385 research outputs found
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
African Farm/Family Forestry and Producer Organizations for improved livelihoods and sustainable landscape management
The paper tries to provide answers to questions on the status of farm/family forestry in Africa
in terms of opportunities and challenges, their contribution to economic development and
sustainable landscape management. In the paper, highlights are made on issues related to land tenure, institutional settings and other supporting policies. In addition, market access and factors that may influence the development of forest products value chains are discussed alongside the possibilities for promoting and strengthening cohesive actions amongst FFPOs in different countries and regions. Answers are provided on questions related to easiness to group formation and registration and their product base. Examples of such groups in terms of associations or federations at local, national and regional levels alongside some examples are briefly described. Information on how FFPOs are currently helping forestry farmers are provided with some examples clearly started including their future potentials and/or pitfalls. At the end of the paper, the outlook in the coming 5 to 10 years for the main opportunities and challenges awaiting farm/family forestry and related FFPOs in Africa are pinpointed
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
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
A new radiative cooling curve based on an up to date plasma emission code
This work presents a new plasma cooling curve that is calculated using the
SPEX package. We compare our cooling rates to those in previous works, and
implement the new cooling function in the grid-adaptive framework `AMRVAC'.
Contributions to the cooling rate by the individual elements are given, to
allow for the creation of cooling curves tailored to specific abundance
requirements. In some situations, it is important to be able to include
radiative losses in the hydrodynamics. The enhanced compression ratio can
trigger instabilities (such as the Vishniac thin-shell instability) that would
otherwise be absent. For gas with temperatures below 10,000 K, the cooling time
becomes very long and does not affect the gas on the timescales that are
generally of interest for hydrodynamical simulations of circumstellar plasmas.
However, above this temperature, a significant fraction of the elements is
ionised, and the cooling rate increases by a factor 1000 relative to lower
temperature plasmas.Comment: 11 pages, 6 figures. Typos fixed to match version on A&A
'forthcoming' website. Tables in text format online available at
http://www.phys.uu.nl/~schure/coolin
Composite structures journal «National Geographic Ukraine»: compliance with existing norms
У статті з’ясовуються особливості системи рубрикації у науково-популярному
журналі «National Geographic. Україна». Досліджуються переваги та недоліки, прийоми та
засоби рубрикації. Аналізуються особливості видання та їх вплив на графічне, композиційне
та смислове виконання рубрик.This article presents aspects of editorial classification systems of «National Geographic.
Ukraine» popular science journal. Benefits and drawbacks, methods and tools of editorial
classification are explored here. Author analyzes edition features, their influence on visual,
compositional and semantic composition of sections
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