Dynamics, nucleosynthesis, and kilonova signature of black hole-neutron star merger ejecta

Academic Article


  • We investigate the ejecta from black hole - neutron star mergers by modeling the formation and interaction of mass ejected in a tidal tail and a disk wind. The outflows are neutron-rich, giving rise to optical/infrared emission powered by the radioactive decay of $r$-process elements (a kilonova). Here we perform an end-to-end study of this phenomenon, where we start from the output of a fully-relativistic merger simulation, calculate the post-merger hydrodynamical evolution of the ejecta and disk winds including neutrino physics, determine the final nucleosynthetic yields using post-processing nuclear reaction network calculations, and compute the kilonova emission with a radiative transfer code. We study the effects of the tail-to-disk mass ratio by scaling the tail density. A larger initial tail mass results in fallback matter becoming mixed into the disk and ejected in the subsequent disk wind. Relative to the case of a disk without dynamical ejecta, the combined outflow has lower mean electron fraction, faster speed, larger total mass, and larger absolute mass free of high-opacity Lanthanides or Actinides. In most cases, the nucleosynthetic yield is dominated by the heavy $r$-process contribution from the unbound part of the tidal tail. A Solar-like abundance distribution can however be obtained when the total mass of the dynamical ejecta is comparable to the mass of the disk outflows. The kilonova has a characteristic duration of 1 week and a luminosity of ~$10^{41}$ erg/s, with orientation effects leading to variations of a factor ~2 in brightness. At early times (< 1 day) the emission includes an optical component from the (hot) Lanthanide-rich material, but the spectrum evolves quickly to the infrared thereafter.
  • Authors

  • Fernandez, Rodrigo
  • Foucart, Francois
  • Kasen, Daniel
  • Lippuner, Jonas
  • Desai, Dhruv
  • Roberts, Luke F
  • Status

    Publication Date

  • August 3, 2017
  • Published In


  • abundances
  • accretion
  • accretion disks
  • dense matter
  • gravitational waves
  • hydrodynamics
  • neutrinos
  • nuclear reactions
  • nucleosynthesis
  • Digital Object Identifier (doi)

    Start Page

  • 154001
  • End Page

  • 154001
  • Volume

  • 34
  • Issue

  • 15