Characterizing Earth-like Planets Using a Combination of High-Dispersion Spectroscopy and High-Contrast Instruments: Doppler-shifted Water and Oxygen Lines
Kawahara et al
Future radial velocity, astrometric and direct imaging surveys will find nearby Earth-sized planets within the habitable zone (HZ) in the near future. How can we search for water and oxygen in those non-transiting planets? We propose a combination of high-dispersion spectroscopy and coronagraphic techniques as a method to detect molecular lines in Earth-like planets (ELPs). In this method, the planetary signals are spectroscopically separated from the telluric absorption due to the Doppler shift. Assuming a long observing campaign (Texp= 20 days) using the high-dispersion spectrometer (R=50,000) with the speckle suppression on a 30 m telescope, we simulate the spectra from the ELPs around M dwarfs at 5 pc. Performing the cross-correlation analysis with the binary template of the molecular lines, we find that the raw contrasts of 10−4 (0.8-1.8 μm) and 10−4.5 (use of J-band only) at 30 mas are required to detect the water vapor for a ∼4−5σ detection. The raw contrast of 10−5 is required for a 4 σ detection of the oxygen 1.27 μm band. For the ELPs around solar-type stars, it is necessary to assume a several hundred times better contrast than that for M dwarfs in order to detect water vapor. This method does not require any additional post-processings and is less sensitive to the terrestrial noise than the low resolution spectroscopy. We conclude that a combination of high-dispersion spectroscopy and high-contrast instruments can be a powerful means to characterize the ELPs in the extremely large telescope era.