Effects of Neutrino Fast Flavor Conversions on Core-Collapse Supernovae

July 01, 2023

Neutrinos are the driving factor for core-collapse supernovae, the violent death of massive stars. According to the neutrino-driven mechanism they are responsible for transferring energy from the hot proto-neutron star (PNS) to the surrounding material. So far, numerical simulations assumed that neutrinos retain their flavor during propagation. Max Planck researchers have now shown that allowing for flavor conversions has a direct influence on the supernova dynamics.

Supernova explosions are the death of massive stars after they have consumed all their raw material and can no longer produced energy through nuclear fusion. Even though the electromagnetic radiation of such supernovae can outshine a whole galaxy, it is faint compared to the energy released in form of neutrinos. These neutrinos only rarely interact with other matter, but interestingly it is just this reluctance to interact that enables neutrinos to play a pivotal role in turning the collapse of the massive stellar core into an explosion.

After a massive star has exhausted its reservoir of material to power nuclear fusion in its core, the missing radiation pressure from the center causes the core to collapse. A dense proto-neutron star forms, which is small (a few tens of kilometers) but massive (more heavy than the sun). Trapped inside is an enormous amount of of heat (a few hundreds of billions of degrees) generated from gravitational energy in the collapse. Neutrinos are the only particles that can escape this monstrous environment as they are interacting very rarely. Therefore they are being produced in vast amounts allowing the proto-neutron star to cool.

Nevertheless, a small fraction of neutrinos will interact with close-by stellar matter and heat-up the region surrounding the core. If the energy transfer is efficient enough (an absorption of about 1% is sufficient), the neutrino heating triggers an expansion strong enough to lead to explode the star, i.e. launching the supernova. If not, the star will finally collapse to a Black Hole.

The neutrino flavor is a quantum mechanical property and influences, how the neutrino interacts with matter. In particular, neutrinos of electron flavor interact more strongly with matter than neutrinos of other flavors (myon and tau). These, in turn, can escape easier making them more energetic on average. Neutrino flavor conversions where individual neutrinos change their flavor are have been known for a few decades already. However, since they are suppressed at very high densities, such as in a collapsing star, so far numerical simulations of supernovae assumed that neutrinos keep their flavor.

Recently scientists discovered that neutrinos can undergo collective self-induced flavor conversions if the neutrino density is sufficiently high, such as in core-collapse supernovae or the early universe. But the conversion length scales are much below the resolution of numerical simulations of supernovae. Also the exact conditions and outcomes of the so-called ‘fast’ neutrino flavor conversions remains incalculable with direct simulations due to the high dimensionality of the problem. Now researchers of the Max Planck Institutes for Astrophysics and for Physics and the Niels Bohr Institute have teamed up to gauge the influence that fast and efficient flavor conversions can have on the dynamics of a core-collapse supernova.

Their new, easy-to-calculate scheme integrates an effective treatment of flavor conversions directly into the numerical simulations. It is designed to find out how big the influence of flavor conversions could be. To do so, it maximizes flavor conversions up to the constraints allowed by the Standard Model of Particle Physics. The scientists inspected the collapse and subsequent accretion phase of an evolved stellar model, which is 20 times more heavy than the sun. In multiple simulations, they increased the stellar volume that is under the influence of flavor conversions and investigated how the system reacts to such strong changes in the neutrino field.

In regions where neutrinos are heating the stellar matter, flavor conversions can transiently intensify this heating because some high-energy neutrinos of non-electron flavor are converted to the more reactive electron flavor. However, the effect is not strong enough to trigger an explosion by itself in the particular model. On the other hand, flavor conversions in the proto-neutron star can even accelerate the collapse to a black hole in the model, because in this region the inverse conversions accelerate the cooling.

This study shows that the influence of neutrino flavor conversions should be taken into account for predictive simulations. Flavor conversions will probably not challenge the neutrino-driven mechanism itself but they might add significant modifications to the dynamics. Yet it is too early to draw final conclusions on the details as the implementation of the effects is schematic and not based on detailed calculations. The simulations assume spherical symmetry without multi-dimensional flows that are known to be an important ingredient for successful explosions. Finally, the detailed structure of the progenitor star is also known to strongly influence important aspects of supernovae in a non-linear fashion.

The latter two limitations are already subject to a follow-up investigation still under peer-review. These multi-dimensional core-collapse simulations now also include stars with lower masses, which usually show explosions. The results indicate that flavor conversions could work both ways: For lower mass progenitors the increased heating can trigger the explosions significantly earlier; in higher mass progenitors the flavor conversions could hamper explosions due to cooling effects. Flavor conversions therefore not only influence the explosion dynamics and modify the neutrino signal measured on Earth, they could also be relevant for the distribution of the masses of black holes and neutron stars.

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