3,284 research outputs found
A new multidimensional, energy-dependent two-moment transport code for neutrino-hydrodynamics
We present the new code ALCAR developed to model multidimensional, multi
energy-group neutrino transport in the context of supernovae and neutron-star
mergers. The algorithm solves the evolution equations of the 0th- and 1st-order
angular moments of the specific intensity, supplemented by an algebraic
relation for the 2nd-moment tensor to close the system. The scheme takes into
account frame-dependent effects of order O(v/c) as well as the most important
types of neutrino interactions. 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
finite-volume discretization of the essentially hyperbolic system of moment
equations employs methods well-known from hydrodynamics. 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. We investigate various problem setups in one
and two dimensions to verify the implementation and to test the quality of the
algebraic closure scheme. 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.Comment: 30 pages, 12 figures. Revised version: several additional comments
and explanations, results remain unchanged. Accepted for publication in MNRA
Three-Dimensional Core-Collapse Supernova Simulations with Multi-Dimensional Neutrino Transport Compared to the Ray-by-Ray-plus Approximation
Self-consistent, time-dependent supernova (SN) simulations in three spatial
dimensions (3D) are conducted with the Aenus-Alcar code, comparing, for the
first time, calculations with fully multi-dimensional (FMD) neutrino transport
and the ray-by-ray-plus (RbR+) approximation, both based on a two-moment solver
with algebraic M1 closure. We find good agreement between 3D results with FMD
and RbR+ transport for both tested grid resolutions in the cases of a 20
solar-mass progenitor, which does not explode with the employed simplified set
of neutrino opacities, and of an exploding 9 solar-mass model. This is in stark
contrast to corresponding axisymmetric (2D) simulations, which confirm previous
claims that the RbR+ approximation can foster explosions in 2D in particular in
models with powerful axial sloshing of the stalled shock due to the standing
accretion shock instability (SASI). However, while local and instantaneous
variations of neutrino fluxes and heating rates can still be considerably
higher with RbR+ transport in 3D, the time-averaged quantities are very similar
to FMD results because of the absence of a fixed, artificial symmetry axis that
channels the flow. Therefore, except for stochastic fluctuations, the neutrino
signals and the post-bounce evolution of 3D simulations with FMD and RbR+
transport are also very similar, in particular for our calculations with the
better grid resolution. Higher spatial resolution has clearly a more important
impact than the differences by the two transport treatments. Our results back
up the use of the RbR+ approximation for neutrino transport in 3D SN modeling.Comment: 25 pages, 16 figures; referee comments included, new appendix added;
accepted by Ap
Effects of LESA in Three-Dimensional Supernova Simulations with Multi-Dimensional and Ray-by-Ray-plus Neutrino Transport
A set of eight self-consistent, time-dependent supernova (SN) simulations in
three spatial dimensions (3D) for 9 solar-mass and 20 solar-mass progenitors is
evaluated for the presence of dipolar asymmetries of the electron lepton-number
emission as discovered by Tamborra et al. and termed lepton-number emission
self-sustained asymmetry (LESA). The simulations were performed with the
Aenus-Alcar neutrino/hydrodynamics code, which treats the energy- and
velocity-dependent transport of neutrinos of all flavors by a two-moment scheme
with algebraic M1 closure. For each of the progenitors, results with fully
multi-dimensional (FMD) neutrino transport and with ray-by-ray-plus (RbR+)
approximation are considered for two different grid resolutions. While the 9
solar-mass models develop explosions, the 20 solar-mass progenitor does not
explode with the employed version of simplified neutrino opacities. In all 3D
models we observe the growth of substantial dipole amplitudes of the
lepton-number (electron neutrino minus antineutrino) flux with stable or slowly
time-evolving direction and overall properties fully consistent with the LESA
phenomenon. Models with RbR+ transport develop LESA dipoles somewhat faster and
with temporarily higher amplitudes, but the FMD calculations exhibit cleaner
hemispheric asymmetries with a far more dominant dipole. In contrast, the RbR+
results display much wider multipole spectra of the neutrino-emission
anisotropies with significant power also in the quadrupole and higher-order
modes. Our results disprove speculations that LESA is a numerical artifact of
RbR+ transport. We also discuss LESA as consequence of a dipolar convection
flow inside of the nascent neutron star and establish, tentatively, a
connection to Chandrasekhar's linear theory of thermal instability in spherical
shells.Comment: 20 pages, 9 figures; revised version accepted by ApJ; new Figs. 6,7,
and new panels in Fig.8 added; Sects. 4,5,6 considerably extended in reply to
referee question
Neutron-star merger ejecta as obstacles to neutrino-powered jets of gamma-ray bursts
We present the first special relativistic, axisymmetric hydrodynamic
simulations of black hole-torus systems (approximating general relativistic
gravity) as remnants of binary-neutron star (NS-NS) and neutron star-black hole
(NS-BH) mergers, in which the viscously driven evolution of the accretion torus
is followed with self-consistent energy-dependent neutrino transport and the
interaction with the cloud of dynamical ejecta expelled during the NS-NS
merging is taken into account. The modeled torus masses, BH masses and spins,
and the ejecta masses, velocities, and spatial distributions are adopted from
relativistic merger simulations. We find that energy deposition by neutrino
annihilation can accelerate outflows with initially high Lorentz factors along
polar low-density funnels, but only in mergers with extremely low baryon
pollution in the polar regions. NS-BH mergers, where polar mass ejection during
the merging phase is absent, provide sufficiently baryon-poor environments to
enable neutrino-powered, ultrarelativistic jets with terminal Lorentz factors
above 100 and considerable dynamical collimation, favoring short gamma-ray
bursts (sGRBs), although their typical energies and durations might be too
small to explain the majority of events. In the case of NS-NS mergers, however,
neutrino emission of the accreting and viscously spreading torus is too short
and too weak to yield enough energy for the outflows to break out from the
surrounding ejecta shell as highly relativistic jets. We conclude that neutrino
annihilation alone cannot power sGRBs from NS-NS mergers.Comment: 7 pages, 4 figures, minor revisions compared to original version,
accepted for publication in ApJ Letter
Nucleosynthesis in dynamical and torus ejecta of compact binary mergers
We present a comprehensive study of r-process element nucleosynthesis in the
ejecta of compact binary mergers (CBMs) and their relic black-hole (BH)-torus
systems. The evolution of the BH-accretion tori is simulated for seconds with a
Newtonian hydrodynamics code including viscosity effects, pseudo-Newtonian
gravity for rotating BHs, and an energy-dependent two-moment closure scheme for
the transport of electron neutrinos and antineutrinos. The investigated cases
are guided by relativistic double neutron star (NS-NS) and NS-BH merger models,
producing ~3-6 Msun BHs with rotation parameters of A~0.8 and tori of 0.03-0.3
Msun. Our nucleosynthesis analysis includes the dynamical (prompt) ejecta
expelled during the CBM phase and the neutrino and viscously driven outflows of
the relic BH-torus systems. While typically ~20-25% of the initial
accretion-torus mass are lost by viscously driven outflows, neutrino-powered
winds contribute at most another ~1%, but neutrino heating enhances the viscous
ejecta significantly. Since BH-torus ejecta possess a wide distribution of
electron fractions (0.1-0.6) and entropies, they produce heavy elements from
A~80 up to the actinides, with relative contributions of A>130 nuclei being
subdominant and sensitively dependent on BH and torus masses and the exact
treatment of shear viscosity. The combined ejecta of CBM and BH-torus phases
can reproduce the solar abundances amazingly well for A>90. Varying
contributions of the torus ejecta might account for observed variations of
lighter elements with 40<Z<56 relative to heavier ones, and a considerable
reduction of the prompt ejecta compared to the torus ejecta, e.g. in highly
asymmetric NS-BH mergers, might explain the composition of heavy-element
deficient stars.Comment: 7 pages, 4 figures, only changed title compared to previous version,
accepted for publication in Proceedings of Science (Nuclei in the Cosmos
XIII, Debrecen
Comprehensive nucleosynthesis analysis for ejecta of compact binary mergers
We present the first comprehensive study of r-process element nucleosynthesis
in the ejecta of compact binary mergers (CBMs) and their relic black-hole
(BH)-torus systems. The evolution of the BH-accretion tori is simulated for
seconds with a Newtonian hydrodynamics code including viscosity effects,
pseudo-Newtonian gravity for rotating BHs, and an energy-dependent two-moment
closure scheme for the transport of electron neutrinos and antineutrinos. The
investigated cases are guided by relativistic double neutron star (NS-NS) and
NS-BH merger models, producing ~3-6 Msun BHs with rotation parameters of A~0.8
and tori of 0.03-0.3 Msun. Our nucleosynthesis analysis includes the dynamical
(prompt) ejecta expelled during the CBM phase and the neutrino and viscously
driven outflows of the relic BH-torus systems. While typically ~20-25% of the
initial accretion-torus mass are lost by viscously driven outflows,
neutrino-powered winds contribute at most another ~1%, but neutrino heating
enhances the viscous ejecta significantly. Since BH-torus ejecta possess a wide
distribution of electron fractions (0.1-0.6) and entropies, they produce heavy
elements from A~80 up to the actinides, with relative contributions of A>130
nuclei being subdominant and sensitively dependent on BH and torus masses and
the exact treatment of shear viscosity. The combined ejecta of CBM and BH-torus
phases can reproduce the solar abundances amazingly well for A>90. Varying
contributions of the torus ejecta might account for observed variations of
lighter elements with 40<Z<56 relative to heavier ones, and a considerable
reduction of the prompt ejecta compared to the torus ejecta, e.g. in highly
asymmetric NS-BH mergers, might explain the composition of heavy-element
deficient stars.Comment: 30 pages, 22 figures; revised version, accepted by MNRAS; appendix
added with test results for neutrino transpor
Continued-fraction expansion of eigenvalues of generalized evolution operators in terms of periodic orbits
A new expansion scheme to evaluate the eigenvalues of the generalized
evolution operator (Frobenius-Perron operator) relevant to the
fluctuation spectrum and poles of the order- power spectrum is proposed. The
``partition function'' is computed in terms of unstable periodic orbits and
then used in a finite pole approximation of the continued fraction expansion
for the evolution operator. A solvable example is presented and the approximate
and exact results are compared; good agreement is found.Comment: CYCLER Paper 93mar00
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