The Hamburg/ESO R-process Enhanced Star survey (HERES). I. Project description, and discovery of two stars with strong enhancements of neutron-capture elements

We report on a dedicated effort to identify and study metal-poor stars strongly enhanced in r-process elements ((r/Fe) > 1 dex; hereafter r-II stars), the Hamburg/ESO R-process Enhanced Star survey (HERES). Moderate-resolution (∼ 2A ) follow-up spectroscopy has been obtained for metal-poor giant candidates selected from the Hamburg/ESO objective-prism sur- vey (HES) as well as the HK survey to identify sharp-lined stars with (Fe/H) < −2.5 dex. For several hundred confirmed metal- poor giants brighter than B ∼ 16.5 mag (most of them from the HES), "snapshot" spectra (R ∼ 20 000; S /N ∼ 30 per pixel) are being obtained with VLT/UVES, with the main aim of finding the 2−3% r-II stars expected to be among them. These are studied in detail by means of higher resolution and higher S /N spectra. In this paper we describe a pilot study based on a set of 35 stars, including 23 from the HK survey, eight from the HES, and four comparison stars. We discovered two new r-II stars, CS 29497−004 ((Eu/Fe) = 1.64 ± 0.22) and CS 29491−069 ((Eu/Fe) = 1.08 ± 0.23). A first abundance analysis of CS 29497−004 yields that its abundances of Ba to Dy are on average enhanced by 1.5 dex with respect to iron and the Sun and match a scaled solar r-process pattern well, while Th is underabundant relative to that pattern by 0.3 dex, which we attribute to radioactive decay. That is, CS 29497−004 seems not to belong to the class of r-process enhanced stars displaying an "actinide boost", like CS 31082−001 (Hill et al. 2002), or CS 30306−132 (Honda et al. 2004b). The abundance pattern agrees well with predictions of the phenomenological model of Qian & Wasserburg.