Neutrino-driven winds from neutron star merger remnants

Abstract

We present a detailed, three-dimensional hydrodynamic study of the neutrino-driven winds that emerge from the remnant of a neutron star merger. Our simulations are performed with the Newtonian, Eulerian code FISH, augmented by a detailed, spectral neutrino leakage scheme that accounts for heating due to neutrino absorption in optically thin conditions. Consistent with the earlier, two-dimensional study of Dessart et al. (2009), we find that a strong baryonic wind is blown out along the original binary rotation axis within ≈100 milliseconds after the merger. We compute a lower limit on the expelled mass of 3.5 × 10 −3 M , large enough to be relevant for heavy element nucleosynthesis. The physical properties vary significantly between different wind regions. For example, due to stronger neutrino irradiation, the polar regions show substantially larger electron fractions than those at lower latitudes. This has its bearings on the nucleosynthesis: the polar ejecta produce interesting r-process contributions from A ≈ 80 to about 130, while the more neutron-rich, lower-latitude parts produce in addition also elements up to the third r-process peak near A ≈ 195. We also calculate the properties of electromagnetic transients that are powered by the radioactivity in the wind, in addition to the "macronova" transient that stems from the dynamic ejecta. The high-latitude (polar) regions produce UV/optical transients reaching luminosities up to 10 41 erg s −1 , which peak around 1 day in optical and 0.3 days in bolometric luminosity. The lower-latitude regions, due to their contamination with high-opacity heavy elements, produce dimmer and more red signals, peaking after ∼ 2 days in optical and infrared. Our numerical experiments indicate that it will be difficult to infer the collapse time-scale of the hypermassive neutron star to a black hole based on the wind electromagnetic transient, at least for collapse time-scales larger than the wind production time-scale.

Impact of Neutrino Interactions on Outflows of Neutron-Star Mergers

“…Heavier elements up to A ∼ 240 are produced as well, but compared to the lighter elements their abundances vary strongly with the torus and BH masses, and the employed viscosity treatment (which is also confirmed by [9]). Our results, in addition to those by [6,10,11] who investigated NS-torus remnants, indicate that merger remnants can throw out ejecta with comparable masses but rather different abundance patterns with respect to the dynamical ejecta. Hence, a consistent follow-up evolution of the post-merger remnant is imperative in order to obtain the entire nucleosynthesis fingerprint of NS mergers.…”

“…[5]). However, it was recently realized that the nucleosynthesis signature of NS mergers is more complex when taking into account ν-interactions in relativistic NS-NS merger simulations and when the ejecta No e ± -capture, no !-interactions for " < " from the merger remnant are consistently included [1,[6][7][8][9][10]. In contrast to NS-BH mergers, in NS-NS mergers the large pressure gradient that is generated by the collision shock building up between both NSs causes a sizable amount of material to become unbound in addition to possible tidal-tail ejecta from the disrupted NS(s).…”

Nucleosynthesis in Supernovae, Hypernovae/Gamma-ray Bursts and Compact Binary Mergers

“…The nucleosynthesis yields of CCSNe are characterized by an average ejected 56 Ni mass of about 0.1 M ⊙ and strong contributions to the so-called alpha elements O, Ne, Mg, Si, S, Ar, Ca, and Ti. They result either from hydrostatic burning in stellar evolution (O, Ne, Mg, and some Si) or from explosive burning (Si, S, Ar, Ca, and Ti) [10].…”

“…In general, explosive Si-burning in CCSNe occurs with a strong alpha-rich freeze-out (dependent on the explosion energy/entropy), which favors for identical Y e conditions those Fe-group nuclei which would result from additional alpha-captures on the nuclei produced in a normal freeze-out. Thus, 56 Ni can be moved up to 64 Ge (decaying to 64 Zn) or 54 Fe up to 58 Ni. The first effect takes place for very high explosion energies, only attained in hypernovae, the second one in all regular CCSNe.…”

“…Recent observations [19] underline that there exist core-collapse supernova explosions whose light curves are not determined by (large) amounts of 56 Ni ejecta, but rather by the energy release of a fast rotating neutron star (pulsar) with extremely strong magnetic fields of the order 10 15 Gauss (magnetars). The question is how can neutron stars of such extremely high magnetic fields (in comparison to the typical 10 12 Gauss) emerge from supernova explosions.…”