6,466 research outputs found
Modeling SNR Cassiopeia A from the Supernova Explosion to its Current Age: The role of post-explosion anisotropies of ejecta
The remnants of core-collapse supernovae (SNe) have complex morphologies that
may reflect asymmetries and structures developed during the progenitor SN
explosion. Here we investigate how the morphology of the SNR Cassiopeia A (Cas
A) reflects the characteristics of the progenitor SN with the aim to derive the
energies and masses of the post-explosion anisotropies responsible for the
observed spatial distribution of Fe and Si/S. We model the evolution of Cas A
from the immediate aftermath of the progenitor SN to the three-dimensional
interaction of the remnant with the surrounding medium. The post-explosion
structure of the ejecta is described by small-scale clumping of material and
larger-scale anisotropies. The hydrodynamic multi-species simulations consider
an appropriate post-explosion isotopic composition of the ejecta. The observed
average expansion rate and shock velocities can be well reproduced by models
with ejecta mass and explosion energy erg. The post-explosion anisotropies (pistons)
reproduce the observed distributions of Fe and Si/S if they had a total mass of
and a total kinetic energy of erg. The pistons produce a spatial inversion of ejecta layers at the
epoch of Cas A, leading to the Si/S-rich ejecta physically interior to the
Fe-rich ejecta. The pistons are also responsible for the development of bright
rings of Si/S-rich material which form at the intersection between the reverse
shock and the material accumulated around the pistons during their propagation.
Our result supports the idea that the bulk of asymmetries observed in Cas A are
intrinsic to the explosion.Comment: 19 pages, 14 Figures; accepted for publication on Ap
Supernova 1987A: a Template to Link Supernovae to their Remnants
The emission of supernova remnants reflects the properties of both the
progenitor supernovae and the surrounding environment. The complex morphology
of the remnants, however, hampers the disentanglement of the two contributions.
Here we aim at identifying the imprint of SN 1987A on the X-ray emission of its
remnant and at constraining the structure of the environment surrounding the
supernova. We performed high-resolution hydrodynamic simulations describing SN
1987A soon after the core-collapse and the following three-dimensional
expansion of its remnant between days 1 and 15000 after the supernova. We
demonstrated that the physical model reproducing the main observables of SN
1987A during the first 250 days of evolution reproduces also the X-ray emission
of the subsequent expanding remnant, thus bridging the gap between supernovae
and supernova remnants. By comparing model results with observations, we
constrained the explosion energy in the range ~erg and
the envelope mass in the range . We found that the shape of
X-ray lightcurves and spectra at early epochs (<15 years) reflects the
structure of outer ejecta: our model reproduces the observations if the
outermost ejecta have a post-explosion radial profile of density approximated
by a power law with index . At later epochs, the shapes of X-ray
lightcurves and spectra reflect the density structure of the nebula around SN
1987A. This enabled us to ascertain the origin of the multi-thermal X-ray
emission, to disentangle the imprint of the supernova on the remnant emission
from the effects of the remnant interaction with the environment, and to
constrain the pre-supernova structure of the nebula.Comment: 16 pages, 11 Figures; accepted for publication on Ap
Shape invariance through Crum transformation
We show in a rigorous way that Crum's result on equal eigenvalue spectrum of
Sturm-Liouville problems can be obtained iteratively by successive Darboux
transformations. It can be shown that all neighbouring Darboux-transformed
potentials of higher order, u_{k} and u_{k+1}, satisfy the condition of shape
invariance provided the original potential u does. We use this result to proof
that under the condition of shape invariance the n-th iteration of the original
Sturm-Liouville problem defined through shape invariance is equal to the n-th
Crum transformationComment: 26 pp, one more reference, J.-M. Sparenberg and D. Baye, J. Phys. A
28, 5079 (1995), has been added as Ref. 18 in the published version, which
has 47 ref
Spin-Electron-Phonon Excitation in Re-based Half-Metallic Double Perovskites
A remarkable hardening (~ 30 cm-1) of the normal mode of vibration associated
with the symmetric stretching of the oxygen octahedra for the Ba2FeReO6 and
Sr2CrReO6 double perovskites is observed below the corresponding magnetic
ordering temperatures. The very large magnitude of this effect and its absence
for the anti-symmetric stretching mode provide evidence against a conventional
spin-phonon coupling mechanism. Our observations are consistent with a
collective excitation formed by the combination of the vibrational mode with
oscillations of local Fe or Cr 3d and Re 5d occupations and spin magnitudes.Comment: 12 pages, 4 figure
Resistivity study of the pseudogap phase for (Hg,Re) - 1223 superconductors
The pseudogap phase above the critical temperature of high
superconductors (HTSC) presents different energy scales and it is currently a
matter of intense study. The complexity of the HTSC normal state requires very
accurate measurements with the purpose of distinguishing different types of
phenomena. Here we have performed systematically studies through electrical
resistivity () measurements by several different current densities in
order to obtain an optimal current for each sample. This approach allows to
determine reliable values of the pseudogap temperature , the layer
coupling temperature between the superconductor layers , the
fluctuation temperature and the critical temperature as
function of the doping . The interpretation of these different temperature
scales allows to characterize possible scenarios for the (Hg,Re) - 1223 normal
state. This method, described in detail here, and used to derive the
(Hg,Re)-1223 phase diagram is general and can be applied to any HTSC.Comment: 31 pages, 12 figures, Latex; 25 pages, LaTeX; 11 figures; rewrited
section II and III; added 18 reference; rewrited title, added discussion
sectio
An Investigation of the -type Lorentz-Symmetry Breaking Gauge Models in -Supersymmetric Scenario
In this work, we present two possible venues to accomodate the -type
Lorentz-symmetry violating Electrodynamics in an -supersymmetric
framework. A chiral and a vector superfield are chosen to describe the
background that signals Lorentz-symmetry breaking. In each case, the -tensor is expressed in terms of the components of the
background superfield that we choose to describe the breaking. We also present
in detail the actions with all fermionic partners of the background that
determine .Comment: 10 page
Radiative accretion shocks along nonuniform stellar magnetic fields in classical T Tauri stars
(abridged) AIMS. We investigate the dynamics and stability of post-shock
plasma streaming along nonuniform stellar magnetic fields at the impact region
of accretion columns. We study how the magnetic field configuration and
strength determine the structure, geometry, and location of the shock-heated
plasma. METHODS. We model the impact of an accretion stream onto the
chromosphere of a CTTS by 2D axisymmetric magnetohydrodynamic simulations. Our
model takes into account the gravity, the radiative cooling, and the
magnetic-field-oriented thermal conduction. RESULTS. The structure, stability,
and location of the shocked plasma strongly depend on the configuration and
strength of the magnetic field. For weak magnetic fields, a large component of
B may develop perpendicular to the stream at the base of the accretion column,
limiting the sinking of the shocked plasma into the chromosphere. An envelope
of dense and cold chromospheric material may also develop around the shocked
column. For strong magnetic fields, the field configuration determines the
position of the shock and its stand-off height. If the field is strongly
tapered close to the chromosphere, an oblique shock may form well above the
stellar surface. In general, a nonuniform magnetic field makes the distribution
of emission measure vs. temperature of the shocked plasma lower than in the
case of uniform magnetic field. CONCLUSIONS. The initial strength and
configuration of the magnetic field in the impact region of the stream are
expected to influence the chromospheric absorption and, therefore, the
observability of the shock-heated plasma in the X-ray band. The field strength
and configuration influence also the energy balance of the shocked plasma, its
emission measure at T > 1 MK being lower than expected for a uniform field. The
above effects contribute in underestimating the mass accretion rates derived in
the X-ray band.Comment: 11 pages, 11 Figures; accepted for publication on A&A. Version with
full resolution images can be found at
http://www.astropa.unipa.it/~orlando/PREPRINTS/sorlando_accretion_shocks.pd
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