Fig. 3: Sequence of volume-rendering images that show the violent non-spherical mass motions that drive the evolution of the collapsing 20 solar-mass star towards the onset of a neutrino-powered explosion. The whitish central sphere indicates the newly formed neutron star, the enveloping bluish surface marks the supernova shock. (Visualization: Elena Erastova and Markus Rampp, Rechenzentrum Garching; copyright (2015) by American Astronomical Society). Movie of the 3D computer simulation (by Aaron Döring)
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.
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.
Dr. Hans-Thomas Janka
Max Planck Institute for Astrophysics, Garching
Tel.: +49 89 30000-2228