Dynamics, nucleosynthesis, and kilonova signature of black hole—neutron star merger ejecta. (4th July 2017)

• Record Type: Journal Article
• Authors: Fernández, Rodrigo; Foucart, Francois; Kasen, Daniel; Lippuner, Jonas; Desai, Dhruv; Roberts, Luke F
• Is Part Of: Classical and quantum gravity. Volume 34:Number 15(2017:Aug.)
• Publication Date: 2017-07-04
• Subjects: accretion -- accretion disks -- dense matter -- gravitational waves -- hydrodynamics -- neutrinos -- nuclear reactions -- nucleosynthesis -- abundances; Quantum gravity -- Periodicals; Gravitation -- Periodicals; Relativity (Physics) -- Periodicals; Space and time -- Periodicals; Periodicals; 521.1
• DOI: 10.1088/1361-6382/aa7a77
• Languages: English
• ISSNs: 0264-9381

Abstract

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