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Neutrino Flavor Conversion Shapes the Rate of Failed Core-collapse Supernovae

By simulating the collapse of 195 massive stars with neutrino flavor conversion, this study demonstrates that flavor transformations significantly alter explosion outcomes and compact remnant distributions, thereby resolving key discrepancies between theoretical predictions and observational data regarding neutron star and black hole populations.

Original authors: Mariam Gogilashvili, Irene Tamborra

Published 2026-05-19

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CC BY 4.0

Original authors: Mariam Gogilashvili, Irene Tamborra

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine a massive star as a giant, cosmic pressure cooker. For most of its life, it balances perfectly: the outward push of nuclear fusion fights against the inward pull of gravity. But when the star runs out of fuel, that balance shatters. The core collapses in on itself, creating a situation where the star either explodes spectacularly as a supernova or quietly implodes into a black hole.

For a long time, scientists thought they could predict which stars would explode and which would collapse just by looking at how heavy they were. They believed heavier stars were more likely to become black holes. However, recent observations have shown this isn't quite right. Some surprisingly heavy stars seem to disappear without a bang (a "failed supernova"), while some lighter ones still manage to explode. This has left astronomers puzzled, creating a few "missing pieces" in our understanding of the universe, such as why we don't see certain types of stars exploding and why the number of observed supernovae is lower than expected.

The New Ingredient: The "Flavor" Switch

This paper introduces a new, crucial ingredient to the recipe: neutrino flavor conversion.

To understand this, imagine neutrinos as tiny, ghostly messengers that carry away almost all the energy from the collapsing star. There are three "flavors" of these messengers: electron, muon, and tau. In the old models, scientists assumed these messengers stayed in their original lanes. They thought the electron messengers were the ones doing the heavy lifting to push the shockwave out and cause the explosion.

The authors of this paper suggest that these messengers are actually very social. As they zoom through the dense heart of the collapsing star, they can suddenly swap identities with each other. An electron neutrino might turn into a muon neutrino, and vice versa. This is called "flavor conversion."

The Experiment: Simulating the Cosmic Kitchen

The researchers ran a massive computer simulation, like a high-tech cosmic kitchen, testing 195 different types of stars (ranging from 9 to 120 times the mass of our Sun). They ran the simulation twice for each star:

The Old Way: Neutrinos kept their original flavors.
The New Way: Neutrinos were allowed to swap flavors instantly in certain regions of the star.

What They Found

The results were like flipping a switch on the star's fate:

More Failed Explosions: When the neutrinos swapped flavors, it changed the energy distribution. It was as if the "push" needed to blow the star apart was suddenly diluted. The simulation showed that this flavor swapping made it much harder for the star to explode. Specifically, stars in the 16 to 30 solar mass range (which were previously thought to be good candidates for exploding) were much more likely to fail and collapse into black holes.
Solving the "Missing Star" Mystery: This helps explain the "Red Supergiant Problem." Astronomers have noticed a gap: they see red supergiant stars exploding, but they don't see the ones in that 16–30 mass range. This paper suggests those stars are collapsing, but they are failing to explode because of the neutrino flavor swap, leaving no bright supernova behind.
Fixing the "Missing Supernova" Count: Similarly, it helps explain why we see fewer supernovae in the universe than we calculate based on how many stars are being born. If a large chunk of stars are failing to explode due to this flavor swap, the numbers finally add up.
Lighter Neutron Stars: For the stars that did manage to explode, the flavor swap actually helped the explosion happen faster and more efficiently. This meant the leftover core (the neutron star) didn't have as much time to gather extra material. The result? The neutron stars left behind were lighter, which fits better with the actual weights of neutron stars we observe in the sky.

The Bottom Line

This paper argues that we can no longer ignore the "social life" of neutrinos. If we want to correctly predict how many black holes and neutron stars exist, and why some stars disappear without a trace, we must include this flavor-swapping behavior in our models. It turns out that the tiny, ghostly particles are the referees deciding whether a star gets a standing ovation (an explosion) or a quiet exit (a black hole).

Problem Statement
The relative rates of neutron star (NS) and black hole (BH) formation from massive star collapse remain highly uncertain. While it was historically assumed that black holes form only from progenitors exceeding $40 M_\odot$ , recent spherically symmetric simulations suggest a non-monotonic relationship between progenitor mass and core-collapse supernova (CCSN) explodability, with failures occurring as low as $13 M_\odot$ . This uncertainty impacts predictions of compact remnant mass distributions, nucleosynthesis, and stellar feedback. Furthermore, observational tensions exist: the "red supergiant problem" (a lack of identified progenitors in the $16\text{--}30 M_\odot$ range for Type IIP supernovae) and the "supernova rate problem" (observed CCSN rates falling below those inferred from cosmic star formation rates). Additionally, the observed neutron star mass distribution peaks at $1.2\text{--}1.4 M_\odot$ , with a scarcity of black holes below $5 M_\odot$ , challenging current theoretical models.

A critical gap in existing CCSN modeling is the treatment of neutrino flavor conversion (FC). While neutrinos drive the explosion mechanism by depositing energy behind the stalled shock, standard multi-dimensional hydrodynamic simulations often neglect FC, assuming its effects behind the shock are negligible. However, recent insights into neutrino self-interactions suggest FC can occur near the proto-neutron star (PNS), potentially altering the energy deposition efficiency.

Methodology
The authors perform 1D+ hydrodynamic simulations of stellar collapse for 195 progenitors with zero-age main-sequence (ZAMS) masses ranging from $9 M_\odot$ to $120 M_\odot$ (solar metallicity), based on the progenitor set from Ref. [12]. The simulations utilize the open-source, spherically symmetric code GR1D, which couples general-relativistic hydrodynamics with energy-dependent neutrino transport.

Physics Setup: The code employs a radial grid resolving the innermost 20 km, uses the SFHo equation of state, and incorporates the Supernova Turbulence in Reduced-dimensionality (STIR) model to approximate multi-dimensional turbulence effects. Neutrino transport is handled via an energy-dependent moment formalism (M1 closure) for three species ( $\nu_e, \bar{\nu}_e, \nu_x$ ) across 18 energy bins.
Flavor Conversion Treatment: The study employs a schematic, parametric approach to FC (based on Refs. [54, 65]). Instead of dynamically solving the quantum kinetic equations, the model imposes instantaneous flavor equipartition shortly after bounce ( $t=0.02$ s) in regions where the matter density falls below a critical threshold $\rho_c$ . This scheme conserves electron lepton number and the number/momentum of (anti)neutrinos for each energy bin while redistributing the flavor spectra.
Scenarios: The authors simulate five scenarios: one without FC ("No FC") and four with FC triggered below critical baryon densities $\rho_c = 10^9, 10^{10}, 10^{11},$ and $10^{13} \text{ g/cm}^3$ .
Criteria: A successful explosion is defined by the shock radius exceeding 1000 km; otherwise, the event is classified as a failed supernova (leading to black hole formation). The explodability is analyzed using the progenitor compactness parameter $\xi_{2.5}$ .

Key Results

Increased Failure Rate: The inclusion of FC systematically increases the fraction of failed explosions. Without FC, 25.6% of the models fail to explode (27.0% when weighted by the Salpeter Initial Mass Function). When FC is included, this fraction rises significantly, reaching 88.3% for $\rho_c = 10^{13} \text{ g/cm}^3$ (88.3% IMF-weighted).
Mass Range Sensitivity: The effect is most pronounced in the $16\text{--}30 M_\odot$ mass range. In this region, many progenitors that would successfully explode without FC are converted into failures when FC is accounted for.
Dependence on Critical Density: The fraction of failed explosions increases as the critical density $\rho_c$ increases (i.e., as FC is triggered deeper within the PNS). This reflects a more efficient redistribution of energy from electron-flavor neutrinos to heavy-lepton neutrinos, reducing the efficiency of charged-current heating behind the shock.
Compact Remnant Masses:
- Successful Explosions: FC leads to earlier and more energetic shock revival, resulting in less efficient matter accretion onto the PNS. Consequently, the resulting neutron stars have systematically lower baryonic masses ( $1.2\text{--}1.9 M_\odot$ ), bringing the theoretical mass distribution into better agreement with the observed peak at $1.2\text{--}1.4 M_\odot$ .
- Failed Explosions: In cases where shock revival is suppressed, accretion continues longer, leading to slightly more massive remnants compared to the no-FC case, though the change is less prominent than in successful explosions.

Significance and Claims
The paper claims that neutrino flavor conversion is a fundamental ingredient in predicting the populations of neutron stars and black holes. By reshaping the explodability landscape, particularly in the $16\text{--}30 M_\odot$ range, FC offers a natural mechanism to:

Alleviate the red supergiant problem by explaining the absence of visible explosions from progenitors in this mass range (via failed explosions).
Address the supernova rate problem by increasing the theoretical fraction of failed explosions, thereby reducing the predicted visible CCSN rate to match observations.
Reconcile theoretical expectations with the low-mass tail of the observed neutron star mass distribution by producing lighter remnants in successful explosions.

The authors emphasize that while these findings are robust relative to their reference progenitor population, the quantitative fraction of failed collapses is sensitive to the details of the simulation and the schematic treatment of FC. They conclude that dynamic, multi-dimensional subgrid models of FC are necessary for precise predictions, but current evidence suggests FC cannot be neglected in modeling the birth rates and mass distributions of compact remnants.

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