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 × 10 −5 M ⊙ <m Eu < 2 × 10 −4 M ⊙ , corresponding to a total r-process mass per event of 6 × 10 −3 M ⊙ < m r−process < 4 × 10 −2 M ⊙ . The rate of r-process events is 2.5 × 10 −4 < R rp/SN < 1.4 ×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.

Formation of Double Neutron Star Systems

“…These velocities can be combined with the merger time for the DNS system to estimate the offset distance of the future DNS merger (short GRB) from its birth place in the Galactic disk (e.g. Bloom et al 1999;Perna & Belczynski 2002;Voss & Tauris 2003;Fong et al 2010;Kelley et al 2010;Beniamini et al 2016b). Assuming, as a first-order approximation, constant motion throughout a galactic potential, an upper limit for this distance can be simply estimated as: d pc ≃ v km/s · τ gwr,Myr .…”

The Intermediate r-process in Core-collapse Supernovae Driven by the Magneto-rotational Instability

“…Tsujimoto & Nishimura (2015) showed that MR-SNe can explain the early growth of Eu in dwarf spheroidal galaxies for [ ] < -Fe H 2 by assuming an event rate of about 0.5% of CC-SNe, while NSMs have problems with doing so. Observation of ultra-faint galaxies (Ji et al 2016;Roederer et al 2016) requires rare r-process events with large mass ejection, for which MR-SNe can be a source as well as NSMs (Beniamini et al 2016).…”

Neutron-Star Implosions as Heavy-Element Sources

“…The rate F UFD 0 implies that the probability of a NS disruption in a single UFD is about 0.1, which explains the uneven distribution. The amount of r-process material supplied by a single event, ∼0.1 M ⊙ , is more than sufficient to explain the observations [21,22,73]. Only a small fraction ∼ðv UFD =v ∞ Þ ∼ 10 −3 of the produced r-process material is likely to remain in the shallow gravitational potential well of a UFD because it is produced with a velocity v ∞ ∼ 0.1v esc .…”

“…Observations imply that 1 in 10 UFDs have been a host to r-process nucleosynthesis events, which must, therefore, be rare [21,22,73]. The rate F UFD 0 implies that the probability of a NS disruption in a single UFD is about 0.1, which explains the uneven distribution.…”

“…This provides a favorable setting for r-process nucleosynthesis, occurring on the Galactic time scales, which can evade several problems that have challenged the leading proposed r-process production sites, such as neutrino-heated winds from core-collapse supernovae or binary compact object mergers (COMs) [19,20]. The unusual distribution of r-process abundances within the ultrafaint dwarf spheroidal galaxies (UFDs) [21,22] is naturally explained by the rates of PBH capture in these systems. The rates are also consistent with the paucity of pulsars in the GC [23].…”