The N2K Consortium. V. Identifying Very Metal-rich Stars with Low-Resolution Spectra: Finding Planet-Search Targets

We present empirical calibrations that provide estimates of stellar metallicity, effective temperature, and surface gravity as a function of Lick IDS indices. These calibrations have been derived from a training set of 261 stars for which (1) high-precision measurements of [Fe/H], T_eff, and log g have been made using spectral-synthesis analysis of HIRES spectra, and (2) Lick indices have also been measured. Estimation of atmospheric parameters with low-resolution spectroscopy rather than photometry has the advantage of producing a highly accurate metallicity calibration, and requires only one observation per star. Our calibrations have identified a number of bright (V < 9) metal-rich stars that are now being screened for hot-Jupiter-type planets. Using the Yonsei-Yale stellar models, we show that the calibrations provide distance estimates accurate to ~20% for nearby stars. We have also investigated the possibility of constructing a "planeticity" calibration to predict the presence of planets based on stellar abundance ratios but find no evidence that a convincing relation of this type can be established. High metallicity remains the best single indicator that a given star is likely to harbor extrasolar planets.