Evidence from Disrupted Halo Dwarfs that $r$-process Enrichment via
  Neutron Star Mergers is Delayed by $\gtrsim500$ Myrs
  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 \textit{Gaia}-Sausage Enceladus (GSE), afford
precisely this opportunity: both have $M_{\star}\approx10^{8}M_{\rm{\odot}}$,
but differing star-formation durations of ${\approx}2$ Gyrs and ${\approx}3.6$
Gyrs. Here we present $R\approx50,000$ 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]$\approx-1$, but Kraken has a median
[Eu/Mg]$\approx-0.1$ while GSE has an elevated [Eu/Mg]$\approx0.2$. With simple
models we argue NSM enrichment must be delayed by $500-1000$ Myrs 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 $\approx50\%$ of the Eu in Kraken, $\approx25\%$ in
GSE, and $\approx15\%$ in dwarfs with extended star-formation durations like
Sagittarius. The inferred delay time for NSM enrichment is $10-100\times$
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 $\approx10^{8}-10^{9}$ years to
cool in these galaxies.
