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

N. Christlieb, T. C. Beers, P. S. Barklem, M. Bessell, V. Hill, J. Holmberg, A. J. Korn, B. Marsteller, L. Mashonkina, Y. Z. Qian, S. Rossi, G. J. Wasserburg, F. J. Zickgraf, K. L. Kratz, B. Nordström, B. Pfeiffer, J. Rhee, S. G. Ryan

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119 Scopus citations

Abstract

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 (∼2 Å) follow-up spectroscopy has been obtained for metal-poor giant candidates selected from the Hamburg/ESO objective-prism survey (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 ∼ 20000; 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.

Original languageEnglish (US)
Pages (from-to)1027-1037
Number of pages11
JournalAstronomy and Astrophysics
Volume428
Issue number3
DOIs
StatePublished - Dec 2004

Keywords

  • Galaxy: abundances
  • Galaxy: halo
  • Stars: abundances
  • Stars: population II
  • Surveys

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