The Hamburg/ESO R-process Enhanced Star survey (HERES): Project Overview, and New r-II Stars

We report on a dedicated effort to identify and study metal-poor stars that are strongly enhanced in r-process elements ([r/Fe]>+1.0dex; hereafter r-II stars), the Hamburg/ESO R-process Enhanced Star survey (HERES). In a sample of 253 confirmed metal-poor stars for which "snapshot" spectra (R∼ 20,000; S/N ∼ 50/1 per pixel) were obtained with VLT/UVES, and abundances were determined in an automated fashion using the methods of Barklem et al. (2005), we identified eight new r-II stars. They are now being studied in detail by means of higher resolution and S/N spectroscopy. The new r-II stars have metallicities in the range -3.2. Future searches for r-II stars should therefore focus on stars in this [Fe/H] range. Moderately r-process enhanced stars (i.e., +0.3 dex; r-I stars) were found at metallicities as high as [Fe/H] = -1.5. The [Fe/H] ranges in which r-I and r-II stars can be found may provide an important constraint for the identification of the site(s) of the r-process(es).