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      Neutron Star Mergers Might not be the Only Source of r-Process Elements in the Milky Way

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          Abstract

          Probing the origin of r-process elements in the universe represents a multi-disciplinary challenge. We review the observational evidence that probe the properties of r-process sites, and address them using galactic chemical evolution simulations, binary population synthesis models, and nucleosynthesis calculations. Our motivation is to define which astrophysical sites have significantly contributed to the total mass of r-process elements present in our Galaxy. We found discrepancies with the neutron star (NS-NS) merger scenario. Assuming they are the only site, the decreasing trend of [Eu/Fe] at [Fe/H] \(>-1\) in the disk of the Milky Way cannot be reproduced while accounting for the delay-time distribution (DTD) of coalescence times (\(\propto~t^{-1}\)) derived from short gamma-ray bursts and population synthesis models. Steeper DTD functions (\(\propto~t^{-1.5}\)) or power laws combined with a strong burst of mergers before the onset of Type~Ia supernovae can reproduce the [Eu/Fe] trend, but this scenario is inconsistent with the similar fraction of short gamma-ray bursts and Type Ia supernovae occurring in early-type galaxies, and reduces the probability of detecting GW170817 in an early-type galaxy. One solution is to assume an extra production site of Eu that would be active in the early universe, but would fade away with increasing metallicity. If this is correct, NS-NS mergers would contribute to about two thirds of the total amount of Eu currently present in the Milky Way. A rare class of supernovae could be this additional source of the r-process, but hydrodynamic simulations still need to ensure the conditions for a robust r-process pattern.

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          Short gamma-ray bursts: A review

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            Multi-messenger picture of compact binary mergers

            (2015)
            In the last decade, enormous progress has been achieved in the understanding of the various facets of coalescing double neutron star and neutron black hole binary systems. One hopes that the mergers of such compact binaries can be routinely detected with the advanced versions of the ground-based gravitational wave detector facilities, maybe as early as in 2016. From the theoretical side, there has also been mounting evidence that compact binary mergers could be major sources of heavy elements and these ideas have gained recent observational support from the detection of an event that has been interpreted as a "macronova", an electromagnetic transient powered by freshly produced, radioactively decaying heavy elements. In addition, compact binaries are the most plausible triggers of short gamma-ray bursts (sGRBs) and the last decade has witnessed the first detection of a sGRB afterglow and subsequent observations have delivered a wealth of information on the environments in which such bursts occur. To date, compact binary mergers can naturally explain most --though not all-- of the observed sGRB properties. This article reviews major recent developments in various areas related to compact binary mergers.
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              Author and article information

              Journal
              10 September 2018
              Article
              1809.03525
              a44e24d2-85dc-46ab-bb92-38448f88079b

              http://arxiv.org/licenses/nonexclusive-distrib/1.0/

              History
              Custom metadata
              25 pages, 7 figures, 1 table, submitted to ApJ
              astro-ph.HE

              High energy astrophysical phenomena
              High energy astrophysical phenomena

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