For the origin of heavy r-process elements, different sources have been proposed, e.g., core-collapse supernovae or neutron star mergers. Old metal-poor stars carry the signature of the astrophysical source(s). Among the elements dominantly made by the r-process, europium (Eu) is relatively easy to observe. In this work we simulate the evolution of europium in our galaxy with the inhomogeneous chemical evolution model 'ICE', and compare our results with spectroscopic observations. We test the most important parameters affecting the chemical evolution of Eu: (a) for neutron star mergers the coalescence time scale of the merger (t coal ) and the probability to experience a neutron star merger event after two supernova explosions occurred and formed a double neutron star system (P NSM ) and (b) for the sub-class of magneto-rotationally driven supernovae ("Jet-SNe"), their occurrence rate compared to standard supernovae (P Jet−SN ). We find that the observed [Eu/Fe] pattern in the galaxy can be reproduced by a combination of neutron star mergers and magneto-rotationally driven supernovae as r-process sources. While neutron star mergers alone seem to set in at too high metallicities, Jet-SNe provide a cure for this deficiency at low metallicities. Furthermore, we confirm that local inhomogeneities can explain the observed large spread in the europium abundances at low metallicities. We also predict the evolution of [O/Fe] to test whether the spread in α-elements for inhomogeneous models agrees with observations and whether this provides constraints on supernova explosion models and their nucleosynthesis.

Neutron star mergers have been predicted since the 1970's, supported by the discovery of the binary pulsar and the observation of its orbital energy loss, consistent with General Relativity. They are considered as nucleosynthesis sites of the rapid neutron-capture process (r-process), being responsible for making about half of all heavy elements beyond Fe and being the only source of elements beyond Pb and Bi. Detailed nucleosynthesis calculations based on the decompression of neutron-star matter are consistent with solar r-process abundances of heavy nuclei. More recently neutron star mergers have also been identified with short duration Gamma-Ray Bursts via their IR afterglow, only explainable by the opacities of heavy (rather than only Fe-group) nuclei. Two other observations support rare events like neutron star mergers as a dominant scenario for the production of the heaviest r-process nuclei: (a)The discrepancy between the latest admixtures of two long-lived radioactivities ( 60 Fe and 244 Pu) found on earth seems to exclude the origin of the latter from core collapse supernovae. (b)The ratio of [Eu/Fe], with Eu being dominated by r-process contributions, shows a strong scatter in low metallicity stars up to [Fe/H]<-2, arguing for a strongly reduced occurrence rate in comparison to core-collapse supernovae. The high neutron densities in ejected matter permit a violent r-process, encountering fission cycling of the heaviest nuclei in regions far from (nuclear) stability. Uncertainties in nuclear properties, like nuclear masses, betadecay half-lives, fission barriers and fission fragment distributions affect the detailed abundance distributions. The modeling of the astrophysical events depends also on the hydrodynamic treatment, i.e. SPH vs. grid calculations, Newtonian vs. GR approaches, the occurrence of a neutrino wind after the merger and before the emergence of a black hole, and finally the properties of black hole accretion disks. We will discuss the effect of both (nuclear and modelling) uncertainties and conclude that binary compact mergers are probably a or the dominant site of the production of r-process nuclei in our Galaxy. A small caveat exists with respect to explaining the behavior of [Eu/Fe] at lowest metallicities and the question whether neutron star mergers can already contribute at such early times in galactic evolution.2 Thielemann et al.

The role of compact binary mergers as the main production site of r-process elements is investigated by combining stellar abundances of Eu observed in the Milky Way, galactic chemical evolution (GCE) simulations, binary population synthesis models, and Advanced LIGO gravitational wave measurements. We compiled and reviewed seven recent GCE studies to extract the frequency of neutron star -neutron star (NS-NS) mergers that is needed in order to reproduce the observed [Eu/Fe] vs [Fe/H] relationship. We used our simple chemical evolution code to explore the impact of different analytical delay-time distribution (DTD) functions for NS-NS mergers. We then combined our metallicity-dependent population synthesis models with our chemical evolution code to bring their predictions, for both NS-NS mergers and black hole -neutron star mergers, into a GCE context. Finally, we convolved our results with the cosmic star formation history to provide a direct comparison with current and upcoming Advanced LIGO measurements. When assuming that NS-NS mergers are the exclusive r-process sites, and that the ejected r-process mass per merger event is 0.01 M , the number of NS-NS mergers needed in GCE studies is about 10 times larger than what is predicted by standard population synthesis models. These two distinct fields can only be consistent with each other when assuming optimistic rates, massive NS-NS merger ejecta, and low Fe yields for massive stars. For now, population synthesis models and GCE simulations are in agreement with the current upper limit (O1) established by Advanced LIGO during their first run of observations. Upcoming measurements will provide an important constraint on the actual local NS-NS merger rate, will provide valuable insights on the plausibility of the GCE requirement, and will help to define whether or not compact binary mergers can be the dominant source of r-process elements in the Universe.

The composition of the early Solar System can be inferred from meteorites. Many elements heavier than iron were formed by the rapid neutron capture process (r-process), but the astrophysical sources where this occurred remain poorly understood. We demonstrate that the near-identical half-lives (≃15.6 million years) of the radioactive r-process nuclei iodine-129 and curium-247 preserve their ratio, irrespective of the time between production and incorporation into the Solar System. We constrain the last r-process source by comparing the measured meteoritic ratio 129I/247Cm = 438 ± 184 with nucleosynthesis calculations based on neutron star merger and magneto-rotational supernova simulations. Moderately neutron-rich conditions, often found in merger disk ejecta simulations, are most consistent with the meteoritic value. Uncertain nuclear physics data limit our confidence in this conclusion.