3,745 research outputs found
The Kinematics of Kepler's Supernova Remnant as revealed by Chandra
I determine the expansion of the supernova remnant of SN1604 (Kepler's
supernova) based on archival Chandra ACIS-S observations made in 2000 and 2006.
The measurements were done in several distinct energy bands, and were made for
the remnant as a whole, and for six individual sectors. The average expansion
parameter indicates that the remnant expands as , but there
are significant differences in different parts of the remnant: the bright
northwestern part expands as , whereas the rest of the
remnant's expansion shows an expansion . The latter is
consistent with an explosion in which the outer part of the ejecta has a
negative power law slope for density () of , or with
an exponential density profile(). The expansion
parameter in the southern region, in conjunction with the shock radius,
indicate a rather low value (<5E50 erg) for the explosion energy of SN1604 for
a distance of 4 kpc. An higher explosion energy is consistent with the results,
if the distance is larger.
The filament in the eastern part of the remnant, which is dominated by X-ray
synchrotron radiation seems to mark a region with a fast shock speed , corresponding to a shock velocity of v= 4200 km/s, for a distance to
SN1604 of 4 kpc. This is consistent with the idea that X-ray synchrotron
emission requires shock velocities in excess of ~2000 km/s.
The X-ray based expansion measurements reported are consistent with results
based on optical and radio measurements, but disagree with previous X-ray
measurements based on ROSAT and Einstein observations.Comment: Accepted for publication in ApJ. This new version is the accepted
version, which differs mainly in the discussion sectio
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
THE COMPETITIVENESS OF WESTERN CAPE WHEAT PRODUCTION: AN INTERNATIONAL COMPARISON
This paper reports the results of an international comparison of the cost of producing wheat in 8 Western Cape, 3 Free State and 7 foreign producing areas. Results show that South African yields are low compared to foreign countries whose production costs are as high as or higher than those in South Africa, while the net margins for South African producers are less than a third of those for countries that have the same or lower yields as South Africa. If the wheat industry in the Western Cape is to survive international competition, it will have to create its international competitiveness.Crop Production/Industries, International Relations/Trade,
The relation between post-shock temperature, cosmic-ray pressure and cosmic-ray escape for non-relativistic shocks
Supernova remnants are thought to be the dominant source of Galactic cosmic
rays. This requires that at least 5% of the available energy is transferred to
cosmic rays, implying a high cosmic-ray pressure downstream of supernova
remnant shocks. Recently, it has been shown that the downstream temperature in
some remnants is low compared to the measured shock velocities, implying that
additional pressure support by accelerated particles is present.
Here we use a two-fluid thermodynamic approach to derive the relation between
post-shock fractional cosmic-ray pressure and post-shock temperature, assuming
no additional heating beyond adiabatic heating in the shock precursor and with
all non-adiabatic heating occurring at the subshock. The derived relations show
that a high fractional cosmic-ray pressure is only possible, if a substantial
fraction of the incoming energy flux escapes from the system. Recently a shock
velocity and a downstream proton temperature were measured for a shock in the
supernova remnant RCW 86. We apply the two-fluid solutions to these
measurements and find that the the downstream fractional cosmic-ray pressure is
at least 50% with a cosmic-ray energy flux escape of at least 20%. In general,
in order to have 5% of the supernova energy go into accelerating cosmic rays,
on average the post-shock cosmic-ray pressure needs to be 30% for an effective
cosmic-ray adiabatic index of 4/3.Comment: 9 pages, 6 color figures. This is updated with a corrected figure 5a
and 5b, reflecting an ApJ erratu
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
Jets as diagnostics of the circumstellar medium and the explosion energetics of supernovae: the case of Cas A
We present hydrodynamical models for the Cassiopeia A (Cas A) supernova
remnant and its observed jet / counter-jet system. We include the evolution of
the progenitor's circumstellar medium, which is shaped by a slow red supergiant
wind that is followed by a fast Wolf-Rayet (WR) wind.
The main parameters of the simulations are the duration of the WR phase and
the jet energy. We find that the jet is destroyed if the WR phase is
sufficiently long and a massive circumstellar shell has formed. We therefore
conclude that the WR phase must have been short (a few thousand yr), if present
at all. Since the actual jet length of Cas A is not known we derive a lower
limit for the jet energy, which is ~10^{48} erg. We discuss the implications
for the progenitor of Cas A and the nature of its explosion.Comment: 9 pages, 5 figures, ApJ accepted. Version with high resolution
figures available at http://www.phys.uu.nl/~schure/CasA_jet.pd
Progenitor's signatures in Type Ia supernova remnants
The remnants of Type Ia supernovae can provide important clues about their
progenitor-histories. We discuss two well-observed supernova remnants (SNRs)
that are believed to result from a Type Ia SN and use various tools to shed
light on the possible progenitor history. We find that Kepler's SNR is
consistent with a symbiotic binary progenitor consisted of a white dwarf and an
AGB star. Our hydrosimulations can reproduce the observed kinematic and
morphological properties. For Tycho's remnant we use the characteristics of the
X-ray spectrum and the kinematics to show that the ejecta has likely interacted
with dense circumstellar gas.Comment: 4 pages, 9 figures, proceedings for IAU Symposium 281, Padova, July
201
Modeling the interaction of thermonuclear supernova remnants with circumstellar structures: The case of Tycho's supernova remnant
The well-established Type Ia remnant of Tycho's supernova (SN 1572) reveals
discrepant ambient medium density estimates based on either the measured
dynamics or on the X-ray emission properties. This discrepancy can potentially
be solved by assuming that the supernova remnant (SNR) shock initially moved
through a stellar wind bubble, but is currently evolving in the uniform
interstellar medium with a relatively low density.
We investigate this scenario by combining hydrodynamical simulations of the
wind-loss phase and the supernova remnant evolution with a coupled X-ray
emission model, which includes non-equilibrium ionization. For the explosion
models we use the well-known W7 deflagration model and the delayed detonation
model that was previously shown to provide good fits to the X-ray emission of
Tycho's SNR.
Our simulations confirm that a uniform ambient density cannot simultaneously
reproduce the dynamical and X-ray emission properties of Tycho. In contrast,
models that considered that the remnant was evolving in a dense, but small,
wind bubble reproduce reasonably well both the measured X-ray emission spectrum
and the expansion parameter of Tycho's SNR. Finally, we discuss possible mass
loss scenarios in the context of single- and double-degenerate models which
possible could form such a small dense wind bubble.Comment: 12 pages, 7 figures, accepted for publication in MNRA
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