Neutrino-driven winds from neutron star merger remnants

Abstract

We present a detailed, 3D hydrodynamics study of the neutrino-driven winds\nthat emerge from the remnant of a NS merger. Our simulations are performed with\nthe Newtonian, Eulerian code FISH, augmented by a detailed, spectral neutrino\nleakage scheme that accounts for heating due to neutrino absorption in\noptically thin conditions. Consistent with the 2D study of Dessart et al.\n(2009), we find that a strong baryonic wind is blown out along the original\nbinary rotation axis within $100$ ms after the merger. We compute a lower limit\non the expelled mass of $3.5 \\times 10^{-3} M_{\\odot}$, large enough to be\nrelevant for heavy element nucleosynthesis. The physical properties vary\nsignificantly between different wind regions. For example, due to stronger\nneutrino irradiation, the polar regions show substantially larger $Y_e$ than\nthose at lower latitudes. This has its bearings on the nucleosynthesis: the\npolar ejecta produce interesting r-process contributions from $A\\sim 80$ to\nabout 130, while the more neutron-rich, lower-latitude parts produce also\nelements up to the third r-process peak near $A\\sim 195$. We also calculate the\nproperties of electromagnetic transients that are powered by the radioactivity\nin the wind, in addition to the macronova transient that stems from the dynamic\nejecta. The high-latitude (polar) regions produce UV/optical transients\nreaching luminosities up to $10^{41} {\\rm erg \\, s^{-1}}$, which peak around 1\nday in optical and 0.3 days in bolometric luminosity. The lower-latitude\nregions, due to their contamination with high-opacity heavy elements, produce\ndimmer and more red signals, peaking after $\\sim 2$ days in optical and\ninfrared. Our numerical experiments indicate that it will be difficult to infer\nthe collapse time-scale of the HMNS to a BH based on the wind electromagnetic\ntransient, at least for collapse time-scales larger than the wind production\ntime-scale.\n