The Hamburg/ESO R-process enhanced star survey (HERES)^*,**

V. Detailed abundance analysis of the r-process enhanced star HE 2327-5642

¹ Universitäts-Sternwarte München, Scheinerstr. 1, 81679 München, Germany e-mail: lyuda@usm.lmu.de
² Institute of Astronomy, Russian Academy of Sciences, 119017 Moscow, Russia e-mail: lima@inasan.ru
³ Zentrum für Astronomie der Universität Heidelberg, Landessternwarte, Königstuhl 12, 69117 Heidelberg, Germany e-mail: N.Christlieb@lsw.uni-heidelberg.de
⁴ Department of Astronomy and Space Physics, Uppsala University, Box 515, 75120 Uppsala, Sweden
⁵ Observatoire de Paris, GEPI and URA 8111 du CNRS, 92195 Meudon Cedex, France
⁶ Department of Physics and Astronomy, Michigan State University, East Lansing, MI 48824, USA

Received: 8 December 2009
Accepted: 8 March 2010

Abstract

Aims. We present a detailed abundance analysis of a strongly r-process enhanced giant star discovered in the HERES project, HE 2327–5642, for which [Fe/H] = -2.78, [r/Fe] = +0.99.

Methods. We determined the stellar parameters and element abundances by analyzing the high-quality VLT/UVES spectra. The surface gravity was calculated from the non-local thermodynamic equilibrium (NLTE) ionization balance between and , and and .

Results. Accurate abundances for a total of 40 elements and for 23 neutron-capture elements beyond Sr and up to Th were determined in HE 2327–5642. For every chemical species, the dispersion in the single line measurements around the mean does not exceed 0.11 dex. The heavy element abundance pattern of HE 2327–5642 is in excellent agreement with those previously derived for other strongly r-process enhanced stars, such as CS 22892-052, CS 31082-001, and HE 1219-0312. Elements in the range from Ba to Hf match the scaled Solar r-process pattern very well. No firm conclusion can be drawn about the relationship between the fisrt neutron-capture peak elements, Sr to Pd, in HE 2327–5642 and the Solar r-process, due to the uncertainty in the Solar r-process. A clear distinction in Sr/Eu abundance ratios was found between the halo stars of different europium enhancement. The strongly r-process enhanced stars contain a low Sr/Eu abundance ratio at [Sr/Eu] = -0.92 ± 0.13, while the stars with 0 < [Eu/Fe] < 1 and [Eu/Fe] < 0 have 0.36 dex and 0.93 dex higher Sr/Eu values, respectively. Radioactive dating for HE 2327–5642 with the observed thorium and rare-earth element abundance pairs results in an average age of 13.3 Gyr, when based on the high-entropy wind calculations, and 5.9 Gyr, when using the Solar r-residuals. We propose that HE 2327–5642 is a radial-velocity variable based on our high-resolution spectra covering ~4.3 years.