r-process Production Sites as inferred from Eu Abundances in Dwarf Galaxies

Abstract

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.