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The Astro-comb: precision astrophysical spectroscopy

Astro-comb on Mt. Hopkins as a wavelength calibrator for the TRES.

- What is an astro-comb?
An astro-comb is a laser system that generates many (~2,000) narrow (<1 MHz), accurate (10-12) and evenly spaced (~25 GHz, tunable) lines. It is an ideal wavelength calibrator for an astrophysical spectrograph. It is expected to improve the accuracy of the stellar velocity measurement to ~1 cm/s level, which is 2 orders of magnitude better than the current best sensitivity. The astro-comb can be tuned to match and optimally calibrate most of the high resolution astrophysical spectrographs (R > 10,000).

- Searches for habitable extra-solar planets (exoplanets)
A habitable exoplanet needs to be terrestrial (mass < 10 Earth mass) and to be in an orbit inside the habitable zone, where water can exist in its liquid form. The smallest exoplanet found to date have been discovered using the precision radial velocity (PRV) technique. The PRV technique observes the periodic Doppler-shifts of the stellar spectrum as the star orbits the planet-star center-of-mass. Planets with only a few Earth mass have been discovered but they are in extremely close-in orbits with periods of days than years. To find a truly Earth-like planet, the stellar spectrum has to be referenced to a light source much more stable and accurate than the thorium-argon lamp now commonly in use. [Ref: Stars with known exoplanets , Nearby stars.]

The equally spaced, dense, and bright dots are the astro-comb spectrum. The irregular, sparse and dim dots are the Th-Ar lamp spectrom. The spectrum are recored by the TRES.

In April 2008, my colleagues and I reported the realization of a revolutionary technique, the astro-comb, which combines a laser frequency comb and a mode-filtering Fabry-Perot (MFFP) cavity to generate many well spaced and ultra-stable calibration lines. In July 2008, we implemented the astro-comb in the Whipple Observatory on Mt. Hopkins in Arizona to calibrate the Tillinghast Reflector Echelle Spectrograph (TRES) for the Tillinghast 60-inch telescope. The astro-comb was designed to generate calibration lines in the NIR bands for the study of M dwarfs.
Why M dwarfs? M dwarfs are dim/red/low-mass main-sequence stars. 75% of the stars in our galaxy are M stars. It is easier to find habitable exoplanets around M stars using the PRV technique for the following two reasons: 1. the habitable zones are closer because M stars are cool and 2. the same gravitaional pull causes bigger velocity wobbles on the stars because M stars are less massive. However, it has been a chanllenge study M stars because most of their photons and spectral features (mostly molecular lines, such as TiO2, VO and FeH) are in the NIR bands and there were no calibrator in NIR before the invention of the astro-comb.

- Dark energy

Related Readings:

  1. Combing the sky for 'earths' [Nature]
  2. A laser frequency comb that enables radial velocity measurements with a precision of 1 cm s-1. [Nature] [arXiv:0804.0955]
    C.-H. Li, A.J. Benedick, P. Fendel, A.G. Glenday, F.X. Kartner, D.F. Phillips, D. Sasselov, A. Szentgyorgyi and R.L. Walsworth
    Nature 452, 610-612 (2008)
  3. With a coarse-tooth comb [Nature]
    G. Walker
    Nature 452, 538 (2008)
  4. Astro-comb on Wikipedia

MRI with hyperpolarized noble gas

Fundamental Symmetry Test:

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