Early Cosmic Evolution of Europium from Core Collapse Supernovae and/or Neutron Star Mergers

E. Vangioni, S. Goriely, F. Daigne, P. Francois, K. Belczynski

Published 2015-01-06Version 1

The rapid neutron-capture process is known to be of fundamental importance for explaining the origin of approximately half of the A > 60 stable nuclei observed in nature. Despite important efforts, the astrophysical site of the r process remains unidentified. The two most promising astrophysical sites of the r process, namely Core Collapse SuperNovae (CCSN) and Neutron Star Mergers (NSM) are considered in the context of the early cosmic chemical evolution through the origin and evolution of a typical r process element, Eu. The Eu abundance in very low metallicity stars is used to shed light on the possible CCSN and NSM contributions in the early Universe. Predictions are made here using a hierarchical model for structure formation for which a special attention is paid to a proper description of the stellar formation rate. Eu yields from NSM are taken from recent nucleosynthesis calculations. Observations of Eu in ultra metal poor stars are considered to constrain the model. We find that the bulk of Eu observations at [Fe/H] > - 2.5 is rather well fitted by both CCSN and NSM scenarios. However, at lower metallicity, the Eu cosmic evolution tends to favor NSM as the main astrophysical site for the r-process since CCSN overproduce Eu at high redshift (corresponding to very low metallicities). Our calculations allow to constrain the coalescence timescale in the NSM scenario: typical timescales of 0.1 - 0.2 Gyr are found to be compatible with observations. The observed evolution of Eu abundances puts also a constraint on the merger rate, which allows an independent prediction of the expected merger rate in the horizon of the gravitational wave detectors advanced Virgo/ad LIGO, as well as a prediction for the expected rate of electromagnetic counterparts to mergers ("kilonovae") in large NIR surveys.