Dynamics, nucleosynthesis, and kilonova signature of black hole—neutron star merger ejecta
Abstract
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 neutronrich, giving rise to optical/infrared emission powered by the radioactive decay of rprocess elements (a kilonova). Here we perform an endtoend study of this phenomenon, where we start from the output of a fullyrelativistic merger simulation, calculate the postmerger hydrodynamical evolution of the ejecta and disk winds including neutrino physics, determine the final nucleosynthetic yields using postprocessing nuclear reaction network calculations, and compute the kilonova emission with a radiative transfer code. We study the effects of the tailtodisk 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 highopacity Lanthanides or Actinides. In most cases, the nucleosynthetic yield is dominated by the heavy rprocess contribution from the unbound part of the dynamical ejecta. A Solarlike 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^{1} , with orientation effects leading to variations of a factor ∼2 in brightness. At early times (< 1 d) the emission includes an optical component from the (hot) Lanthaniderich material, but the spectrum evolves quickly to the infrared thereafter.
 Publication:

Classical and Quantum Gravity
 Pub Date:
 August 2017
 DOI:
 10.1088/13616382/aa7a77
 arXiv:
 arXiv:1612.04829
 Bibcode:
 2017CQGra..34o4001F
 Keywords:

 Astrophysics  High Energy Astrophysical Phenomena;
 Astrophysics  Solar and Stellar Astrophysics;
 General Relativity and Quantum Cosmology;
 Nuclear Theory
 EPrint:
 Accepted by Classical &