Stochastic Chemical Evolution of Galactic Subhalos and the Origin of r-Process Elements

TL;DR

This paper presents a stochastic chemical evolution model of galactic subhalos showing that neutron star mergers with a 100 Myr coalescence time can explain the observed r-process element distribution in the Galactic halo, including ultra-faint dwarf galaxies.

Contribution

It introduces a stochastic subhalo model incorporating neutron star mergers with long coalescence times to explain r-process element dispersions in the Galactic halo.

Findings

Less massive subhalos produce highly r-process-enhanced stars.

The model reproduces the star-to-star scatter of [r/Fe] in the halo.

It suggests neutron star mergers are the main sources of r-process elements.

Abstract

Mergers of compact binaries (of a neutron star and another neutron star or a black hole, NSMs) are suggested to be the promising astrophysical site of the r-process. While the average coalescence timescale of NSMs appears to be > 100 Myr, most of previous chemical evolution models indicate that the observed early appearance and large dispersion of [r/Fe] in Galactic halo stars at [Fe/H] < -2.5 favors shorter coalescence times of 1-10 Myr. We argue that this is not the case for the models assuming the formation of the Galactic halo from clustering of subhalos with different star formation histories as suggested by Ishimaru et al. (2015). We present a stochastic chemical evolution model of the subhalos, in which the site of the r-process is assumed to be mainly NSMs with a coalescence timescale of 100 Myr. In view of the scarcity of NSMs, their occurrence in each subhalo is computed with a Monte Carlo method. Our results show that the less massive subhalos evolve at lower metallicities and generate highly r-process-enhanced stars. An assembly of these subhalos leaves behind the large star-to-star scatters of [r/Fe] in the Galactic halo as observed. However, the observed scatters of [Sr/Ba] at low metallicities indicate the presence of an additional site that partially contributes to the enrichment of light neutron-capture elements such as Sr. The high enhancements of [r/Fe] at low metallicities found in our low-mass subhalo models also qualitatively reproduce the abundance signatures of the stars in the recently discovered ultra-faint dwarf galaxy Reticulum II. Therefore, our results suggest NSMs as the dominant sources of r-process elements in the Galactic halo.