Contribution of Neutron Star Mergers to the R-process Chemical Evolution in the Hierarchical Galaxy Formation

Abstract

The main astronomical source of r-process elements has not yet been identified. One plausible site is neutron star mergers (NSMs), but from perspective of the Galactic chemical evolution, it has been pointed out that NSMs cannot reproduce the observed r-process abundance distribution of metal-poor stars at [Fe/H] $<-3$. Recently, Tsujimoto & Shigeyama (2014) pointed out that NSM ejecta can spread into much larger volume than ejecta from a supernova. We re-examine the enrichment of r-process elements by NSMs considering this difference in propagation using the chemical evolution model under the hierarchical galaxy formation. The observed r-process enhanced stars around [Fe/H] $\sim -3$ are reproduced if the star formation efficiency is lower for low-mass galaxies under a realistic delay time distribution for NSMs. We show that a significant fraction of NSM ejecta escape from its host proto-galaxy to pollute intergalactic matter and other proto-galaxies. The propagation of r-process elements over proto-galaxies changes the abundance distribution at [Fe/H] $<-3$ and obtains distribution compatible with observations of the Milky Way halo stars. In particular, the pre-enrichment of intergalactic medium explains the observed scarcity of EMP stars without Ba and abundance distribution of r-process elements at [Fe/H] $\lesssim -3.5$.

11 pages, 7 figures, ApJ accepted

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• Early chemo-dynamical evolution of dwarf galaxies deduced from enrichment of r-process elements
• Neutron star mergers as the astrophysical site of the r-process in the Milky Way and its satellite galaxies
• Common envelope jets supernova r-process yields can reproduce $[Eu/Fe]$ abundance evolution in the Galaxy
• Are Faint Supernovae Responsible for Carbon-Enhanced Metal-Poor Stars?
• Chemical evolution of 244Pu in the solar vicinity and its implication for the properties of r-process production
• Stellar Signatures of Inhomogeneous Big Bang Nucleosynthesis
• Formation of globular clusters with internal abundance spreads in r-process elements: strong evidence for prolonged star formation
• On the Impact of Neutron Star Binaries Natal-Kick Distribution on the Galactic r-process Enrichment
• R-process Element Cosmic Rays from Neutron Star Mergers
• Modelling the chemical evolution of the Milky Way
• A statistical method for the identification of stars enriched in neutron-capture elements from medium-resolution spectra