5,077 research outputs found
Non-Radial Instabilities and Progenitor Asphericities in Core-Collapse Supernovae
Since core-collapse supernova simulations still struggle to produce robust
neutrino-driven explosions in 3D, it has been proposed that asphericities
caused by convection in the progenitor might facilitate shock revival by
boosting the activity of non-radial hydrodynamic instabilities in the
post-shock region. We investigate this scenario in depth using 42 relativistic
2D simulations with multi-group neutrino transport to examine the effects of
velocity and density perturbations in the progenitor for different perturbation
geometries that obey fundamental physical constraints (like the anelastic
condition). As a framework for analysing our results, we introduce
semi-empirical scaling laws relating neutrino heating, average turbulent
velocities in the gain region, and the shock deformation in the saturation
limit of non-radial instabilities. The squared turbulent Mach number, ,
reflects the violence of aspherical motions in the gain layer, and explosive
runaway occurs for ~0.3, corresponding to a reduction of the critical
neutrino luminosity by ~25% compared to 1D. In the light of this theory,
progenitor asphericities aid shock revival mainly by creating anisotropic mass
flux onto the shock: Differential infall efficiently converts velocity
perturbations in the progenitor into density perturbations (Delta rho/rho) at
the shock of the order of the initial convective Mach number Ma. The
anisotropic mass flux and ram pressure deform the shock and thereby amplify
post-shock turbulence. Large-scale (l=2,l=1) modes prove most conducive to
shock revival, whereas small-scale perturbations require unrealistically high
convective Mach numbers. Initial density perturbations in the progenitor are
only of order Ma^2 and therefore play a subdominant role.Comment: revised version, 34 pages, 24 figure
Making big steps in trajectories
We consider the solution of initial value problems within the context of
hybrid systems and emphasise the use of high precision approximations (in
software for exact real arithmetic). We propose a novel algorithm for the
computation of trajectories up to the area where discontinuous jumps appear,
applicable for holomorphic flow functions. Examples with a prototypical
implementation illustrate that the algorithm might provide results with higher
precision than well-known ODE solvers at a similar computation time
Supernova Simulations from a 3D Progenitor Model -- Impact of Perturbations and Evolution of Explosion Properties
We study the impact of large-scale perturbations from convective shell
burning on the core-collapse supernova explosion mechanism using
three-dimensional (3D) multi-group neutrino hydrodynamics simulations of an 18
solar mass progenitor. Seed asphericities in the O shell, obtained from a
recent 3D model of O shell burning, help trigger a neutrino-driven explosion
330ms after bounce whereas the shock is not revived in a model based on a
spherically symmetric progenitor for at least another 300ms. We tentatively
infer a reduction of the critical luminosity for shock revival by ~20% due to
pre-collapse perturbations. This indicates that convective seed perturbations
play an important role in the explosion mechanism in some progenitors. We
follow the evolution of the 18 solar mass model into the explosion phase for
more than 2s and find that the cycle of accretion and mass ejection is still
ongoing at this stage. With a preliminary value of 0.77 Bethe for the
diagnostic explosion energy, a baryonic neutron star mass of 1.85 solar masses,
a neutron star kick of ~600km/s and a neutron star spin period of ~20ms at the
end of the simulation, the explosion and remnant properties are slightly
atypical, but still lie comfortably within the observed distribution. Although
more refined simulations and a larger survey of progenitors are still called
for, this suggests that a solution to the problem of shock revival and
explosion energies in the ballpark of observations are within reach for
neutrino-driven explosions in 3D.Comment: 23 pages, 22 figures, accepted for publication in MNRA
Parameterized Uniform Complexity in Numerics: from Smooth to Analytic, from NP-hard to Polytime
The synthesis of classical Computational Complexity Theory with Recursive
Analysis provides a quantitative foundation to reliable numerics. Here the
operators of maximization, integration, and solving ordinary differential
equations are known to map (even high-order differentiable) polynomial-time
computable functions to instances which are `hard' for classical complexity
classes NP, #P, and CH; but, restricted to analytic functions, map
polynomial-time computable ones to polynomial-time computable ones --
non-uniformly!
We investigate the uniform parameterized complexity of the above operators in
the setting of Weihrauch's TTE and its second-order extension due to
Kawamura&Cook (2010). That is, we explore which (both continuous and discrete,
first and second order) information and parameters on some given f is
sufficient to obtain similar data on Max(f) and int(f); and within what running
time, in terms of these parameters and the guaranteed output precision 2^(-n).
It turns out that Gevrey's hierarchy of functions climbing from analytic to
smooth corresponds to the computational complexity of maximization growing from
polytime to NP-hard. Proof techniques involve mainly the Theory of (discrete)
Computation, Hard Analysis, and Information-Based Complexity
Magnetic properties of the three-band Hubbard model
We present magnetic properties of the three-band Hubbard model in the para-
and antiferromagnetic phase on a hypercubic lattice calculated with the
Dynamical Mean-Field Theory (DMFT). To allow for solutions with broken
spin-symmetry we extended the approach to lattices with AB-like structure.
Above a critical sublattice magnetization m_d=0.5 one can observe rich
structures in the spectral-functions similar to the t-J model which can be
related to the well known bound states for one hole in the Neel-background. In
addition to the one-particle properties we discuss the static
spin-susceptiblity in the paramagnetic state at the points q=0 and
q=(pi,pi,pi,...) for different dopings delta. The delta-T-phase-diagram
exhibits an enhanced stability of the antiferromagnetic state for
electron-doped systems in comparison to hole-doped. This asymmetry in the phase
diagram is in qualitative agreement with experiments for high-T_c materials.Comment: revised version, to be publishe
Innovative Anwendungen von polymeren Werkstoffen in der Medizintechnik : Formgedächtnis und Oberflächenmodifizierung
Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG geförderten) Allianz- bzw. Nationallizenz frei zugänglich.This publication is with permission of the rights owner freely accessible due to an Alliance licence and a national licence (funded by the DFG, German Research Foundation) respectively.Das bekannte Prinzip des Kunststoff-Schrumpfschlauches, der bei etwa 120°C eine Formänderung vollzieht, wurde vom Fachgebiet Polymertechnik der TU-Berlin in der Art weiterentwickelt, daß es möglich ist, schon eine Formänderung eines Teiles bei Temperaturen zu erreichen, die im menschlichen Organismus vorliegen können
Global Anisotropies in Supernova Explosions and Pulsar Recoil
We show by two-dimensional and first three-dimensional simulations of
neutrino-driven supernova explosions that low (l=1,2) modes can dominate the
flow pattern in the convective postshock region on timescales of hundreds of
milliseconds after core bounce. This can lead to large global anisotropy of the
supernova explosion and pulsar kicks in excess of 500 km/s.Comment: 3 pages, 2 figures, contribution to Procs. 12th Workshop on Nuclear
Astrophysics, Ringberg Castle, March 22-27, 200
Dynamical Magnetic Susceptibility for the - Model
We present results for the {\em dynamical}\/ magnetic susceptibility of the
- model, calculated with the dynamical mean field theory. For we
find enhanced ferromagnetic correlations but an otherwise relatively
-independent dynamical magnetic susceptibility. For the explicit
antiferromagnetic exchange leads to a dynamic spin structure factor with the
expected peak at the antiferromagnetic Bragg point.Comment: 3 pages LaTeX, postscript figures included, submitted as contribution
to SCES' 96, to appear in Physica
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