Neutrinos Can Alter Exploding Star Physics
August 22, 2012

When A Star Explodes, Neutrinos Change Everything

April Flowers for - Your Universe Online

Within the heart of exploding stars, sparse halos of neutrinos exert a previously unrecognized influence on the physics of the explosion and may alter which elements can be forged by these violent events.

Neutrinos are one of the fundamental particles that make up the Universe. Although neutrinos are similar to the electron, they have one important difference; they carry no electrical charge. Being electrically neutral, they are not affected by the electromagnetic forces which act on electrons.

John Cherry, a graduate student at UC San Diego, models stellar explosions. His work includes a type of explosion called a core-collapse supernova. As these stars run out of fuel, their cores suddenly collapse to form a neutron star, which quickly rebounds sending seas of neutrinos through the surrounding stellar envelope and out into space.

Even as this rebound is happening, the rest of the star is still falling inward. Plumes of matter sink, accreting onto the core.

"This matter is actually causing some small fraction of neutrinos to bounce at wide angles and cross the trajectories of neutrinos coming from the core," Cherry said.

Astrophysicists knew that the heart of that envelope contained these scattered neutrinos, but because they are relatively few compared with the numbers streaming from the core, they thought neutrino influence on the physics of these explosions would be so minor it could be ignored.  Cherry and his colleagues say not so in a paper published in Physics Review Letters.

The research team showed that neutrinos streaming from the core interacted with halo neutrinos far more often than anticipated. They also calculated how often that might occur and how large a difference it would make to their models of neutrinos within supernovae.

"What was so startling about this is that nowhere was the correction less than 14 percent. That's enough that you need to worry about it," he said. Indeed, some places in the outer regions of the envelope require as much as a 10-fold correction.

Neutrinos are famously aloof particles that seldom interact with matter.

"The way neutrinos interact in matter depends on what we call 'flavor'," said George Fuller, professor of physics at UC San Diego who leads the neutrino-modeling research group.

Neutrinos meet and "scatter" off one another and in the process change their flavor. The influence is much greater than physicists thought in the outer halo of neutrinos.

"Even though few neutrinos are scattered in funny directions, they can completely dominate how the neutrinos change their favors," Fuller said.

Many important things are determined by this balance of neutrino flavors, such as what new elements are forged when a star explodes.

"The neutrinos are the engine that drives the exploding star," Cherry said. "What's going on with neutrinos sets the entire stage for what's happening in the explosion. Those neutrino flavor states allow the neutrinos to change protons to neutrons or neutrons to protons. What matter is produced, what kinds of atoms, elements are produced by these supernovas are changed dramatically if you change the flavor content of neutrinos."