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A Search for Metal-poor Stars Pre-enriched by Pair-instability Supernovae I. A Pilot Study for Target Selection from Sloan Digital Sky Survey (SDSS)

We report on a pilot study on identifying metal-poor stars pre-enriched by Pair-Instability Supernovae (PISNe). Very massive, first generation (Population III) stars (140M\odot \leq M \leq 260M\odot) end their lives as PISNe, which have been predicted by theories, but no relics of PISNe have been observed yet. Among the distinct characteristics of the yields of PISN, as predicted by theoretical calculations, are a strong odd-even effect, and a strong overabundance of Ca with respect to iron and the Solar ratio. We use the latter characteristic to identify metal-poor stars in the Galactic halo that have been pre-enriched by PISN, by comparing metallicites derived from strong, co-added Fe lines detected in low-resolution (i.e., R \sim 2000) spectra of the Sloan Digital Sky Survey (SDSS), with metallicities determined by the SDSS Stellar Parameters Pipeline (SSPP). The latter are based on the strength of the Ca II K line and assumptions on the Ca/Fe abundance ratio. Stars are selected as candidates if their metallicity derived from Fe lines is significantly lower than the SSPP metallicities. In a sample of 12,300 stars for which SDSS spectroscopy is available, we have identified 18 candidate stars. Higher resolution and signal-to-noise ratio spectra of these candidates are being obtained with the Very Large Telescope of the European Southern Observatory and the XSHOOTER spectrograph, to determine their abundance patterns, and to verify our selection method. We plan to apply our method to the data base of several million stellar spectra to be acquired with the LAMOST telescope in the next five years.