Protoneutron star evolution and the neutrino-driven wind in general relativistic neutrino radiation hydrodynamics simulations

Abstract

Massive stars end their lives in explosions with kinetic energies on the order of 10 51 erg. Immediately after the explosion has been launched, a region of low density and high entropy forms behind the ejecta, which is continuously subject to neutrino heating. The neutrinos emitted from the remnant at the center, the protoneutron star (PNS), heat the material above the PNS surface. This heat is partly converted into kinetic energy, and the material accelerates to an outflow that is known as the neutrino-driven wind. For the first time we simulate the collapse, bounce, explosion, and the neutrino-driven wind phases consistently over more than 20 s. Our numerical model is based on spherically symmetric general relativistic radiation hydrodynamics using spectral three-flavor Boltzmann neutrino transport. In simulations where no explosions are obtained naturally, we model neutrino-driven explosions for low-and intermediatemass Fe-core progenitor stars by enhancing the charged current reaction rates. In the case of a special progenitor star, the 8.8 M O-Ne-Mg-core, the explosion in spherical symmetry was obtained without enhanced opacities. The post-explosion evolution is in qualitative agreement with static steady-state and parametrized dynamic models of the neutrino-driven wind. On the other hand, we generally find lower neutrino luminosities and mean neutrino energies, as well as a different evolutionary behavior of the neutrino luminosities and mean neutrino energies. The neutrino-driven wind is proton-rich for more than 10 s and the contraction of the PNS differs from the assumptions made for the conditions at the inner boundary in previous neutrino-driven wind studies. Despite the moderately high entropies of about 100 k B /baryon and the fast expansion timescales, the conditions found in our models are unlikely to favor r-process nucleosynthesis. The simulations are carried out until the neutrino-driven wind settles down to a quasi-stationary state. About 5 s after the bounce, the peak temperature inside the PNS already starts to decrease because of the continued deleptonization. This moment determines the beginning of a cooling phase dominated by the emission of neutrinos. We discuss the physical conditions of the quasi-static PNS evolution and take the effects of deleptonization and mass accretion from early fallback into account.

Neutrino-driven wind simulations and nucleosynthesis of heavy elements

“…When the wind becomes supersonic and the early ejecta move slowly, the collision of both results in a wind termination shock or reverse shock. This hydrodynamical feature has been found in several supernova simulations [38,39,40,11,41,12]. Qian & Woosley (1996) [5] first used in a steady state model an outer boundary with constant pressure at a radius corresponding to a temperature of ≈ 2 GK.…”

“…In order to have a neutron-rich wind, i.e., Y e < 0.5, the neutrino and antineutrino energies have to approximately fulfill ǫν e −ǫ νe 4∆ ≈ 5 MeV. Indeed, this is not found in recent spherically symmetric simulations [13,12] where the neutrino outflow stays proton-rich. Under such conditions the rapid neutron capture process to form the heaviest nuclei cannot occur, ruling out the spherically symmetric neutrino-driven winds as its astrophysical site.…”

“…Thus, for the the wind one can assume that the electron fraction is given by [5]: (12) where L νe , ǫ νe and Lν e , ǫν e are the electron neutrino and antineutrino luminosities and mean energies (ǫ ν = ε 2 / ε ), respectively. The neutron-proton mass difference is ∆ = m n − m p = 1.293 MeV.…”

Origin of r-Process Elements and Galactic Chemodynamical Evolution

“…Although the original formulation was quite successful [23], it has been shown [24,25] that the desired conditions of high entropy and neutron-rich composition [4] do not occur in the neutrino energized wind. Nevertheless, it is quite likely that the so-called "weak r-process" occurs in the NDW producing neutron rich nuclei up to about A ∼ 125 [26,27].…”

Explosion Dynamics of Parametrized Spherically Symmetric Core-Collapse Supernova Simulations

“…The explosion mechanism of CCSNe is still not fully understood and self consistent one-dimensional simulations of CCSNe, including general relativity and detailed neutrino transport, do not lead to explosions, with the exception of the lowest-mass CCSN progenitors [2]. Even though multi-dimensional simulations are promising to explode and well suited to investigate the explosion mechanism they are computationally too expensive to explore a large set of progenitors.…”