![]() However, multidimensional applications of these ‘Boltzmann solvers’ are severely constrained by their complexity and computational expense and force modern simulations to employ miscellaneous restrictions, such as the ‘ray-by-ray’ approach (e.g. The most accurate neutrino-transport schemes follow the full spatial, energetic and directional dependence of the neutrino distribution, described by the Boltzmann equation. ![]() The level of simplification is typically chosen to provide the optimal balance between accuracy and computational expense, given the constraints of the available computational resources and the considered physical effects to be captured to a sufficient degree. Unfortunately, most multidimensional results for the aforementioned scenarios stem from more or less idealized investigations, mainly owing to the enormous computational requirements of a time-dependent, multidimensional treatment of neutrino transport. Another astrophysical scenario in which neutrino transport may be crucial is the launching of a gamma-ray burst jet, which could be powered to some degree by neutrino pairs annihilating in the polar regions of a BH–torus system (e.g. Wanajo & Janka 2012 Fernández & Metzger 2013). 2009), or similarly a black hole (BH) torus configuration also produced by such a merger or by the merger of an NS and a BH (e.g. Another example is a massive NS formed during the merger of two NSs (Dessart et al. Such outflows are believed to occur during a CCSN in the form of a neutrino-driven wind expelled from the surface of the proto-NS (e.g. Genuine neutrino-transport effects can also be crucial for determining the properties of potentially nucleosynthesis-relevant outflows and may even give rise to these outflows to begin with. A prominent example is a core-collapse supernova (CCSN), in which according to the present standard model the explosion is essentially only made possible by the energy deposition due to the re-absorption of neutrinos just produced in the proto-neutron star (proto-NS see Janka 2012 Burrows 2013 for recent reviews). In various astrophysical scenarios involving matter in a hot and dense phase, neutrino interactions take place in a way that the full transport problem – which consistently follows the emission, propagation and absorption of neutrinos – needs to be taken into account to correctly describe these systems. Hydrodynamics, neutrinos, radiative transfer, methods: numerical, stars: neutron, supernovae: general 1 INTRODUCTION In our most detailed test, we compare a fully dynamic, one-dimensional core-collapse simulation with two published calculations performed with well-known Boltzmann-type neutrino-hydrodynamics codes and we find very satisfactory agreement. We investigate various problem set-ups in one and two dimensions to verify the implementation and to test the quality of the algebraic closure scheme. For the time integration of the potentially stiff moment equations we employ a scheme in which only the local source terms are treated implicitly, while the advection terms are kept explicit, thereby allowing for an efficient computational parallelization of the algorithm. The finite-volume discretization of the essentially hyperbolic system of moment equations employs methods well-known from hydrodynamics. ![]() The transport scheme is significantly more efficient than a multidimensional solver of the Boltzmann equation, while it is more accurate and consistent than the flux-limited diffusion method. The scheme takes into account frame-dependent effects of the order |$\mathcal (v/c)$| as well as the most important types of neutrino interactions. The algorithm solves the evolution equations of the zeroth- and first-order angular moments of the specific intensity, supplemented by an algebraic relation for the second-moment tensor to close the system. ![]() We present the new code alcar developed to model multidimensional, multienergy-group neutrino transport in the context of supernovae and neutron-star mergers. ![]()
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