Fig. 1:
SuperMUC supercomputer of the Leibniz Computing Center (LRZ)
Image copyrights: LRZ 2012
Fig. 2:
MareNostrum supercomputer of the Barcelona Supercomputing Center (BSC)
Image copyrights: BSC 2013
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Fig. 4:
Interactive 3D graphics corresponding to the images of Fig. 3.
Latest three-dimensional computer simulations are closing in on the solution of an decades-old problem: how do massive stars die in gigantic supernova explosions? Since the mid-1960s, astronomers thought that neutrinos, elementary particles that are radiated in huge numbers by the newly formed neutron star, could be the ones to energize the blast wave that disrupts the star. However, only now the power of modern supercomputers has made it possible to actually demonstrate the viability of this neutrino-driven mechanism.
Supernovae are among the brightest and most violent explosive events in the Universe. They are not only the birth sites of neutron stars and black holes; they also produce and disseminate heavy chemical elements up to iron and possibly even nuclear species heavier than iron, which could be forged during the explosion. Understanding the explosion mechanism of massive stars is therefore of fundamental importance to better define the role of supernovae in the cosmic cycle of matter.
Stars with more than about eight times the mass of our sun evolve
by "burning" nuclear fuel to successively heavier chemical elements,
thus converting hydrogen to helium, carbon, oxygen, sulfur and silicon,
until a dense, degenerate core mostly made of iron builds up in the
center. At this stage no further energy gain by nuclear fusion is
possible, because neutrons and protons in iron nuclei possess the
highest nuclear binding energies.
Lacking its central energy source, the stellar iron core cannot escape
gravitational instability when its mass grows to a critical limit
by ongoing silicon burning in a surrounding shell. A catastrophic
collapse sets in and stops abruptly only when the stellar matter
reaches densities higher than in atomic nuclei. At this
moment repulsive forces between the neutrons and protons resist
further compression and the central region bounces back to send a
strong shock wave into the overlying, still infalling matter of
the iron core.
For more than 30 years there had been hope that ever more improved
computer models would finally be able to demonstrate that this
"core-bounce shock" is able to trigger a successful supernova
explosion by reversing the infall of the outer stellar layers.
However, the opposite turned out to be the case: Better models showed
that the energy losses of the bounce shock are so dramatic that
its outward propagation comes to a halt still well inside of the
iron core. It became clear that something has to help reviving
the stalled shock.
Some mechanism has to supply the shock with
fresh energy so that it reaccelerates and expels the stellar mantle
and envelope in the supernova blast.
Already in the 1960's it was speculated (in a seminal publication
by Stirling Colgate and Richard White) that neutrinos might be
involved. Myriads of these high-energy elementary particles are
radiated by the extremely hot, newly formed neutron star. If less than
one percent of them gets absorbed in the matter behind the stalled
shock, a healthy supernova explosion will be the consequence
(see MPA research highlight 2001).
This was shown, in principle, already in the mid 1980's with first sufficiently detailed numerical simulations by Jim Wilson and interpretative work by Wilson and Hans Bethe.
This was shown, in principle, already in the mid 1980's with first sufficiently detailed numerical simulations by Jim Wilson and interpretative work by Wilson and Hans Bethe.
However, many aspects of the involved physics were still too crude and too approximate to be realistic. In particular, with the observation of Supernova 1987A it became clear that stellar explosions are highly asymmetric phenomena and non-spherical plasma flows must play an important role already at the very beginning of the explosion. Early multi-dimensional computer models ---mostly still in two dimensions, i.e., assuming rotational symmetry around a chosen axis for reasons of computational efficiency--- indeed showed that convection and non-radial mass motions provide crucial support to the neutrino-heating mechanism and enhance the energy deposition by neutrinos. Thus explosions could be obtained although spherical models did not find shock revival and did not lead to explosions (see MPA press release 2009).
Nature, however, has three spatial dimensions and therefore these early successful models were critisized to be unrealistic and not reliable. In fact, not only the assumed axial symmetry is artificial, also the physics of turbulent flows differs in two dimensions compared to the 3D case.
Only very recently the increasing power of modern supercomputers has now made it possible to perform supernova simulations without artificial constraints of the symmetry. A new level of realism in such simulations is thus reached and brings us closer to the solution of a 50 year old problem.
The stellar collapse group at the Max Planck Institute for Astrophysics
(MPA) plays a leading role in the worldwide race for such models.
With all relevant physics included, in particular using a highly
complex treatment of neutrino transport and interactions, such
computations are at the very limit of what is currently feasible
on the biggest available computers. The model simulations are
performed on 16,000 cores (equivalent to a similar number of the
fastest existing PCs) in parallel, which is the largest
share of SuperMUC at the Leibniz-Rechenzentrum (LRZ) in Garching
(Fig. 1) and of MareNostrum at the Barcelona Supercomputing
Center (BSC; Fig. 2) that the MPA team is granted access to.
Nevertheless, one full supernova run, conducted over an evolution
time of typically half a second, consumes up to 50 million core
hours and takes more than 1/2 year of project time to be completed.
The enormous effort has payed off! The MPA team has recently
been able to report a first successful 3D explosion for a 9.6
solar-mass star (see MPA research highlight 2015;
Movie of the 3D explosion of a star with 9.6 solar masses by Aaron Döring)
and has now also obtained a 3D explosion of a 20 solar-mass
progenitor (Figs. 3, 4 and Movie).
Based on the presently most advanced
description of the neutrino physics in collapsing stellar cores
worldwide, these results are a true milestone in supernova
modeling. They confirm the viability of the neutrino-heating
mechanism in principle, applying our currently best knowledge of
all processes that play a role in the center of dying stars,
whose extreme conditions in temperature and density are hardly
accessible by laboratory experiments on Earth. Since not all
aspects of the complex neutrino reactions in the newly formed
neutron star are finally understood, the 3D models
demonstrate that within existing uncertainties neutrinos
can indeed transfer enough energy to revive the stalled
shock. As known from previous models in two dimensions, violent
nonradial fluid flows must provide crucial support to relaunch
the blast wave and will function as seeds of the later,
large-scale asymmetries that are observed in supernova explosions.
Further work on the theoretical models is necessary. So far the
successful 3D simulations could only be done with rather coarse
resolution, because bigger computers would be needed to perform
more refined supernova calculations.
Moreover, a wider range of stellar masses must be investigated,
varying the initial conditions in the pre-collapse cores.
A final confirmation of our theoretical picture of the explosion
mechanism and the role of neutrinos, however, can only come from
observations. On the one hand this demands a closer link of
the explosion models to observable supernova properties,
on the other hand much hope rests on a next supernova that
will occur in our Milky Way galaxy.
Such a nearby event will flood the Earth with
1030 neutrinos, of which several thousand to tens of thousands
will be captured in huge underground experiments like
Super-Kamiokande in Japan and IceCube at the South Pole.
Neutrinos (besides gravitational waves) will thus serve as
unique messengers: since they escape from the center of the
supernova they will bring us information directly from the
very heart of the explosion.
Hans-Thomas Janka
Publications:
T. Melson, H.-T. Janka, & A. Marek: Neutrino-driven supernova of a
low-mass iron-core progenitor boosted by three-dimensional
turbulent convection, Astrophysical Journal Letters, 801, L24 (2015);
e-print arXiv:1501.01961
T. Melson, H.-T. Janka, R. Bollig, F. Hanke, A. Marek, & B. Müller:
Neutrino-driven explosion of a 20 solar-mass star in three dimensions
enabled by strange-quark contributions to neutrino-nucleon scattering,
Astrophysical Journal Letters, 808, L42 (2015); e-print arXiv:1504.07631
Acknowledgments:
Elena Erastova and Markus Rampp (Rechenzentrum Garching) are
acknowledged for the images of Figs. 3 and 4, Aaron Döring for
the movies of our supernova simulations.
This project was partly funded by the European Research Council
through grant ERC-AdG No. 341157-COCO2CASA. Computing time was
kindly provided by the European PRACE Initiative on SuperMUC
(GCS@LRZ, Germany) and MareNostrum (BSC, Spain).
The postprocessing of the simulation data was conducted
on the IBM iDataPlex System hydra of the Rechenzentrum Garching.
Contact:Dr. Hans-Thomas Janka
Max Planck Institute for Astrophysics, Garching
Tel.: +49 89 30000-2228
email: hjanka@mpa-garching.mpg.de
