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

• Published in: Classical and quantum gravity Vol. 34; no. 15; pp. 154001 - 154024
• Main Authors: Fernández, Rodrigo, Foucart, Francois, Kasen, Daniel, Lippuner, Jonas, Desai, Dhruv, Roberts, Luke F
• Format: Journal Article
• Language: English
• Published: United States IOP Publishing 04.07.2017
• Subjects: abundances; accretion; accretion disks; ASTRONOMY AND ASTROPHYSICS; dense matter; gravitational waves; hydrodynamics; neutrinos; nuclear reactions; nucleosynthesis
• ISSN: 0264-9381; 1361-6382
• DOI: 10.1088/1361-6382/aa7a77

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 dynamical ejecta. 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   1041 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) Lanthanide-rich material, but the spectrum evolves quickly to the infrared thereafter.