This award is for a project to build an ultra-stable frequency calibrator for high resolution optical spectrography using a comb laser. The purpose of developing such a system is to be able to extend the indirect detection of extra-solar planets to lower mass objects, i.e. those comparable to the Solar System's terrestrial planets rather than the gas-giants. The indirect spectroscopic techniques have proven to be successful at finding extra-solar planets and the technology to be developed in this award will permit the spectral line positional changes of the parent star, which infer the existence and mass of a planetary companion, to be measured to significantly higher accuracy than before, thereby extending the technique to lower mass planets.
The award will demonstrate that comb lasers can be used for this application and the technology and techniques will be distributed to the general community. In addition, undergraduate and graduate students will undergo training in instrumentation. This is a very topical field and the existence of extra-solar planets, especially those resembling the Earth, will capture the public imagination.
(AST-0804311) Astronomers are keen to find Earth-like planets around solar-type stars in the "habitable zone" where the planetary surface conditions are suitable for water to exist in liquid phase—a prerequisite for exobiotic life as we know it. As a planet orbits around its host star, its gravitational pull causes a reflex motion of the host star. Due to the Doppler effect, such a tiny change in the star’s motion causes a spectrum-shift of the star light, which can be detected by precise radial velocity (PRV) measurements. Optimal PRV measurements are achieved by calibrating the wavelength scale of high dispersion spectrographs on the largest telescopes, with the greatest achievable precision and stability. At present, the best achievable PRV measurements are >50 cm/s, substantially limited by the wavelength calibrator, i.e., ThAr (thorium argon) hollow cathode lamp. These calibrators are limited by their unevenness in line spacing and intensity as well as the slow variation of their line wavelengths with time. The discovery of exoearths around solar-type stars requires PRV measurements at the 5-10 cm/s level, which depends largely on developing bright, stable wavelength calibrators that produce spectra with regularly-spaced spectral lines and feature densities well matched to the resolution of astronomical spectrographs. The power of such improved wavelength calibration is not limited to exoplanet research — it will open up new discovery space over a broad cross-section of astrophysical problems, including perhaps the nature and dynamics of dark energy, and the constancy of fundamental constants over cosmological time scales. We are developing such a wavelength calibrator ("astro-comb") which consists of tens of thousands of narrow (kHz-wide) emission lines, with equal frequency spacing. Referenced to the Global Position System (GPS), astro-combs can achieve long-term accuracy better than 10-12, corresponding to uncertainties in astronomical Doppler shifts well below 1 cm/s. With support from the NSF-ATI program, we have demonstrate such an astro-comb operating in the red part of the spectrum (750-900 nm) for the 1.5 m telescope at Mt. Hopkins, and used it successfully for wavelength calibration of the TRES spectrograph with radial velocity precision <50 m/s. This red astro-comb employs a Ti:sapphire comb laser, which is filtered with a Fabry-Perot cavity to a line density matched to astronomical spectrographs with resolutions in the range of R=40,000-150,000. We also demonstrated an in-situ technique to determine the systematic shifts of astro-comb lines due to finite Fabry-Perot cavity dispersion. The technique can be employed at a telescope-based spectrograph to enable wavelength calibration accuracy better than 10 cm/s.
