Development of a differential stellar coronograph for direct search of exoplanets

我们进行了多次直接探测系外行星和观测系外行星光谱的光学模拟实验。我们的方法是基于Baba和Murakami提出的偏振微分恒星日冕仪。星光通常被认为是非偏振的。另一方面，从它的行星发出的光预计是部分偏振光，因为行星的光是被母恒星散射和反射的光。当我们用相互正交的偏振光观测恒星-行星系统时，取这两幅图像的差值，非偏振分量被抵消，部分偏振分量保留下来。这就是偏振微分法的原理。然而，简单的极化分化并不总是产生良好的结果。因此，我们提出对像和物镜光谱进行偏振度分析。在这里，偏振度是指两个正交线性偏振之间的差值，由该点的强度归一化。我们可以从嘈杂的星光中分辨出行星的光。在我们的光学模拟中，我们将行星模型的偏振度设置为50%。恒星模型和行星模型之间的角距离设为4.8λ/D，其中λ为观测的中心波长，D为望远镜的直径。行星模型与恒星模型的强度比为1.1x10^<-5>。在偏振差图像中，我们无法从散斑噪声中区分行星图像，但可以从偏振度的表示中清晰地识别行星图像。我们还进行了实验，以获得系外行星的客观光谱。实验结果表明，偏振度的分析对区分行星光谱和有噪声的恒星光谱也很有帮助。

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.