First stars^*

VIII. Enrichment of the neutron-capture elements in the early Galaxy

¹ European Southern Observatory (ESO), Alonso de Cordova 3107, Vitacura, Casilla 19001, Santiago 19, Chile
² GEPI, Observatoire de Paris-Meudon, CNRS, Univ. de Paris Diderot, Place Jules Janssen, 92190 Meudon, France e-mail: Patrick.Francois@obspm.fr
³ Las Cumbres Observatory, Santa Barbara, California, USA
⁴ GRAAL, Université de Montpellier II, 34095 Montpellier Cedex 05, France
⁵ Dept. of Physics & Astronomy, CSCE: Center for the Study of Cosmic Evolution, and JINA: Joint Institute for Nuclear Astrophysics, Michigan State University, E. Lansing, MI 48824, USA
⁶ The Niels Bohr Institute, Astronomy Group, Juliane Maries Vej 30, 2100 Copenhagen, Denmark
⁷ Nordic Optical Telescope, Apartado 474, 38700 Santa Cruz de La Palma, Spain
⁸ IAG, Universidade de São Paulo, Departamento de Astronomia, CP 3386, 01060-970 São Paulo, Brazil
⁹ Istituto Nazionale di Astrofisica - Osservatorio Astronomico di Trieste, via G.B. Tiepolo 11, 34131 Trieste, Italy
¹⁰ European Southern Observatory (ESO), Karl-Schwarschild-Str. 2, 85749 Garching b. München, Germany

Received: 24 April 2007
Accepted: 18 September 2007

Abstract

Context.Extremely metal-poor (EMP) stars in the halo of the Galaxy are sensitive probes of the production of the first heavy elements and the efficiency of mixing in the early interstellar medium. The heaviest measurable elements in such stars are our main guides to understanding the nature and astrophysical site(s) of early neutron-capture nucleosynthesis.

Aims.Our aim is to measure accurate, homogeneous neutron-capture element abundances for the sample of 32 EMP giant stars studied earlier in this series, including 22 stars with .

Methods.Based on high-resolution, high S/N spectra from the ESO VLT/UVES, 1D, LTE model atmospheres, and synthetic spectrum fits, we determine abundances or upper limits for the 16 elements Sr, Y, Zr, Ba, La, Ce, Pr, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, and Yb in all stars.

Results.As found earlier, [Sr/Fe], [Y/Fe], [Zr/Fe] and [Ba/Fe] are below Solar in the EMP stars, with very large scatter. However, we find a tight anti-correlation of [Sr/Ba], [Y/Ba], and [Zr/Ba] with [Ba/H] for , also when subtracting the contribution of the main r-process as measured by [Ba/H]. Spectra of even higher S/N ratio are needed to confirm and extend these results below . The huge, well-characterised scatter of the [n-capture/Fe] ratios in our EMP stars is in stark contrast to the negligible dispersion in the [ α/Fe] and [Fe-peak/Fe] ratios for the same stars found in Paper V.

Conclusions.These results demonstrate that a second (“weak” or LEPP) r-process dominates the production of the lighter neutron-capture elements for . The combination of very consistent [ α/Fe] and erratic [n-capture/Fe] ratios indicates that inhomogeneous models for the early evolution of the halo are needed. Our accurate data provide strong constraints on future models of the production and mixing of the heavy elements in the early Galaxy.