6,506 research outputs found
Host Galaxies of Gamma-Ray Bursts
Host galaxies are an excellent means of probing the natal environments that
generate gamma-ray bursts (GRBs). Recent work on the host galaxies of
short-duration GRBs has offered new insights into the parent stellar
populations and ages of their enigmatic progenitors. Similarly, surveys of
long-duration GRB (LGRB) host environments and their ISM properties have
produced intriguing new results with important implications for long GRB
progenitor models. These host studies are also critical in evaluating the
utility of LGRBs as potential tracers of star formation and metallicity at high
redshifts. I will summarize the latest research on LGRB host galaxies, and
discuss the resulting impact on our understanding of these events' progenitors,
energetics, and cosmological applications.Comment: 8 pages, 3 figures; to appear in Proceedings of IAU 279 "Death of
Massive Stars: Supernovae and Gamma-ray Bursts
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
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
Fluids with quenched disorder: Scaling of the free energy barrier near critical points
In the context of Monte Carlo simulations, the analysis of the probability
distribution of the order parameter , as obtained in simulation
boxes of finite linear extension , allows for an easy estimation of the
location of the critical point and the critical exponents. For Ising-like
systems without quenched disorder, becomes scale invariant at the
critical point, where it assumes a characteristic bimodal shape featuring two
overlapping peaks. In particular, the ratio between the value of at
the peaks () and the value at the minimum in-between ()
becomes -independent at criticality. However, for Ising-like systems with
quenched random fields, we argue that instead should be observed, where is the
"violation of hyperscaling" exponent. Since is substantially non-zero,
the scaling of with system size should be easily detectable in
simulations. For two fluid models with quenched disorder, versus
was measured, and the expected scaling was confirmed. This provides further
evidence that fluids with quenched disorder belong to the universality class of
the random-field Ising model.Comment: sent to J. Phys. Cond. Mat
An explanation for the curious mass loss history of massive stars: from OB stars, through Luminous Blue Variables to Wolf-Rayet stars
The stellar winds of massive stars show large changes in mass-loss rates and
terminal velocities during their evolution from O-star through the Luminous
Blue Variable phase to the Wolf-Rayet phase. The luminosity remains
approximately unchanged during these phases. These large changes in wind
properties are explained in the context of the radiation driven wind theory, of
which we consider four different models. They are due to the evolutionary
changes in radius, gravity and surface composition and to the change from
optically thin (in continuum) line driven winds to optically thick radiation
driven winds.Comment: Accepted for publication in Astronomy and Astrophysics (Letter to the
Editor
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
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