Colloquium: Formation of Binary Neutron Stars and Implications for Heavy Element Production

Observations of double neutron star systems in our Galaxy suggest that the formation of the second born NS in most systems involves a very weak explosion, rather than a more "traditional" supernova. This allows us to estimate the typical velocities and time between formation to merger for double neutron star systems. More than half of the systems are slow enough and merge rapidly enough to be able to account for the recent observations of r-process elements in ultra-faint dwarf galaxies. Furthermore, assuming that the explosion energy is less than 10⁵² erg and that the galaxies haven't lost most of their gas before the r-process event, we can expect most of the r-process material to be retained even in faint dwarfs. At the same time, observations of the Eu abundances in dwarf galaxies lead to model-independent estimates of the rate and mass per event of r-process formation. These suggest rates between 2x10^-4 and 10^-2 per core-collapse supernova and Eu masses between 10^-5M_sun and 2x10^-4M_sun. Both values rule out regular supernovae as the sources of r-process events and are consistent with double neutron star mergers. Finally, we show that faint dwarf galaxies are consistent with being the "building blocks" of the Galactic halo and provide a natural explanation for the mean value and fluctuations in the abundances of Galactic halo metal-poor stars. . Both values rule out regular supernovae as the sources of r-process events and are consistent with double neutron star mergers. Finally, we show that faint dwarf galaxies are consistent with being the "building blocks" of the Galactic halo and provide a natural explanation for the mean value and fluctuations in the abundances of Galactic halo metal-poor stars.