The dynamic ejecta of compact object mergers and eccentric collisions

Abstract

Compact object mergers eject neutron-rich matter in a number of ways: by the dynamical ejection mediated by gravitational torques, as neutrino-driven winds and probably also a good fraction of the resulting accretion disc finally becomes unbound by a combination of viscous and nuclear processes. If compact binary mergers produce indeed gamma-ray bursts there should also be an interaction region where an ultra-relativistic outflow interacts with the neutrino-driven wind and produces moderately relativistic ejecta. Each type of ejecta has different physical properties and therefore plays a different role for nucleosynthesis and for the electromagnetic transients that go along with compact object encounters. Here we focus on the dynamic ejecta and present results for over 30 hydrodynamical simulations of both gravitational wave-driven mergers and parabolic encounters as they may occur in globular clusters. We find that mergers eject $\sim 1$% of a solar mass of extremely neutron-rich material. The exact amount as well as the ejection velocity depends on the involved masses with asymmetric systems ejecting more material at higher velocities. This material undergoes a robust r-process and both ejecta amount and abundance pattern are consistent with neutron star mergers being a major source of the "heavy" ($A>130$) r-process isotopes. Parabolic collisions, especially those between neutron stars and black holes, eject substantially larger amounts of mass and therefore cannot occur frequently without overproducing galactic r-process matter. We also discuss the electromagnetic transients that are powered by radioactive decays within the ejecta ("Macronovae"), and the radio flares that emerge when the ejecta dissipate their large kinetic energies in the ambient medium.