Issue |
A&A
Volume 408, Number 2, September III 2003
|
|
---|---|---|
Page(s) | 621 - 649 | |
Section | Stellar structure and evolution | |
DOI | https://doi.org/10.1051/0004-6361:20030863 | |
Published online | 17 November 2003 |
Non-spherical core collapse supernovae
I. Neutrino-driven convection, Rayleigh-Taylor instabilities, and the formation and propagation of metal clumps
1
Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Straße 1, 85741 Garching, Germany
2
Nicolaus Copernicus Astronomical Center, Bartycka 18, 00716 Warsaw, Poland
3
Center for Astrophysical Thermonuclear Flashes, University of Chicago, 5640 S. Ellis Avenue, Chicago, IL 60637, USA
K. Kifonidis
Received:
11
February
2003
Accepted:
28
May
2003
We have performed two-dimensional simulations of core
collapse supernovae that encompass shock revival by neutrino
heating, neutrino-driven convection, explosive
nucleosynthesis, the growth of Rayleigh-Taylor
instabilities, and the propagation of newly formed metal
clumps through the exploding star. A simulation of a type II
explosion in a blue supergiant
progenitor is presented, that confirms our earlier type II
models and extends their validity to times as late as 5.5
hours after core bounce. We also study a type Ib-like
explosion, by simply removing the hydrogen envelope of the
progenitor model. This allows for a first comparison of
type II and type Ib evolution. We present evidence that the
hydrodynamics of core collapse supernovae beyond shock
revival differs markedly from the results of simulations
that have followed the Rayleigh-Taylor mixing starting from
ad hoc energy deposition schemes to initiate the
explosion. We find iron group elements to be synthesized in
an anisotropic, dense, low-entropy shell that expands with
velocities of ~
km s-1 shortly after shock
revival. The growth of Rayleigh-Taylor instabilities at the
Si/O and (C+O)/He composition interfaces of the progenitor,
seeded by the flow-structures resulting from neutrino-driven
convection, leads to a fragmentation of this shell into
metal-rich “clumps”. This fragmentation starts already
~
s after core bounce and is complete within the
first few minutes of the explosion. During this time the
clumps are slowed down by drag, and by the positive pressure
gradient in the unstable layers. However, at
s they decouple from the flow and start to propagate
ballistically and subsonically through the He core, with the
maximum velocities of metals remaining constant at ~
km s-1. This early “clump decoupling” leads to
significantly higher
velocities at
s than in one-dimensional models of the explosion,
demonstrating that multi-dimensional effects which are at
work within the first minutes, and which have been neglected
in previous studies (especially in those which dealt with
the mixing in type II supernovae), are crucial. Despite
comparably high initial maximum nickel velocities in both
our type II and our type Ib-like model, we find that there
are large differences in the final maximum nickel velocities
between both cases. In the “type Ib” model the maximum
velocities of metals remain frozen in at ~
km s-1 for
s, while in the type II model
they drop significantly for
s. In the latter
case, the massive hydrogen envelope of the progenitor forces
the supernova shock to slow down strongly, leaving behind a
reverse shock and a dense helium shell (or “wall”) below
the He/H interface. After penetrating into this dense
material the metal-rich clumps possess supersonic speeds,
before they are slowed down by drag forces to ~
km s-1 at a time of 20 000 s post-bounce. While, due
to this deceleration, the maximum velocities of iron-group
elements in SN 1987 A cannot be reproduced in case of the
considered
progenitor, the “type Ib”
model is in fairly good agreement with observed clump
velocities and the amount of mixing inferred for type Ib
supernovae. Thus it appears promising for calculations of
synthetic spectra and light curves. Furthermore, our
simulations indicate that for type Ib explosions the pattern
of clump formation in the ejecta is correlated with the
structure of the convective pattern prevailing during the
shock-revival phase. This might be used to deduce
observational constraints for the dynamics during this early
phase of the evolution, and the role of neutrino heating in
initiating the explosion.
Key words: hydrodynamics / instabilities / nuclear reactions, nucleosynthesis abundances / shock waves / stars: supernovae: general
© ESO, 2003
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