Evidence from Disrupted Halo Dwarfs that r-process Enrichment via Neutron Star Mergers is Delayed by greater than or similar to 500 Myr

Abstract

The astrophysical origins of r-process elements remain elusive. Neutron star mergers (NSMs) and special classes of core-collapse supernovae (rCCSNe) are leading candidates. Due to these channels' distinct characteristic timescales (rCCSNe: prompt, NSMs: delayed), measuring r-process enrichment in galaxies of similar mass but differing star formation durations might prove informative. Two recently discovered disrupted dwarfs in the Milky Way's stellar halo, Kraken and Gaia-Sausage Enceladus (GSE), afford precisely this opportunity: Both have M-* approximate to 10(8) M (circle dot) but differing star formation durations of approximate to 2 Gyr and approximate to 3.6 Gyr. Here we present R approximate to 50,000 Magellan/MIKE spectroscopy for 31 stars from these systems, detecting the r-process element Eu in all stars. Stars from both systems have similar [Mg/H] approximate to -1, but Kraken has a median [Eu/Mg] approximate to -0.1 while GSE has an elevated [Eu/Mg] approximate to 0.2. With simple models, we argue NSM enrichment must be delayed by 500-1000 Myr to produce this difference. rCCSNe must also contribute, especially at early epochs, otherwise stars formed during the delay period would be Eu free. In this picture, rCCSNe account for approximate to 50% of the Eu in Kraken, approximate to 25% in GSE, and approximate to 15% in dwarfs with extended star formation durations like Sagittarius. The inferred delay time for NSM enrichment is 10x-100x longer than merger delay times from stellar population synthesis-this is not necessarily surprising because the enrichment delay includes time taken for NSM ejecta to be incorporated into subsequent generations of stars. For example, this may be due to natal kicks that result in r-enriched material deposited far from star-forming gas, which then takes approximate to 10(8)-10(9) yr to cool in these galaxies.