1,060 research outputs found
Neutrino-driven wind and wind termination shock in supernova cores
The neutrino-driven wind from a nascent neutron star at the center of a
supernova expands into the earlier ejecta of the explosion. Upon collision with
this slower matter the wind material is decelerated in a wind termination
shock. By means of hydrodynamic simulations in spherical symmetry we
demonstrate that this can lead to a large increase of the wind entropy,
density, and temperature, and to a strong deceleration of the wind expansion.
The consequences of this phenomenon for the possible r-process nucleosynthesis
in the late wind still need to be explored in detail. Two-dimensional models
show that the wind-ejecta collision is highly anisotropic and could lead to a
directional dependence of the nucleosynthesis even if the neutrino-driven wind
itself is spherically symmetric.Comment: 6 pages, 3 figures, International Symposium on Nuclear Astrophysics -
Nuclei in the Cosmos - IX, CERN, Geneva, Switzerland, 25-30 June, 200
Nucleosynthesis-relevant conditions in neutrino-driven supernova outflows: I. Spherically symmetric hydrodynamic simulations
We investigate the behavior and consequences of the reverse shock that terminates the supersonic expansion of the baryonic wind which is driven by neutrino heating off the surface of (non-magnetized) new-born neutron stars in supernova cores. To this end we perform long-time hydrodynamic simulations in spherical symmetry. In agreement with previous relativistic wind studies, we find that the neutrino-driven outflow accelerates to supersonic velocities and in case of a compact, about 1.4 solar mass (gravitational mass) neutron star with a radius of about 10 km, the wind reaches entropies of about 100 k_B per nucleon. The wind, however, is strongly influenced by the environment of the supernova core. It is decelerated and shock-heated abruptly by a termination shock that forms when the supersonic outflow collides with the slower preceding supernova ejecta. The radial position of this reverse shock varies with time and depends on the strength of the neutrino wind and the different conditions in progenitor stars with different masses and structure. Its basic properties and behavior can be understood by simple analytic considerations. We demonstrate that the entropy of matter going through the reverse shock can increase to a multiple of the asymptotic wind value. Seconds after the onset of the explosion it therefore can exceed 400 k_B per nucleon. The temperature of the shocked wind has typically dropped to about or less than 10^9 K, and density and temperature in the shock-decelerated matter continue to decrease only very slowly. Such conditions might strongly affect the important phases of supernova nucleosynthesis in a time and progenitor dependent way. (abridged
Supernova explosions and the birth of neutron stars
We report here on recent progress in understanding the birth conditions of
neutron stars and the way how supernovae explode. More sophisticated numerical
models have led to the discovery of new phenomena in the supernova core, for
example a generic hydrodynamic instability of the stagnant supernova shock
against low-mode nonradial deformation and the excitation of gravity-wave
activity in the surface and core of the nascent neutron star. Both can have
supportive or decisive influence on the inauguration of the explosion, the
former by improving the conditions for energy deposition by neutrino heating in
the postshock gas, the latter by supplying the developing blast with a flux of
acoustic power that adds to the energy transfer by neutrinos. While recent
two-dimensional models suggest that the neutrino-driven mechanism may be viable
for stars from about 8 solar masses to at least 15 solar masses, acoustic
energy input has been advocated as an alternative if neutrino heating fails.
Magnetohydrodynamic effects constitute another way to trigger explosions in
connection with the collapse of sufficiently rapidly rotating stellar cores,
perhaps linked to the birth of magnetars. The global explosion asymmetries seen
in the recent simulations offer an explanation of even the highest measured
kick velocities of young neutron stars.Comment: 10 pages, 8 figures, 19 ps files; to be published in Proc. of Conf.
"40 Years of Pulsars: Millisecond Pulsars, Magnetars, and More", August
12-17, 2007, McGill Univ., Montreal, Canada; high-resolution images can be
obtained upon request; incorrect panel in fig.8 replace
3D simulations of Rayleigh-Taylor mixing in core-collapse SNe with CASTRO
We present multidimensional simulations of the post-explosion hydrodynamics
in three different 15 solar mass supernova models with zero, 10^{-4} solar
metallicity, and solar metallicities. We follow the growth of the
Rayleigh-Taylor instability that mixes together the stellar layers in the wake
of the explosion. Models are initialized with spherically symmetric explosions
and perturbations are seeded by the grid. Calculations are performed in
two-dimensional axisymmetric and three-dimensional Cartesian coordinates using
the new Eulerian hydrodynamics code, CASTRO. We find as in previous work, that
Rayleigh-Taylor perturbations initially grow faster in 3D than in 2D. As the
Rayleigh-Taylor fingers interact with one another, mixing proceeds to a greater
degree in 3D than in 2D, reducing the local Atwood number and slowing the
growth rate of the instability in 3D relative to 2D. By the time mixing has
stopped, the width of the mixed region is similar in 2D and 3D simulations
provided the Rayleigh-Taylor fingers show significant interaction. Our results
imply that 2D simulations of light curves and nucleosynthesis in supernovae
(SNe) that die as red giants may capture the features of an initially
spherically symmetric explosion in far less computational time than required by
a full 3D simulation. However, capturing large departures from spherical
symmetry requires a significantly perturbed explosion. Large scale asymmetries
cannot develop through an inverse cascade of merging Rayleigh-Taylor
structures; they must arise from asymmetries in the initial explosion.Comment: 12 pages, 5 figures, ApJ accepte
An axis-free overset grid in spherical polar coordinates for simulating 3D self-gravitating flows
A type of overlapping grid in spherical coordinates called the Yin-Yang grid
is successfully implemented into a 3D version of the explicit Eulerian
grid-based code PROMETHEUS including self-gravity. The modified code
successfully passed several standard hydrodynamic tests producing results which
are in very good agreement with analytic solutions. Moreover, the solutions
obtained with the Yin-Yang grid exhibit no peculiar behaviour at the boundary
between the two grid patches. The code has also been successfully used to model
astrophysically relevant situations, namely equilibrium polytropes, a
Taylor-Sedov explosion, and Rayleigh-Taylor instabilities. According to our
results, the usage of the Yin-Yang grid greatly enhances the suitability and
efficiency of 3D explicit Eulerian codes based on spherical polar coordinates
for astrophysical flows.Comment: 15 pages, 17 figures, 2 tables, accepted for publication in A&
Does gravity modelling justify a rifted "Ligurian Basin"?
The geo-historical development of the Ligurian Basin and the structure of the crust and upper mantle in this area are still being discussed. Yet it remains unclear if rifting caused continental break-up and seafloor spreading and one of the key questions is whether rifting can be identified in geophysical measurements. For our investigations we had the following updated data sets at our disposal: the new gravity maps of the AlpArray Gravity Working Group (complete Bouguer - CBA, Free air, and isostatic anomalies) the seismic results of the Lobster campaigns of our GEOMAR partners in the SPP MB4D as well as the dynamic modelling results from our own subproject. The constraining data are supplemented with seismic profile data from French and Italian offshore campaigns, as far as they are usable in publications for us. The GFZ modelling software IGMAS+ was used for an interactive 3D modelling. The resulting model contains density inhomogeneities in the crust as well as in the upper mantle down to a depth of 300 km following the results of dynamic models of our own subproject. Due to the special hybrid modelling of the crust (by polygonal structures) and the upper mantle (by voxels of recent velocity models), the individual contributions to the gravity field are clearly separable. As a further special feature, we point out that the density model used is based on the gravity modelling from the first phase of the SPP MB4D (our former subproject INTEGRATE). Thus, a largely consistent 3D density model for both the Alps and the Ligurian Sea is available for interpretation. The constrained 3D modelling of the gravity field, as well as numerical analyses of the fields (terracing, clustering, filtering, curvature), calculations of the vertical stress and Gravity Potential Energy (GPE) suggest that a rift structure in the area of the Ligurian Sea can be identified and mapped. The interactive modelling is supported by the use of geological maps in the Ligurian Sea area. By overlaying the model gravity maps and the geological maps, the good agreement becomes visible â refer to the attached figure
Over-representation of specific regions of chromosome 22 in cells from human glioma correlate with resistance to 1,3-bis(2-chloroethyl)-1-nitrosourea
BACKGROUND: Glioblastoma multiforme is the most malignant form of brain tumor. Despite treatment including surgical resection, adjuvant chemotherapy, and radiation, these tumors typically recur. The recurrent tumor is often resistant to further therapy with the same agent, suggesting that the surviving cells that repopulate the tumor mass have an intrinsic genetic advantage. We previously demonstrated that cells selected for resistance to 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU) are near-diploid, with over-representation of part or all of chromosomes 7 and 22. While cells from untreated gliomas often have over-representation of chromosome 7, chromosome 22 is typically under-represented. METHODS: We have analyzed cells from primary and recurrent tumors from the same patient before and after in vitro selection for resistance to clinically relevant doses of BCNU. Karyotypic analyses were done to demonstrate the genetic makeup of these cells, and fluorescent in situ hybridization analyses have defined the region(s) of chromosome 22 retained in these BCNU-resistant cells. RESULTS: Karyotypic analyses demonstrated that cells selected for BCNU resistance were near-diploid with over-representation of chromosomes 7 and 22. In cells where whole copies of chromosome 22 were not identified, numerous fragments of this chromosome were retained and inserted into several marker and derivative chromosomes. Fluorescent in situ hybridization analyses using whole chromosome paints confirmed this finding. Additional FISH analysis using bacterial artificial chromosome probes spanning the length of chromosome 22 have allowed us to map the over-represented region to 22q12.3â13.32. CONCLUSION: Cells selected for BCNU resistance either in vivo or in vitro retain sequences mapped to chromosome 22. The specific over-representation of sequences mapped to 22q12.3â13.32 suggest the presence of a DNA sequence important to BCNU survival and/or resistance located in this region of chromosome 22
Neutrino signatures of supernova shock and reverse shock propagation
A few seconds after bounce in a core-collapse supernova, the shock wave
passes the density region corresponding to resonant neutrino oscillations with
the ``atmospheric'' neutrino mass difference. The transient violation of the
adiabaticity condition manifests itself in an observable modulation of the
neutrino signal from a future galactic supernova. In addition to the shock wave
propagation effects that were previously studied, a reverse shock forms when
the supersonically expanding neutrino-driven wind collides with the slower
earlier supernova ejecta. This implies that for some period the neutrinos pass
two subsequent density discontinuities, giving rise to a ``double dip'' feature
in the average neutrino energy as a function of time. We study this effect both
analytically and numerically and find that it allows one to trace the positions
of the forward and reverse shocks. We show that the energy dependent neutrino
conversion probabilities allow one to detect oscillations even if the energy
spectra of different neutrino flavors are the same as long as the fluxes
differ. These features are observable in the \bar\nu_e signal for an inverted
and in the \nu_e signal for a normal neutrino mass hierarchy, provided the
13-mixing angle is ``large'' (sin^2\theta_{13}\gg 10^{-5}).Comment: 23 pages, 27 eps figures (high resolution plots are available on
request), JCAP style; v2: figure 8 extended, matches published versio
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