Neutron Star Mergers Are the Dominant Source of the r-process in the Early Evolution of Dwarf Galaxies

TL;DR

This study uses chemical abundance measurements in dwarf galaxies to demonstrate that neutron star mergers are the primary source of r-process elements during early galaxy evolution.

Contribution

It provides the largest dataset of barium abundances in dwarf galaxies and offers evidence that neutron star mergers dominate early r-process enrichment.

Findings

Large sample of ~250 stars analyzed for barium abundances.

Observed [Ba/Fe] trends suggest delayed enrichment consistent with neutron star mergers.

Neutron star mergers are identified as the main early r-process source in dwarf galaxies.

Abstract

There are many candidate sites of the r-process: core-collapse supernovae (including rare magnetorotational core-collapse supernovae), neutron star mergers, and neutron star/black hole mergers. The chemical enrichment of galaxies---specifically dwarf galaxies---helps distinguish between these sources based on the continual build-up of r-process elements. This technique can distinguish between the r-process candidate sites by the clearest observational difference---how quickly these events occur after the stars are created. The existence of several nearby dwarf galaxies allows us to measure robust chemical abundances for galaxies with different star formation histories. Dwarf galaxies are especially useful because simple chemical evolution models can be used to determine the sources of r-process material. We have measured the r-process element barium with Keck/DEIMOS medium-resolution spectroscopy. We present the largest sample of barium abundances (almost 250 stars) in dwarf galaxies ever assembled. We measure [Ba/Fe] as a function of [Fe/H] in this sample and compare with existing [alpha/Fe] measurements. We have found that a large contribution of barium needs to occur at more delayed timescales than core-collapse supernovae in order to explain our observed abundances, namely the significantly more positive trend of the r-process component of [Ba/Fe] vs. [Fe/H] seen for [Fe/H] <~ -1.6 when compared to the [Mg/Fe] vs. [Fe/H] trend. We conclude that neutron star mergers are the most likely source of r-process enrichment in dwarf galaxies at early times.