利用高色散光谱学和高对比度仪器组合表征类地行星：多普勒频移水和氧谱线
Characterizing Earth-like Planets Using a Combination of High-Dispersion Spectroscopy and High-Contrast Instruments: Doppler-shifted Water and Oxygen Lines
摘要：
未来的径向速度、天体测量和直接成像调查将在不久的将来在宜居带内发现附近地球大小的行星。我们如何在那些非凌日行星上寻找水和氧气呢？我们表明，高色散光谱和coronagraphic技术相结合，是一种很有前途的技术来检测分子线印在散射光的类地行星（ELP）。在该方法中，行星信号通过多普勒频移从大地吸收光谱分离。假设一个长期的观测活动（10天的曝光），使用高色散光谱仪（R=50,000）与斑点抑制30米的望远镜，我们模拟的光谱ELP周围的M矮星（其恒星有效温度为2750-3750 K）在5 pc。进行互相关分析的分子线的光谱模板，我们发现，原始对比的$10^{-4}$和$10^{-5}$（使用Y，J，和H带）需要检测水蒸气在3 $σ$和16 $σ$水平，分别为$T_\star$ =3000 K。氧1.27 $μ$ m波段的6 $σ$检测需要$10^{-5}$的原始对比度。我们还研究了可能的系统，不完整的斑点减法，大地线的校正。当它们不完美时，在交叉关联函数中出现大地水信号。然而，我们发现行星信号与速度差产生的信号是分离的。我们还发现，凤凰光谱中的本征水线太弱，影响水检测的结果。我们的结论是，高色散光谱和高对比度的仪器相结合，可以是一个强大的手段来表征ELP在极大的望远镜时代。
英文摘要：
Future radial velocity, astrometric, and direct-imaging surveys will find nearby Earth-sized planets within the habitable zone in the near future. How can we search for water and oxygen in those nontransiting planets? We show that a combination of high-dispersion spectroscopic and coronagraphic techniques is a promising technique to detect molecular lines imprinted in the scattered light of Earth-like planets (ELPs). In this method, the planetary signals are spectroscopically separated from telluric absorption by using the Doppler shift. Assuming a long observing campaign (a 10-day exposure) using a high-dispersion spectrometer (R=50,000) with speckle suppression on a 30-m telescope, we simulate the spectra from ELPs around M dwarfs (whose stellar effective temperature is 2750-3750 K) at 5 pc. Performing a cross-correlation analysis with the spectral template of the molecular lines, we find that raw contrasts of $10^{-4}$ and $10^{-5}$ (using Y, J, and H bands) are required to detect water vapor at the 3 $σ$ and 16 $σ$ levels, respectively, for $T_\star$=3000 K. The raw contrast of $10^{-5}$ is required for a 6 $σ$ detection of the oxygen 1.27 $μ$m band. We also examine possible systematics, incomplete speckle subtraction, and the correction for telluric lines. When those are not perfect, a telluric water signal appears in the cross-correlation function. However, we find the planetary signal is separated from that resulting from the velocity difference. We also find that the intrinsic water lines in the Phoenix spectra are too weak to affect the results for water detection. We conclude that a combination of high-dispersion spectroscopy and high-contrast instruments can be a powerful means to characterize ELPs in the extremely large telescope era.
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