4,984 research outputs found
Thermalisation of inhomogeneous quantum scalar fields in 1+1D
Using an improved version of the Hartree approximation, allowing for
ensembles of inhomogeneous configurations, we show in a
theory, that initially the system thermalises with a Bose-Einstein
distribution. For later times and larger couplings we see deviations.Comment: Presented at CAPP 2000, 4 pages including figures, typo's correcte
Evolution of Magnetic Fields in Supernova Remnants
Supernova remnants (SNR) are now widely believed to be a source of cosmic
rays (CRs) up to an energy of 1 PeV. The magnetic fields required to accelerate
CRs to sufficiently high energies need to be much higher than can result from
compression of the circumstellar medium (CSM) by a factor 4, as is the case in
strong shocks. Non-thermal synchrotron maps of these regions indicate that
indeed the magnetic field is much stronger, and for young SNRs has a dominant
radial component while for old SNRs it is mainly toroidal. How these magnetic
fields get enhanced, or why the field orientation is mainly radial for young
remnants, is not yet fully understood. We use an adaptive mesh refinement MHD
code, AMRVAC, to simulate the evolution of supernova remnants and to see if we
can reproduce a mainly radial magnetic field in early stages of evolution. We
follow the evolution of the SNR with three different configurations of the
initial magnetic field in the CSM: an initially mainly toroidal field, a
turbulent magnetic field, and a field parallel to the symmetry axis. Although
for the latter two topologies a significant radial field component arises at
the contact discontinuity due to the Rayleigh-Taylor instability, no radial
component can be seen out to the forward shock. Ideal MHD appears not
sufficient to explain observations. Possibly a higher compression ratio and
additional turbulence due to dominant presence of CRs can help us to better
reproduce the observations in future studies.Comment: 5 pages, 3 figures. To appear in conference proceedings of "Magnetic
Fields in the Universe II" (2008), RevMexA
Revealing the obscured supernova remnant Kes 32 with Chandra
I report here on the analysis and interpretation of a Chandra observation of
the supernova remnant Kes 32. Kes 32 is rather weak in X-rays due to a large
interstellar absorption, which is found to be ~4E22 cm^-2, larger than
previously reported. Spectral analysis indicates that the ionization age of
this object is very young, with n_e t ~ 4E9 cm^-3s, and a temperature of kT_e ~
1 keV. The X-ray emission peaks at a smaller radius than in the radio. The low
ionization age suggests that Kes 32 is a young remnant. However, a young age is
in contradiction with the relatively large apparent size, which indicates an
age of several thousand years, instead of a few hundred years. This problem is
discussed in connection with Kes 32's unknown distance and its possible
association with the Norma galactic arm.Comment: Accepted for publication in the Astrophysical Journal. 7 pages, 7
figure
Scalar Field Dynamics: Classical, Quantum and in Between
Using a Hartree ensemble approximation, we investigate the dynamics of the
\f^4 model in 1+1 dimensions. We find that the fields initially thermalize with
a Bose-Einstein distribution for the fields. Gradually, however, the
distribution changes towards classical equipartition. Using suitable initial
conditions quantum thermalization is achieved much faster than the onset of
this undesirable equipartition. We also show how the numerical efficiency of
our method can be significantly improved.Comment: Presented at SEWM 2000, 6 pages including figures, reference
correcte
Twin Peaks
The on-shell imaginary part of the retarded selfenergy of massive \vr^4
theory in 1+1 dimensions is logarithmically infrared divergent. This leads to a
zero in the spectral function, separating its usual bump into two. The twin
peaks interfere in time-dependent correlation functions, which causes
oscillating modulations on top of exponential-like decay, while the usual
formulas for the decay rate fail. We see similar modulations in our numerical
results for a mean field correlator, using a Hartree ensemble approximation.Comment: Presented at SEWM 2000, language correction
Simulation and theory of fluid demixing and interfacial tension of mixtures of colloids and non-ideal polymers
An extension of the Asakura-Oosawa-Vrij model of hard sphere colloids and
non-adsorbing polymers, that takes polymer non-ideality into account through a
repulsive stepfunction pair potential between polymers, is studied with grand
canonical Monte Carlo simulations and density functional theory. Simulation
results validate previous theoretical findings for the shift of the bulk fluid
demixing binodal upon increasing strength of polymer-polymer repulsion,
promoting the tendency to mix. For increasing strength of the polymer-polymer
repulsion, simulation and theory consistently predict the interfacial tension
of the free colloidal liquid-gas interface to decrease significantly for fixed
colloid density difference in the coexisting phases, and to increase for fixed
polymer reservoir packing fraction.Comment: 10 pages, 4 figure
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