7,628 research outputs found
Delayed neutrino-driven supernova explosions aided by the standing accretion-shock instability
We present results of 2D hydrodynamic simulations of stellar core collapse,
which confirm that the neutrino-heating mechanism remains viable for the
explosion of a wider mass range of supernova progenitors with iron cores. We
used an energy-dependent treatment of the neutrino transport based on the
"ray-by-ray plus" approximation, in which the number, energy, and momentum
equations are closed with a variable Eddington factor obtained by iteratively
solving a model Boltzmann equation. We focus on the evolution of a 15 Msun
progenitor and show that shock revival and the explosion are initiated at about
600 ms post bounce, powered by neutrino energy deposition. Similar to previous
findings for an 11.2 Msun star, but significantly later, the onset of the
explosion is fostered by the standing accretion shock instability (SASI). This
instability exhibits highest growth rates for the dipole and quadrupole modes,
which lead to large-amplitude bipolar shock oscillations and push the shock to
larger radii, thus increasing the time accreted matter is exposed to neutrino
heating in the gain layer. Therefore also convective overturn behind the shock
is strengthened. A "soft" nuclear equation of state that causes a rapid
contraction and a smaller radius of the forming neutron star and thus a fast
release of gravitational binding energy, seems to be more favorable for an
explosion. Rotation has the opposite effect because it leads to a more extended
and cooler neutron star and thus lower neutrino luminosities and mean energies
and overall less neutrino heating. Neutron star g-mode oscillations and the
acoustic mechanism play no important role in our simulations. (abridged)Comment: 46 pages, 20 figures, 59 eps files; submitted to ApJ; significantly
extended and revised version to account for referee comments; high-resolution
images can be obtained upon reques
Dynamics of the Mott Insulator to Superfluid quantum phase transition in the truncated Wigner approximation
The quantum phase transition from the Mott insulator state to the superfluid
in the Bose-Hubbard model is investigated. We research one, two and three
dimensional lattices in the truncated Wigner approximation. We compute both
kinetic and potential energy and they turn out to have a power law behaviour as
a function of the transition rate, with the power equal to 1/3. The same
applies to the total energy in a system with a harmonic trap, which is usually
present in the experimental set-up. These observations are in agreement with
the experiment of [8], where such scalings were also observed and the power of
the decay was numerically close to 1/3. The results confirm the Kibble-Zurek
(adiabatic-impulse-adiabatic approximation) scenario for this transition.Comment: 14 pages, 11 figures; Laser Physics Workshop (LPHYS'12) Proceedings,
Calgary 201
Radiative Corrections to the top quark width
Calculations of radiative corrections to the top quark width are reviewed.
QCD effects are discussed for systems produced in
annihilation near the energy threshold.}Comment: Karlsruhe Univ. preprint TTP 93-21, 6p., 2 figures not included (Talk
presented by M. Je\.zabek at the Workshop on Physics and Experiments at
Linear Colliders, Waikoloa, Hawaii, April 1993
A ring of BEC pools as a trap for persistent flow
Mott insulator - superfluid transition in a periodic lattice of Josephson
junctions can be driven by tunneling rate increase. Resulting winding numbers
of the condensate wavefunction decrease with increasing quench time in
accord with the Kibble-Zurek mechanism (KZM). However, in very slow quenches
Bose-Hubbard dynamics rearranges wavefunction phase so that its random walk
cools, decreases and eventually the wavefunction becomes too cold
to overcome potential barriers separating different . Thus, in contrast with
KZM, in very slow quenches is set by random walk with "critical"
step size, independently of .Comment: Decompressed version to appear in Phys. Rev.
Neutrino-driven explosions twenty years after SN1987A
The neutrino-heating mechanism remains a viable possibility for the cause of
the explosion in a wide mass range of supernova progenitors. This is
demonstrated by recent two-dimensional hydrodynamic simulations with detailed,
energy-dependent neutrino transport. Neutrino-driven explosions were not only
found for stars in the range of 8-10 solar masses with ONeMg cores and in case
of the iron core collapse of a progenitor with 11 solar masses, but also for a
``typical'' progenitor model of 15 solar masses. For such more massive stars,
however, the explosion occurs significantly later than so far thought, and is
crucially supported by large-amplitude bipolar oscillations due to the
nonradial standing accretion shock instability (SASI), whose low (dipole and
quadrupole) modes can develop large growth rates in conditions where convective
instability is damped or even suppressed. The dominance of low-mode deformation
at the time of shock revival has been recognized as a possible explanation of
large pulsar kicks and of large-scale mixing phenomena observed in supernovae
like SN 1987A.Comment: 11 pages, 6 figures; review proceeding for "Supernova 1987A: 20 Years
After: Supernovae and Gamma-Ray Bursters" AIP, New York, eds. S. Immler, K.W.
Weiler, and R. McCra
Quench from Mott Insulator to Superfluid
We study a linear ramp of the nearest-neighbor tunneling rate in the
Bose-Hubbard model driving the system from the Mott insulator state into the
superfluid phase. We employ the truncated Wigner approximation to simulate
linear quenches of a uniform system in 1,2, and 3 dimensions, and in a harmonic
trap in 3 dimensions. In all these setups the excitation energy decays like one
over third root of the quench time. The -1/3 scaling arises from an
impulse-adiabatic approximation - a variant of the Kibble-Zurek mechanism -
describing a crossover from non-adiabatic to adiabatic evolution when the
system begins to keep pace with the increasing tunneling rate.Comment: 10 pages, 13 figures; version published in Phys. Rev.
Symmetry breaking bias and the dynamics of a quantum phase transition
The Kibble-Zurek mechanism predicts the formation of topological defects and
other excitations that quantify how much a quantum system driven across a
quantum critical point fails to be adiabatic. We point out that, thanks to the
divergent linear susceptibility at the critical point, even a tiny symmetry
breaking bias can restore the adiabaticity. The minimal required bias scales
like , where are the
critical exponents and is a quench time. We test this prediction by
DMRG simulations of the quantum Ising chain. It is directly applicable to the
recent emulation of quantum phase transition dynamics in the Ising chain with
ultracold Rydberg atoms.Comment: 5+1 pages, 5+1 figures; close to published versio
High-frequency QPOs as a problem in physics: non-linear resonance
The presence of a kHz frequency in LMXBs has been expected from scaling laws,
by analogy with the QPO phenomenon in HMXB X-ray pulsars. Interpretation of the
two kHz frequencies, observed in accreting neutron stars, in terms of
non-linear resonance in strong-field gravity led to the prediction of twin QPOs
in black hole systems, in a definite frequency ratio (such as 2/3). The imprint
of a subharmonic of the 401 Hz rotation rate in the frequencies of the QPOs
detected in the accreting millisecond pulsar is at once a signature of
non-linear resonance and of coupling between accretion disk modes and the
neutron star spin.Comment: presented at X-ray Timing 2003: Rossi and Beyond, Boston, November
200
- …