On Neutron Star Mergers as the Source of r-process-enhanced Metal-poor Stars in the Milky Way

Abstract

We model the history of Galactic r -process enrichment using high-redshift, high-resolution zoom cosmological simulations of a Milky Way–type halo. We assume that all r -process sources are neutron star mergers (NSMs) with a power-law delay time distribution. We model the time to mix pollutants at subgrid scales, which allows us to better compute the properties of metal-poor (MP) and carbon-enhanced metal-poor (CEMP) stars, along with statistics of their r -process-enhanced subclasses. Our simulations underpredict the cumulative ratios of r -process-enhanced MP and CEMP stars (MP- r , CEMP- r ) over MP and CEMP stars by about one order of magnitude, even when the minimum coalescence time of the double neutron stars (DNSs), t min , is set to 1 Myr. No r -process-enhanced stars form if t min  = 100 Myr. Our results show that even when we adopt the r -process yield estimates observed in GW170817, NSMs by themselves can only explain the observed frequency of r -process-enhanced stars if the birth rate of DNSs per unit mass of stars is boosted to .