165 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 III. From Pop I to Pop III stars
Degenerate ignition of helium in low-mass stars at the end of the red giant
branch phase leads to dynamic convection in their helium cores. One-dimensional
(1D) stellar modeling of this intrinsically multi-dimensional dynamic event is
likely to be inadequate. Previous hydrodynamic simulations imply that the
single convection zone in the helium core of metal-rich Pop I stars grows
during the flash on a dynamic timescale. This may lead to hydrogen injection
into the core, and a double convection zone structure as known from
one-dimensional core helium flash simulations of low-mass Pop III stars. We
perform hydrodynamic simulations of the core helium flash in two and three
dimensions to better constrain the nature of these events. To this end we study
the hydrodynamics of convection within the helium cores of a 1.25 \Msun
metal-rich Pop I star (Z=0.02), and a 0.85 \Msun metal-free Pop III star (Z=0)
near the peak of the flash. These models possess single and double convection
zones, respectively. We use 1D stellar models of the core helium flash computed
with state-of-the-art stellar evolution codes as initial models for our
multidimensional hydrodynamic study, and simulate the evolution of these models
with the Riemann solver based hydrodynamics code Herakles which integrates the
Euler equations coupled with source terms corresponding to gravity and nuclear
burning. The hydrodynamic simulation of the Pop I model involving a single
convection zone covers 27 hours of stellar evolution, while the first
hydrodynamic simulations of a double convection zone, in the Pop III model,
span 1.8 hours of stellar life. We find differences between the predictions of
mixing length theory and our hydrodynamic simulations. The simulation of the
single convection zone in the Pop I model shows a strong growth of the size of
the convection zone due to turbulent entrainment. Hence we predict that for the
Pop I model a hydrogen injection phase (i.e. hydrogen injection into the helium
core) will commence after about 23 days, which should eventually lead to a
double convection zone structure known from 1D stellar modeling of low-mass Pop
III stars. Our two and three-dimensional hydrodynamic simulations of the double
(Pop III) convection zone model show that the velocity field in the convection
zones is different from that predicted by stellar evolutionary calculations.
The simulations suggest that the double convection zone decays quickly, the
flow eventually being dominated by internal gravity waves.Comment: 16 pages, 18 figures, submitted to Aa
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
Two-Dimensional Hydrodynamic Core-Collapse Supernova Simulations with Spectral Neutrino Transport II. Models for Different Progenitor Stars
1D and 2D supernova simulations for stars between 11 and 25 solar masses are
presented, making use of the Prometheus/Vertex neutrino-hydrodynamics code,
which employs a full spectral treatment of the neutrino transport.
Multi-dimensional transport aspects are treated by the ``ray-by-ray plus''
approximation described in Paper I. Our set of models includes a 2D calculation
for a 15 solar mass star whose iron core is assumed to rotate rigidly with an
angular frequency of 0.5 rad/s before collapse. No important differences were
found depending on whether random seed perturbations for triggering convection
are included already during core collapse, or whether they are imposed on a 1D
collapse model shortly after bounce. Convection below the neutrinosphere sets
in about 40 ms p.b. at a density above 10**12 g/cm^3 in all 2D models, and
encompasses a layer of growing mass as time goes on. It leads to a more
extended proto-neutron star structure with accelerated lepton number and energy
loss and significantly higher muon and tau neutrino luminosities, but reduced
mean energies of the radiated neutrinos, at times later than ~100 ms p.b. In
case of an 11.2 solar mass star we find that low (l = 1,2) convective modes
cause a probably rather weak explosion by the convectively supported
neutrino-heating mechanism after ~150 ms p.b. when the 2D simulation is
performed with a full 180 degree grid, whereas the same simulation with 90
degree wedge fails to explode like all other models. This sensitivity
demonstrates the proximity of our 2D models to the borderline between success
and failure, and stresses the need of simulations in 3D, ultimately without the
axis singularity of a polar grid. (abridged)Comment: 42 pages, 44 figures; revised according to referee comments; accepted
to Astronomy & Astrophysic
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
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
Multi-dimensional nucleosynthesis calculations of Type II SNe
We investigate explosive nuclear burning in core collapse supernovae by
coupling a tracer particle method to one and two-dimensional Eulerian
hydrodynamic calculations. Adopting the most recent experimental and
theoretical nuclear data, we compute the nucleosynthetic yields for 15 Msun
stars with solar metallicity, by post-processing the temperature and density
history of advected tracer particles. We compare our results to 1D calculations
published in the literature.Comment: 10 pages, to appear in Carnegie Observatories Astrophysics Series,
Vol. 4: Origin and Evolution of the Elements, ed. A. McWilliam and M. Rauch
(Pasadena: Carnegie Observatories
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