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

X. HE 2252−4225, one more r-process enhanced and actinide-boost halo star

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

Received: 17 April 2014
Accepted: 27 June 2014

Abstract

Context. Studies of the r-process enhanced stars are important for understanding the nature and origin of the r-process better.

Aims. We present a detailed abundance analysis of a very metal-poor giant star discovered in the HERES project, HE 2252–4225, which exhibits overabundances of the r-process elements with [r/Fe] = +0.80.

Methods. We determined the stellar atmosphere parameters, T_eff = 4710 K, log  g = 1.65, and [ Fe/H ] = −2.63, and chemical abundances by analysing the high-quality VLT/UVES spectra. The surface gravity was calculated from the non-local thermodynamic equilibrium (NLTE) ionisation balance between Fe i and Fe ii.

Results. Accurate abundances for a total of 38 elements, including 22 neutron-capture elements beyond Sr and up to Th, were determined in HE 2252−4225. For every chemical species, the dispersion in the single line measurements around the mean does not exceed 0.12 dex. This object is deficient in carbon, as expected for a giant star with T_eff < 4800 K. The stellar Na–Zn abundances are well fitted by the yields of a single supernova of 14.4 M_⊙. For the neutron-capture elements in the Sr–Ru, Ba–Yb, and Os–Ir regions, the abundance pattern of HE 2252−4225 is in excellent agreement with the average abundance pattern of the strongly r-process enhanced stars CS 22892-052, CS 31082-001, HE 1219-0312, and HE 1523-091. This suggests a common origin of the first, second, and third r-process peak elements in HE 2252−4225 in the classical r-process. We tested the solar r-process pattern based on the most recent s-process calculations of Bisterzo, Travaglio, Gallino, Wiescher, and Käppeler and found that elements in the range from Ba to Ir match it very well. No firm conclusion can be drawn about the relationship between the first neutron-capture peak elements, Sr to Ru, in HE 2252−4225 and the solar r-process, owing to the uncertainty in the solar r-process. The investigated star has an anomalously high Th/Eu abundance ratio, so that radioactive dating results in a stellar age of τ = 1.5 ± 1.5 Gyr that is not expected for a very metal-poor halo star.