based on the polarization differential stellar coronagraph that has been proposed by Baba and Murakami. Starlight is generally considered to be unpolarized. On the other hand the light from its planet is expected to be partially polarized, because the planetary light is the scattered and reflected one by the parent starlight. When we observe a star-planet system with mutually orthogonal polarized light and take the difference of these two images, the unpolarized component is cancelled out and the partially polarized component remains. This is the principle of the polarization differential method.However, the simple polarization differentiation does not always produce good results. Therefore, we have proposed to conduct the analysis of the degree of polarization for image and objective spectra. Here, the degree of polarization means a value of the difference between two orthogonally linear polarizations normalized by the intensity at the point. We can discriminate planetary light from noisy starlight.In our optical simulation we set the degree of polarization of a planet model to 50%. An angular distance between a star model and its planet model is set to 4.8λ/D, where λ is the central wavelength for observation and D a diameter of a telescope. An intensity ratio of the planet model to the star model is 1.1x10^<-5>. In the polarization differential image we could not discriminate the planetary image from speckle noise, but we could clearly identify the planetary image from the representation of the degree of polarization.We also conducted the experiment to obtain objective spectra of exoplanets. Our experimental results show that the analysis of the polarization degree is also very useful to distinguish planetary spectrum from noisy stellar spectra.