用SPHERE / ZIMPOL探测地球质量行星半人马座α B b的偏振特征的前景
Prospects of detecting the polarimetric signature of the Earth-mass planet alpha Centauri B b with SPHERE / ZIMPOL
摘要：
在过去的五年里，径向速度和凌日技术揭示了一个新的类地行星群，其质量只有几个地球质量。它们围绕主星的轨道非常接近，需要一个精致的内部工作角度才能在直接成像中被检测到，这对在可见光范围内工作的直接成像仪（如SPHERE / ZIMPOL）提出了挑战。在所有已知的小于25个地球质量的系外行星中，我们首先预测了直接成像的最佳候选者。然后，我们的主要目标是提供最好的仪器设置和观测策略，以检测这样一个特殊的对象与ZIMPOL。作为第二步，我们的目标是预测其探测率。利用系外行星的径向速度测量，偏振模型和衍射传播代码CAOS的约束下的属性，我们估计的探测灵敏度ZIMPOL这样的行星在不同的观测模式的仪器。我们展示了如何优化观察策略，以获得对特定目标的最佳检测性能。在我们目前对系外行星系统的了解中，半人马座α B b是ZIMPOL最有希望的目标，其质量小于25个地球。由于有气体瑞利散射的大气层和良好的倾角，这颗行星可以在大约4小时的观测时间内被发现，使用I波段的四象限相位掩模日冕仪。然而，如果半人马座α B b显示出与我们的月球相似的不利的偏振和反射特性，那么它比ZIMPOL的最佳灵敏度要暗50倍左右。Alpha Centauri B是SPHERE的主要目标。如果行星的偏振特性是有利的，专门针对径向速度检测的行星进行专门的深度观测可以导致检测。
英文摘要：
Over the past five years, radial-velocity and transit techniques have revealed a new population of Earth-like planets with masses of a few Earth masses. Their very close orbit around their host star requires an exquisite inner working angle to be detected in direct imaging and sets a challenge for direct imagers that work in the visible range, like SPHERE / ZIMPOL. Among all known exoplanets with less than twenty-five Earth-masses we first predict the best candidate for direct imaging. Our primary objective is then to provide the best instrument setup and observing strategy for detecting such a peculiar object with ZIMPOL. As a second step, we aim at predicting its detectivity. Using exoplanet properties constrained by radial velocity measurements, polarimetric models and the diffraction propagation code CAOS, we estimate the detection sensitivity of ZIMPOL for such a planet in different observing modes of the instrument. We show how observing strategies can be optimized to yield the best detection performance on a specific target. In our current knowledge of exoplanetary systems, alpha Centauri B b is the most promising target with less than twenty-five Earth-masses for ZIMPOL. With a gaseous Rayleigh-scattering atmosphere and favorable inclinations, the planet could be detected in about four hours of observing time, using the four-quadrant phase-mask coronograph in the I band. However, if alpha Centauri B b should display unfavorable polarimetric and reflective properties similar to that of our Moon, it is around 50 times fainter than the best sensitivity of ZIMPOL. Alpha Centauri B is a primary target for SPHERE. Dedicated deep observations specifically targeting the radial velocity-detected planet can lead to a detection if the polarimetric properties of the planet are favorable.
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