The high-dispersion optical spectrographs that have thus far enabled the discovery of more than 400 extrasolar planets have a common susceptibility: the need for ultrastable, broadband, high-line-density, bright, and continuous wavelength calibration so that the small reflex velocities of the host stars can be precisely characterized over periods of weeks to years. One crucial technology for these investigations utilizes a mode-locked femtosecond pulsed laser - which produces a bright, regularly spaced set of emission features in frequency space - together with a Fabry-Perot filtering cavity to create the appropriate line-density, all locked to an atomic clock for long-term stability. These so-called "laser combs" have the potential for enabling radial velocity measurements to a precision <10 cm/s; thus the determination of the mass of Earth-like exoplanets should eventually be possible.
Dr. Ronald Walsworth of the Harvard-Smithsonian Center for Astrophysics and Dr. Franz Kaertner of the Massachusetts Institute of Technology are planning to construct such a laser comb for the orange-green spectrum, a region well-suited to the detection of solar-type stars and therefore of stars in which a planet with an orbital period of ~1 yr would exist in the "life-zone", where surface water would be in liquid form. The team of investigators has an excellent track record in this area, having developed laser combs for the optical red and blue spectral regions. The new comb will be tested at the 1.5-m Whipple Observatory telescope on Mt. Hopkins, Az, and, when proven reliable, applied to an ongoing project at the 1.5-m telescope at Cerro Tololo Interamerican Observatory that is searching for exo-Earths orbiting stars in the very nearby alpha/beta-Centaurus star system. The activity will involve both women as both scientists and students, and the resulting time-controlled laser technology should have application well beyond astronomy. Funding for this work is being provided by NSF's Division of Astronomical Sciences through its Advanced Technologies and Instrumentation program.
(AST-1006507) Precision radial velocity (PRV) measurement — a successful method for identifying planets in other star systems (exoplanets) — exploits small, periodic Doppler shifts in the spectrum of a target star to infer the existence of orbiting planets and determine characteristics such as orbital period and a lower limit to the planet’s mass. Utilizing the RV method to find small, rocky exoplanets similar to the Earth in the habitable zone requires wavelength calibration precision and stability ~10 cm/s for the astrophysical spectrograph used to measure the stellar spectrum; measurements of time-variation of the fundamental constants or of the expansion of the universe requires ~1 cm/s sensitivity over decadal timescales. We are developing a next-generation wavelength calibrator ("astro-comb") consisting of tens of thousands of narrow, equally-spaced spectral-lines. When referenced to the Global Position System (GPS), astro-combs are able to provide long-term accuracy better than 10-12, corresponding to uncertainties in astronomical Doppler shifts well below 1 cm/s. With support from the NSF-AST program, we have demonstrated two astro-comb systems: An astro-comb operating in the range of 750-900 nm, and used it successfully for wavelength calibration of the TRES spectrograph at Mt. Hopkins, AZ. This red astro-comb is designed to have a line density matched to astronomical spectrographs with resolutions in the range of R=40,000-150,000. A visible-wavelength (400-700 nm) astro-comb, which is of particular importance because this wavelength region provides the largest photon flux with rich, high-quality spectral features most suitable for the PRV method. Specifically during the collaborative research supported by the grant AST-1006507, we have developed this broadband visible-wavelength astro-comb by up-shifting a Ti:Sapphire laser frequency comb using fiber-optic Cherenkov radiation. An astro-comb is then implemented by filtering the visible-wavelength frequency comb with two Fabry-Perot filtering cavities, creating >40 dB side-mode suppression. Currently we are deploying this astro-comb to the HARPS-N spectrograph at Canary Islands to search for Earth-like exoplanets.
