Focus: Supernovae that Don’t Explode
There’s no avoiding the tragic end of a large star’s life: it dies in a catastrophic explosion called a supernova. But in the most complete computer simulation to date, those final moments don’t end with a bang, reports a team in the 20 June PRL. Ignorance about the physics of neutrinos is the main problem. The researchers were surprised not to get an explosion, even though they accounted for neutrinos more accurately than previous attempts had. Only a better understanding of neutrino behavior in the extreme environment of a dying star is likely to lead to explosive virtual supernovae.
Astrophysicists still don’t know exactly why supernovae explode, although they have a rough idea. First, the old iron core collapses under its own weight, which sends a shock wave blasting out through the rest of the star. But this shock alone would peter out, according to computer models. Current theories suggest that neutrinos expelled by the collapsing core provide the extra oomph the star needs to blow up. The idea is that free protons and neutrons near the shock front absorb the neutrinos and heat up, creating enough pressure behind the shock front to prevent it from dissipating. Instead, it speeds up and rips through the star supersonically. The force of the shock overcomes the core’s gravitational pull, and the outer layers of the star blow out into a huge shell of glowing gas.
Previous simulations in two and three dimensions have assumed the neutrinos equilibrate to a uniform temperature because more accurate representations would take too much computer power. Now Hans-Thomas Janka of the Max Planck Institute for Astrophysics in Garching, Germany, and his colleagues have used the most complete theoretical description of neutrinos in stars, called the time-dependent Boltzmann equation, in a two-dimensional supernova simulation. The two dimensions are the distance from the center and the stellar equivalent of latitude; the computer program assumes all longitudes to be identical.
The simulation–one of the most computationally intensive calculations ever performed–ran on a German supercomputer. It allowed for convection and other fluid flows, and with such an accurate description of neutrinos, the researchers expected to see it detonate. Instead, the shock wave petered out, leaving only a black hole.
The failure to blow up suggests that physicists are missing something essential about neutrinos and nuclear physics, says Janka. He thinks the problem may be that in Earth-bound experiments neutrinos interact in a vacuum with a few protons, neutrons, or small nuclei, whereas a giant collapsing star contains superdense matter where nuclear particles may form exotic structures. These conditions could lead to unexpected neutrino behavior. Another missing piece may be magnetic effects. Very high fields are required to influence neutrino motion, but such fields might exist in supernovae.
Researchers are currently trying to observe neutrinos in dense matter environments like those within supernovae. Meanwhile, Janka and his colleagues at the Max Planck Institute are working on a three-dimensional supernova simulation they hope will be more complete.
“The riddle is over 50 years old and apparently the jury is still out,” quips Stan Woosely of the University of California at Santa Cruz. “Nature still does something very dramatic that we don’t understand.”
Kim Krieger is a freelance science writer in Montreal.