Astrophysical wavelength calibrators based on the broad, stable LFC spectrum (“astro-combs”) may solve this problem, improving the accuracy and stability of astrophysical spectroscopy by two orders of magnitude and thereby enabling the discovery and characterization of Earth-like exoplanets. Over the last few years, several groups have developed prototype astro-combs operating in the visible spectrum, including a collaboration of which we are members between the Harvard-Smithsonian Center for Astrophysics (CfA) and MIT, as well as a collaboration between the Max Planck Institute for Quantum-Optics (MPQ) and the European Southern Observatory (ESO). The work of the CfA/MIT and MPQ/ESO teams is directed primarily at the search for Earth-like exoplanets orbiting in the liquid-water habitable zone around Sun-like stars, which are brightest in the visible spectrum. Promising initial tests of these visible-wavelength astro-combs have been performed with high-resolution astrophysical spectrographs located at the Whipple Observatory in Arizona and the La Silla Observatory in Chile, for the CfA/MIT and MPQ/ESO collaborations, respectively. With further development and optimization, visible-wavelength astro-combs should enable astrophysical spectroscopy with accuracy and long-term stability sufficient to measure slow changes in stellar RV smaller than 5 cm/s, which is equivalent to <100 kHz shifts in the stellar visible emission spectrum. As a comparison, the Earth induces an RV change in the Sun, relative to a distant observer, of ±9 cm/s with a period of one year.
In the present paper, Ycas et al. report a complementary RV technique for the search for Earth-like exoplanets. They developed an astro-comb operating in the near-infrared (NIR) and used it for wavelength calibration of the high-resolution PathFinder spectrograph operated with the 9.2 m Hobby-Eberly telescope at the McDonald Observatory in southwest Texas. NIR astrophysical spectroscopy is optimal for the search for small exoplanets orbiting M dwarf (also known as red dwarf) stars, which have peak emission in the NIR and are the most numerous type of stars (~60% of stars near our solar system). The liquid-water habitable zone is much closer to an M dwarf than to a Sun-like star, because M dwarfs are relatively cool and low mass. Consequently, the RV change caused by an Earth-mass exoplanet in the habitable zone around an M dwarf star is more than an order of magnitude larger (>1 m/s) than for a Sun-like star, greatly easing the technical challenge of detection.
Ycas et al. used an erbium-fiber LFC, two Fabry-Perot mode-filtering cavities, and nonlinear fibers to produce a broadband (1450-1700 nm) NIR frequency comb with 25 GHz line spacing, optimized for the resolution of the PathFinder spectrograph. Long-term stability was guaranteed by referencing all control loops to an atomic clock governed by the Global Positioning System (GPS). Laboratory tests of the NIR astro-comb demonstrated optical frequency measurements equivalent to an RV accuracy of 6 cm/s, which would be more than sufficient for the detection of an Earth-like exoplanet around an M dwarf star. Ycas et al. then employed the NIR astro-comb in proof-of-principle calibrations of the PathFinder spectrograph during a two week program of observations of several nearby stars, yielding a stellar RV precision of 10 m/s limited by technical issues such as guiding of the star and astro-comb calibration light to the spectrograph. The authors believe that the lessons learned from their work will inform the development of a broader-bandwidth NIR astro-comb (covering ~950 to 1700 nm) to be used as the wavelength calibrator for the planned Habitable Zone Planet Finder spectrograph, which is expected to achieve RV accuracy <1 m/s, suitable for the detection of Earth-like exoplanets in the habitable zone around M dwarf stars.
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