88 research outputs found
Non-spherical core collapse supernovae and nucleosynthesis
Motivated by observations of supernova SN 1987A, various authors have
simulated Rayleigh-Taylor (RT) instabilities in the envelopes of core collapse
supernovae (for a review, see Mueller 1998). The non-radial motion found in
these simulations qualitatively agreed with observations in SN 1987A, but
failed to explain the extent of mixing of newly synthesized 56Ni
quantitatively. Here we present results of a 2D hydrodynamic simulation which
re-addresses this failure and covers the entire evolution of the first 5 hours
after core bounce.Comment: 4 pages, 1 figure, LaTeX, requires espcrc1.sty. To appear in Nucl.
Phys. A., the proceedings of the conference "Nuclei in the Cosmos 2000", held
in Aarhus, Denmark, June 27-July 1, 200
The core helium flash revisited: II. Two and three-dimensional hydrodynamic simulations
We study turbulent convection during the core helium flash close to its peak
by comparing the results of two and three-dimensional hydrodynamic simulations.
We use a multidimensional Eulerian hydrodynamics code based on
state-of-the-art numerical techniques to simulate the evolution of the helium
core of a Pop I star.
Our three-dimensional hydrodynamic simulations of the evolution of a star
during the peak of the core helium flash do not show any explosive behavior.
The convective flow patterns developing in the three-dimensional models are
structurally different from those of the corresponding two-dimensional models,
and the typical convective velocities are smaller than those found in their
two-dimensional counterparts. Three-dimensional models also tend to agree
better with the predictions of mixing length theory. Our hydrodynamic
simulations show the presence of turbulent entrainment that results in a growth
of the convection zone on a dynamic time scale. Contrary to mixing length
theory, the outer part of the convection zone is characterized by a
sub-adiabatic temperature gradient.Comment: 19 pages, 18 figure
Global Anisotropies in Supernova Explosions and Pulsar Recoil
We show by two-dimensional and first three-dimensional simulations of
neutrino-driven supernova explosions that low (l=1,2) modes can dominate the
flow pattern in the convective postshock region on timescales of hundreds of
milliseconds after core bounce. This can lead to large global anisotropy of the
supernova explosion and pulsar kicks in excess of 500 km/s.Comment: 3 pages, 2 figures, contribution to Procs. 12th Workshop on Nuclear
Astrophysics, Ringberg Castle, March 22-27, 200
Core Collapse and Then? The Route to Massive Star Explosions
The rapidly growing base of observational data for supernova explosions of
massive stars demands theoretical explanations. Central of these is a
self-consistent model for the physical mechanism that provides the energy to
start and drive the disruption of the star. We give arguments why the delayed
neutrino-heating mechanism should still be regarded as the standard paradigm to
explain most explosions of massive stars and show how large-scale and even
global asymmetries can result as a natural consequence of convective overturn
in the neutrino-heating region behind the supernova shock. Since the explosion
is a threshold phenomenon and depends sensitively on the efficiency of the
energy transfer by neutrinos, even relatively minor differences in numerical
simulations can matter on the secular timescale of the delayed mechanism. To
enhance this point, we present some results of recent one- and two-dimensional
computations, which we have performed with a Boltzmann solver for the neutrino
transport and a state-of-the-art description of neutrino-matter interactions.
Although our most complete models fail to explode, the simulations demonstrate
that one is encouragingly close to the critical threshold because a modest
variation of the neutrino transport in combination with postshock convection
leads to a weak neutrino-driven explosion with properties that fulfill
important requirements from observations.Comment: 14 pages; 3 figures. Invited Review, in: ``From Twilight to
Highlight: The Physics of Supernovae'', Eds. W. Hillebrandt and B.
Leibundgut, Springer Series ``ESO Astrophysics Symposia'', Berli
Non-spherical core collapse supernovae III. Evolution towards homology and dependence on the numerical resolution
(abridged) We study the hydrodynamic evolution of a non-spherical
core-collapse supernova in two spatial dimensions. We find that our model
displays a strong tendency to expand toward the pole. We demonstrate that this
expansion is a physical property of the low-mode, SASI instability. The SASI
leaves behind a large lateral velocity gradient in the post shock layer which
affects the evolution for minutes and hours later. This results in a prolate
deformation of the ejecta and a fast advection of Ni-rich material from
moderate latitudes to the polar regions. This effect might actually be
responsible for the global asymmetry of the nickel lines in SN 1987A. The
simulations demonstrate that significant radial and lateral motions in the
post-shock region, produced by convective overturn and the SASI during the
early explosion phase, contribute to the evolution for minutes and hours after
shock revival. They lead to both later clump formation, and a significant
prolate deformation of the ejecta which are observed even as late as one week
after the explosion. As pointed out recently by Kjaer et al., such an ejecta
morphology is in good agreement with the observational data of SN 1987A.
Systematic future studies are needed to investigate how the SASI-induced
late-time lateral expansion depends on the dominant mode of the SASI, and to
which extent it is affected by the dimensionality of the simulations. The
impact on and importance of the SASI for the distribution of iron group nuclei
and the morphology of the young SNR argues for future three-dimensional
explosion and post-explosion studies on singularity-free grids that cover the
entire sphere. Given the results of our 2D resolution study, present 3D
simulations must be regarded as underresolved, and their conclusions must be
verified by a proper numerical convergence analysis in three dimensions.Comment: 16 pages, 20 figures, accepted for publication in Astronomy &
Astrophysic
Hydrodynamic simulations of the core helium flash
We describe and discuss hydrodynamic simulations of the core helium flash
using an initial model of a 1.25 M_sol star with a metallicity of 0.02 near at
its peak. Past research concerned with the dynamics of the core helium flash is
inconclusive. Its results range from a confirmation of the standard picture,
where the star remains in hydrostatic equilibrium during the flash (Deupree
1996), to a disruption or a significant mass loss of the star (Edwards 1969;
Cole & Deupree 1980). However, the most recent multidimensional hydrodynamic
study (Dearborn 2006) suggests a quiescent behavior of the core helium flash
and seems to rule out an explosive scenario. Here we present partial results of
a new comprehensive study of the core helium flash, which seem to confirm this
qualitative behavior and give a better insight into operation of the convection
zone powered by helium burning during the flash. The hydrodynamic evolution is
followed on a computational grid in spherical coordinates using our new version
of the multi-dimensional hydrodynamic code HERAKLES, which is based on a direct
Eulerian implementation of the piecewise parabolic method.Comment: 6 pages, 5 figures. IAUS 252 Conference Proceeding (Sanya, China):
"The art of modeling stars in the 21st century
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