37,708 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
Theory and computation of higher gradient elasticity theories based on action principles
In continuum mechanics, there exists a unique theory for elasticity, which includes the first gradient of displacement. The corresponding generalization of elasticity is referred to as strain gradient elasticity or higher gradient theories, where the second and higher gradients of displacement are involved. Unfortunately, there is a lack of consensus among scientists how to achieve the generalization. Various suggestions were made, in order to compare or even verify these, we need a generic computational tool. In this paper, we follow an unusual but quite convenient way of formulation based on action principles. First, in order to present its benefits, we start with the action principle leading to the well-known form of elasticity theory and present a variational formulation in order to obtain a weak form. Second, we generalize elasticity and point out, in which term the suggested formalism differs. By using the same approach, we obtain a weak form for strain gradient elasticity. The weak forms for elasticity and for strain gradient elasticity are solved numerically by using open-source packages—by using the finite element method in space and finite difference method in time. We present some applications from elasticity as well as strain gradient elasticity and simulate the so-called size effect
Casimir energy density in closed hyperbolic universes
The original Casimir effect results from the difference in the vacuum
energies of the electromagnetic field, between that in a region of space with
boundary conditions and that in the same region without boundary conditions. In
this paper we develop the theory of a similar situation, involving a scalar
field in spacetimes with compact spatial sections of negative spatial
curvature.Comment: 10 pages. Contribution to the "Fifth Alexander Friedmann
International Seminar on Gravitation and Cosmology," Joao Pessoa, Brazil,
2002. Revised version, with altered Abstract and one new referenc
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
Butterfly hysteresis loop and dissipative spin reversal in the S=1/2, V15 molecular complex
Time resolved magnetization measurements have been performed on a spin 1/2
molecular complex, so called V. Despite the absence of a barrier,
magnetic hysteresis is observed over a timescale of several seconds. A detailed
analysis in terms of a dissipative two level model is given, in which
fluctuations and splittings are of same energy. Spin-phonon coupling leads to
long relaxation times and to a particular "butterfly" hysteresis loop.Comment: LaTeX/RevTeX, 3 figures.Approved for publication in PR
Observation of quantum spin noise in a 1D light-atoms quantum interface
We observe collective quantum spin states of an ensemble of atoms in a
one-dimensional light-atom interface. Strings of hundreds of cesium atoms
trapped in the evanescent fiel of a tapered nanofiber are prepared in a
coherent spin state, a superposition of the two clock states. A weak quantum
nondemolition measurement of one projection of the collective spin is performed
using a detuned probe dispersively coupled to the collective atomic observable,
followed by a strong destructive measurement of the same spin projection. For
the coherent spin state we achieve the value of the quantum projection noise 40
dB above the detection noise, well above the 3 dB required for reconstruction
of the negative Wigner function of nonclassical states. We analyze the effects
of strong spatial inhomogeneity inherent to atoms trapped and probed by the
evanescent waves. We furthermore study temporal dynamics of quantum
fluctuations relevant for measurement-induced spin squeezing and assess the
impact of thermal atomic motion. This work paves the road towards observation
of spin squeezed and entangled states and many-body interactions in 1D spin
ensembles
- …