The Hamburg/ESO R-process Enhanced Star survey (HERES)^*

I. Project description, and discovery of two stars with strong enhancements of neutron-capture elements

¹ Hamburger Sternwarte, Universität Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany e-mail: nchristlieb@hs.uni-hamburg.de
² Department of Astronomy and Space Physics, Uppsala University, Box 515, 75120 Uppsala, Sweden
³ Department of Physics and Astronomy, Michigan State University, East Lansing, MI 48824, USA
⁴ Research School of Astronomy & Astrophysics, Australian National University, Cotter Road, Weston, ACT 2611, Australia
⁵ Observatoire de Paris, GEPI and URA 8111 du CNRS, 92195 Meudon Cedex, France
⁶ Astronomical Observatory, NBIfAFG, Juliane Meries Vej 30, 2100 Copenhagen, Denmark
⁷ Nordic Optical Telescope Scientific Association, Apartado 474, 38 700 Santa Cruz de La Palma, Spain
⁸ Universitäts-Sternwarte München, Scheinerstrasse 1, 81679 München, Germany
⁹ Institute of Astronomy, Russian Academy of Sciences, Pyatnitskaya 48, 109017 Moscow, Russia
¹⁰ School of Physics and Astronomy, University of Minnesota, Minneapolis, MN 55455, USA
¹¹ Departamento de Astronomia Instituto de Astronomia, Geofísica e Ciências Atmosféricas, Universidade de São Paulo, 05508-900 São Paulo SP, Brazil
¹² Division of Geophysics and Planetary Sciences, California Institute of Technology, Pasadena, CA91125, USA
¹³ Institut für Kernchemie, Universität Mainz, 55128 Mainz, Germany
¹⁴ VISTARS HGF – Virtual Institute for Nuclear Structure and Nuclear Astrophysics, Mainz, Germany
¹⁵ Lund University Box 43, 22100 Lund, Sweden
¹⁶ Center for Space Astrophysics, Yonsei University, Seoul 120-749, Korea
¹⁷ Space Astrophysics Laboratory, California Institute of Technology, MC 405-47, Pasadena, CA 91125, USA
¹⁸ Department of Physics and Astronomy, Open University, Walton Hall, Milton Keynes, MK7 6AA, UK

Received: 25 June 2004
Accepted: 11 August 2004

Abstract

We report on a dedicated effort to identify and study metal-poor stars strongly enhanced in r-process elements ( 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  dex. For several hundred confirmed metal-poor giants brighter than  mag (most of them from the HES), “snapshot” spectra (;  per pixel) are being obtained with VLT/UVES, with the main aim of finding the % r-II stars expected to be among them. These are studied in detail by means of higher resolution and higher  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 () and CS 29491-069 (). 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.