r-process Production Sites as inferred from Eu Abundances in Dwarf
  Galaxies
  Recent observations of $r$-process material in ultra-faint dwarf galaxies
(UFDs) shed light on the sources of these elements. Strong upper limits on the
Eu mass in some UFDs combined with detections of much larger masses in a UFD,
Reticulum II, and other dwarf galaxies imply that Eu production is dominated by
rare events, and that the minimal Eu mass observed in any UFD is approximately
the amount of Eu mass produced per event. This is consistent with other
independent observations in the Galaxy. We estimate, using a model independent
likelihood analysis, the rate and Eu (Fe) mass produced per $r$-process (Fe
production) event in dwarf galaxies including classical dwarfs and UFDs. The
mass and rate of the Fe production events are consistent with the normal
core-collapse supernova~(ccSN) scenario. The Eu mass per event is $3\times
10^{-5}M_{\odot}<\tilde{m}_{\rm Eu}<2\times 10^{-4}M_{\odot}$, corresponding to
a total $r$-process mass per event of $6\times
10^{-3}M_{\odot}<\tilde{m}_{r-process}<4\times 10^{-2}M_{\odot}$. The rate of
$r$-process events is $2.5\times 10^{-4}<R_{rp/SN}<1.4\times 10^{-3}$ as
compared with the ccSNe rate. These values are consistent with the total Eu
mass observed in our own Galaxy, suggesting that the same mechanism is behind
the production of $r$-process events in both dwarf galaxies and the Milky Way,
and that it may be the dominant mechanism for production of $r$-process
elements in the Universe. The results are consistent with neutron star mergers
estimates, but cannot rule out other rare core collapse scenarios, provided
that they produce a significant amounts of $r$-process material per event.
