Proceedings Article | 16 December 2002
KEYWORDS: Doppler effect, Interferometers, Stars, Telescopes, Planets, Error analysis, Absorption, Germanium, Iodine, Spectrographs
Characterization of extra-solar planetary systems requires surveying for planets around hundreds of thousands of nearby stars of all types, with different metalicities, environments (star cluster and multiple star systems), ages etc. Space missions such as SIM, NGST and TPF will identify many of these systems. However, these missions need ground-based surveys to find candidates to improve their efficiency and provide complementary work. Among these surveys, Doppler radial velocity (RV) surveys, which have detected almost all of ~ 100 known planetary systems, will continue to be the most efficient for detecting planets. Though the cross-dispersed echelle spectroscopy has demonstrated high sensitivity and good efficiency for observing thousands of stars, (limited to late F,G, K and M type), it would be tremendously challenging to search for hundreds of thousands of stars since this would require more than 2 orders of magnitude improvement in observing efficiency. New techniques with high throughput and multi-object capability for high precision RV surveys are crucial in solving this problem. Here we introduce a new technique based on a multi-object fixed-delay interferometer with a first order grating postdisperser which provides the potential for all sky radial velocity surveys for planets.
This kind of instrument is a combination of a fixed-delay interferometer with a moderate resolution post-disperser. Doppler measurements are conducted by monitoring stellar interferometric fringe phase shifts instead of absorption line centroid shifts as in the echelle. High Doppler sensitivity is achieved by optimizing the optical delay in the interferometer and reducing photon noise by measuring multiple fringes over a broadband realized by the post-disperser. Since the resulting Doppler sensitivity is independent of the dispersion power of the post-disperser, the whole instrument can be designed for multiple objects, high throughput, and high Doppler sensitivity, while the instrument can be made very compact, thermally and mechanically rigid, and low-cost compared to the echelles. Its superior stability and simple instrument response allow its easy adaptation to other wavelengths such as UV and IR. Once a multi-object instrument of this type, with possible UV, visible and near-IR instrument channels, is coupled with a wide field telescope (a few degree, such as Sloan and WIYN), it will produce hundreds of fringing spectra to allow simultaneous searching for planets around late type F, G, and K stars in the visible, early type B and A-type stars, and white dwarfs in UV and late M-dwarfs in near-IR.
The first light observations of our prototype interferometer at the Hobby-Eberly 9m and Palomar 5m telescopes in 2001 have demonstrated that this new technique can approach high Doppler precision mainly determined by photon statistics (Ge et al. 2001; van Eyken et al. 2001; Ge et al. 2002). For instance, a stellar intrinsic Doppler precision of ~ 3 m/s has been achieved with a wavelength coverage of ~ 140 Å and S/N ~ 120 per pixel. The overall short-term Doppler measurement error is ~ 9 m/s. This is mainly caused by low fringe contrast (or visibility) of the iodine absorption lines (~ 2.5% vs. ~7% in stellar lines) for wavelength calibration. Recent observing at the KPNO 2.1-m telescope demonstrated good instrument throughput and increased wavelength coverage. The total detection efficiency including the sky, telescope and fiber transmission losses, the instrument and iodine transmission losses and detector quantum efficiency is 3.4% under 1.5 arcsec seeing conditions. This efficiency is already comparable to all of the echelle spectrometers for planet detection.
