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.Comment: accepted in Philosophical Transactions A; references update

Galactic evolution of rapid neutron capture process abundances: the inhomogeneous approach

“…(i) The production of heavy r-process matter in NSM is evident since many years (see Freiburghaus et al 1999 and many later investigations up to Korobkin et al 2012, Rosswog 2013, Just et al 2014, Wanajo et al 2014, Eichler et al 2014, Mendoza et al 2015. Our implementation of NSM in the inhomogeneous chemical evolution model "ICE" can explain the bulk of Eu (r-process) contributions in the galaxy for [Fe/H]> −2.5, but have problems to explain the amount and the spread of [Eu/Fe] at lower metallicities.…”

“…Such a system emits gravitational waves and the two NS spiral inwards towards their common center of mass with a coalescence time (t coal ) until they merge. The actual merging event is accompanied by an ejection of matter and (r-process) nucleosynthesis (Rosswog 2013, Freiburghaus et al 1999, Panov et al 2008, Korobkin et al 2012, Rosswog 2013, Eichler et al 2014, Wanajo et al 2014. As all of these publications show the emergence of a strong r-process, in the mass region of Eu they suffer partially from nuclear uncertainties related to fission fragment distributions (see e.g., Eichler et al 2014).…”

“…As all of these publications show the emergence of a strong r-process, in the mass region of Eu they suffer partially from nuclear uncertainties related to fission fragment distributions (see e.g., Eichler et al 2014). For our purposes we chose to utilize as total amount of r-process ejecta 1.28 · 10 −2 M⊙ (consistent with the 1.4M⊙ + 1.4M⊙ NS collision in Korobkin et al 2012 andRosswog 2013), but distributed in solar r-process proportions, which leads for Eu to a total amount of 10 −4 M⊙ per merger. This value is relatively high in comparison to other investigations in the literature.…”

Mass ejection from neutron star mergers: different components and expected radio signals

“…In many senses it is the easiest to calculate and as such it is the most robust element. It was investigated using Newtonian simulations (e.g., Davies et al 1994;Ruffert et al 1997;Rosswog et al 1999;Rosswog 2013) and using general relativistic simulations (e.g., Oechslin et al 2007;Bauswein et al 2013). According to these numerical simulations, the mass and kinetic energy of the dynamical ejecta are expected to be in the range 10 −4 Mej 10 −2 M⊙ and 10 49 E 10 51 erg, respectively.…”

“…[1] Goriely et al (2011); Korobkin et al (2012); ; Bauswein et al (2013); Rosswog (2013); Piran et al (2013); Wanajo et al (2014), b This component includes two cases depending on whether the remnant is a black hole or a neutron star. In the former the wind is just from the surrounding disk while in the latter it arises from the neutron star as well.…”