The way in which the ghostly particles known as neutrinos can switch from one kind to another might help explain which stars explode and which collapse to form black holes, a new study finds.
These new findings might also help explain a number of other cosmic mysteries, such as why astronomers have detected fewer supernovas than expected.
When a massive star finally depletes its store of fuel, without nuclear reactions driving the star to expand, its core will implode under its own weight. The extraordinarily high heat and pressure resulting from this collapse will crush protons and electrons together, forming neutrons. This in turn releases a flood of ghostly particles known as neutrinos.
Generally, neutrinos barely interact with all forms of matter. However, when a massive star's heart collapses, it releases a staggering amount of neutrinos — enough for many to slam into and heat the layers just outside the star's core. In some cases, such neutrino heating can lead the rest of the star to explode as a supernova, and leave behind an extraordinarily dense cluster of neutrons known as a neutron star. In other cases, neutrino heating fails to lead to an explosion, and the star collapses, forming a black hole.
"Neutrinos are not a side detail in supernovas," study co-author Mariam Gogilashvili, a particle astrophysicist at the University of Copenhagen's Niels Bohr Institute in Denmark, told Space.com. "They carry away about 99% of the energy released when the core collapses, and a small change in how they behave can decide the fate of the whole star."
Much remains uncertain about which massive stars explode to birth neutron stars and which collapse to form black holes. Uncovering these details could help shed light on a wide range of activity in the cosmos, like the way in which dying stars filled the universe with the kind of matter that nowadays makes up everything from planets to people.
To yield insights on these mysteries, scientists investigated the role the strange nature of neutrinos might play. There are three "flavors" of neutrino — electron, muon and tau — which are named after the kind of particle each flavor generates if they ever collide with matter. In 1998, physicists discovered that neutrinos can "oscillate," or change from one flavor to another, a finding that won the 2015 Nobel Prize in Physics. (There are three corresponding flavors of antineutrino that can similarly change flavor.)
Both muon and tau neutrinos (and antineutrinos) are much less likely to interact with regular matter than electron neutrinos (and antineutrinos). This suggests neutrino flavor change might potentially influence how much neutrinos can heat up matter in a dying star and trigger an explosion.
Previously, scientists thought flavor change would only play a negligible role in the way dying stars collapsed. However, over the past decade, physicists have found that given the astoundingly large number of neutrinos that dying stars generate, many neutrinos are concentrated enough to interact with each other, triggering flavor changes within the core of the collapsing star.
In the new study, the researchers simulated the collapse of 195 stars, ranging in size from nine to 120 solar masses. They analyzed different assumptions about where neutrino flavor conversion occurs in the cores of collapsing stars.
The scientists found that neutrino flavor change might make dying massive stars significantly less likely to explode as supernovas and form neutron stars, and more likely to collapse to create black holes.
"What surprised us most was that the stars between 16 and 30 times the mass of the sun, many of which explode comfortably in our standard simulations, turned out to be particularly sensitive to neutrino physics," study co-author Irene Tamborra, a particle astrophysicist at the University of Copenhagen's Niels Bohr Institute in Denmark, told Space.com "A large number of them fail once flavor conversion is included."
These new findings might help solve a number of longstanding mysteries. For instance, astronomers have detected fewer supernovas than theoretical predictions — neutrino flavor change might help explain these findings, Gogilashvili said. Neutrino flavor change might also help reveal why the biggest red supergiant stars appear to vanish without generating supernova explosions — they may collapse as black holes.
Moreover, the scientists estimated that neutrino flavor change might yield less massive neutron stars. This could help explain why astronomers have detected neutron stars with lower masses than previously theoretically predicted, Tamborra said.
In the future, the scientists want to include more realistic models of neutrino behavior in 3-D computer simulations of massive stars, so neutrino flavor change starts and evolves over time as the stars die.
Gogilashvili and Tamborra detailed their findings in September in the journal Physical Review D.
