Arcones, A.

Perego, A.

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Neutrino-driven winds from binary neutron star mergers∗A. Perego1,2 and A. Arcones1,21Institut fu¨r Kernphysik (Theoriezentrum), Technische Universita¨t Darmstadt, Darmstadt, Germany;2GSI-Helmholtzzentrum fu¨r Schwerionenforschung, Darmstadt, GermanyThe merger of two neutron stars in a binary system is oneof the most powerful events in the Universe. Within a fewtens of milliseconds, an extremely hot, hypermassive neu-tron star (possibly, gravitationally unstable), surrounded bya thick accretion disc, has formed. Besides being a promis-ing progenitor for short gamma-ray-bursts, the expectedneutron-rich ejecta can produce a robust r-process. Thesynthesised neutron-rich nuclei can power an electromag-netic transients, known as kilonova (or macronova) and po-tentially observed in 2013 [1].In the post-merger phase, a large fraction of the gravi-tational energy released during the coalescence process isradiated away by neutrinos, with peak luminosities in ex-cess of 1053 erg/s. The energy deposition caused by theabsorption of neutrinos inside the disc, mainly via the pro-cesses:n+ νe → p+ e− p+ ν¯e → n+ e+,drives a wind on a time scale:twind ∼ egrave˙ν,heat≈ 0.07 s ,for typical remnant conditions.We performed detailed, three-dimensional radiation hy-drodynamical simulations of the neutrino-driven wind thatemerges from the remnant of a neutron star binary merger[2]. We used the Newtonian, Eulerian code FISH [3], aug-mented by a detailed neutrino leakage scheme that accountsfor the heating due to neutrino absorption in optically thinconditions. We evolve the remnant for 100ms under the in-fluence of neutrino cooling and heating. The initial condi-tions of our simulation are taken from high resolution SPHsimulations of the merger of two neutron stars [4].We find that a strong baryonic wind is blown out alongthe original binary rotation axis within a few tens of mil-liseconds after the merger. After 100 ms, the outer part ofthe disc has reached an almost stationary state, character-ized by smooth gradients in all the relevant quantities. Thephysical properties of the wind vary significantly betweendifferent regions. Due to stronger neutrino irradiation, thepolar regions show substantially larger electron fractions(Ye ∼ 0.3 - 0.4) than those at lower latitudes (Ye ∼ 0.2 -0.3). Because of the radiation absorption, the entropy in-side the wind first increases, then stays constant during theadiabatic expansion (s ∼ 15 - 20 kB/baryon).∗Work supported by Helmholtz-University Investigator grant No. VH-NG-825Figure 1: Vertical slices of the 3D domain (y = 0 plane) 85ms after the beginning of the simulation. We represent theelectron fraction (left) and the fluid velocity (right).Around 100 ms after the merger, 2.1×10−3M⊙ of matterin the wind is unbound. The polar ejecta produce interest-ing weak r-process contributions, from A ∼ 80 to about130, while the more neutron-rich, lower-latitude parts pro-duce in addition also elements up to the third r-processpeak near A ∼ 195. We also calculated the properties ofelectromagnetic transients that are powered by the radioac-tivity in the wind. The polar regions produce a transientswhose bolometric luminosity peaks around 0.3 days in theUV band. The lower-latitude regions are significantly lessbright and peak after ∼ 2 days in UV/optical band, due totheir much larger content in very heavy elements and theconsequently larger matter opacities.We conclude that the inclusion of the wind contributionsin the merger nucleosynthesis and in the light curve pre-diction is crucial for detailed theoretical predictions, to becompared with present and future observations.References[1] N.-R. Tanvir et al., Nature, 2013, 500, p. 547[2] A. Perego et al., MNRAS, 2014, 443, p. 3134[3] R. Ka¨ppeli et al., ApJS, 2011, 195, p. 20[4] D. J. Price, S. Rosswog, Science, 2006, 312, p. 719GSI SCIENTIFIC REPORT 2014 MU-THEORY-07DOI:10.15120/GR-2015-1-MU-THEORY-07 343

Neutrino-driven winds from binary neutron star mergers

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Neutrino-driven winds from binary neutron star mergers

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