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

Introduction to the show: ident: Astrophysics Radio.

Vera: Next we'll be talking about the paper "Neutrino Flavor Conversion Shapes the Rate of Failed Core-collapse Supernovae".

Jocelyn: The paper was written by Mariam Gogilashvili and Irene Tamborra from University of Copenhagen.

Vera: Stay tuned as we take you through the paper and discuss its implications.

Title: Vera: Welcome back to the show. Today we're looking at a paper that's been making waves in the astrophysics community, and it has a bit of a mouthful for a title: "Neutrino Flavor Conversion Shapes the Rate of Failed Core-collapse Supernovae." Jocelyn, I think this is one of those papers where the title really does tell you exactly what's going on, but let's unpack it for our listeners.

Jocelyn: Absolutely, Vera. So when a massive star runs out of fuel, its core collapses, and whether it explodes as a supernova or just collapses straight into a black hole depends on neutrinos. These are nearly massless particles that are produced in huge numbers during the collapse. And the key thing this paper investigates is something called neutrino flavor conversion — basically, neutrinos can change from one type, or flavor, to another as they're streaming out of the collapsing core.

Vera: And that matters because not all neutrino flavors are equally good at driving the explosion. The electron neutrinos and antineutrinos are the ones that deposit energy behind the shock wave and help push it outward. If some of those convert into other flavors before they get to the shock, the energy deposition changes. And this paper, by Mariam Gogilashvili and Irene Tamborra at the Niels Bohr Institute, simulates what that does to the fate of the star.

Jocelyn: Right. They took one hundred ninety-five different progenitor stars, with masses ranging from nine up to one hundred twenty times the mass of the Sun, and ran simulations of the collapse with and without this flavor conversion included. And the results are pretty dramatic. Without flavor conversion, about twenty-five point six percent of their models fail to explode. But when they turn on flavor conversion, that fraction jumps up significantly — depending on how deep in the star they trigger the conversion, it goes from about fifty-one percent all the way up to ninety-six percent.

Vera: So essentially, neutrinos changing flavor can turn a would-be supernova into a quiet collapse into a black hole. And that's a huge shift in our understanding of what happens to massive stars at the end of their lives. Now, one thing I want to emphasize is that this is a schematic treatment — they're not simulating the full quantum mechanics of neutrino oscillations, but they're using a parametric model that captures the essential effect. And the fact that even this simplified approach has such a big impact suggests the real thing could be just as important.

Jocelyn: And that's the hook for our next segment, because the implications go beyond just individual stars — this could actually help solve some long-standing puzzles about the rates of supernovae we observe in the universe.

Paper discussion segment 2: Vera: So we've established that "Neutrino Flavor Conversion Shapes the Rate of Failed Core-collapse Supernovae" shows that flavor conversion makes failed explosions much more common. But why should we care about the rate of failed supernovae in the first place? Jocelyn, what's the observational situation?

Jocelyn: Well, Vera, there are actually a couple of known problems in the field. The first is called the red supergiant problem. When astronomers look at the progenitors of Type IIP supernovae — these are explosions from red supergiant stars — they notice that there are very few progenitors with masses between about sixteen and thirty solar masses. But stellar evolution theory says there should be plenty of stars in that range. So either those stars are exploding in a way we're not seeing, or they're not exploding at all.

Vera: And this paper suggests that flavor conversion could be exactly what's suppressing those explosions. In their simulations, the sixteen to thirty solar mass range is particularly sensitive to flavor conversion — many of those stars that would have exploded without flavor conversion end up failing when it's included. That would explain why we don't see them as supernovae: they're quietly collapsing into black holes instead.

Jocelyn: Exactly. And there's also the supernova rate problem. The observed cosmic rate of core-collapse supernovae is systematically lower than what we'd expect from the rate of star formation. If a significant fraction of massive stars are failing to explode, that discrepancy starts to make sense. The paper even calculates this using the Salpeter initial mass function, which weights the importance of different stellar masses. Without flavor conversion, about twenty-seven percent of collapses fail. With flavor conversion at the lowest threshold they consider, that jumps to about forty-eight percent.

Vera: Now, I should note that the paper is careful here — they're not claiming this definitively solves these problems. The fraction of failed explosions they get for the more aggressive flavor conversion scenarios, like seventy-four or eighty-eight percent, would actually be too high compared to observational constraints. So it's not that more flavor conversion is always better. It's that the right amount of flavor conversion could naturally explain why we see fewer supernovae than we expect.

Jocelyn: And that's a really important nuance. The observational constraints on failed supernovae — from surveys that look for disappearing stars, and from the diffuse supernova neutrino background — suggest that somewhere between five and fifty percent of collapses fail. So the no-flavor-conversion case at twenty-seven percent is actually consistent with that range. But the flavor conversion cases push it higher, and the question is whether the real physics lands in the sweet spot.

Vera: So the next question is: what does this mean for the actual objects that are left behind? Because if flavor conversion changes which stars explode and which don't, it should also change the masses of the neutron stars and black holes that form. And that's exactly what we're going to dig into next.

Paper discussion segment 3: Vera: We're back with "Neutrino Flavor Conversion Shapes the Rate of Failed Core-collapse Supernovae," and we've talked about how flavor conversion increases the rate of failed explosions. But the paper also looks at something more subtle: the masses of the compact remnants that are left behind. Jocelyn, what did they find there?

Jocelyn: This is where things get really interesting, Vera. For the stars that do successfully explode, the presence of flavor conversion actually leads to less massive neutron stars. In their simulations, the baryonic mass of the remnant — that's the mass before you account for the gravitational binding energy — ends up lower when flavor conversion is included. And the reason is that flavor conversion, when it happens at the right place, can actually help revive the shock earlier.

Vera: That seems counterintuitive at first, because we just said flavor conversion makes explosions fail more often. But the paper distinguishes between different scenarios. When flavor conversion happens deep in the star, near the neutrinosphere, it tends to suppress the explosion. But when it happens closer to the stalled shock, it can actually enhance the neutrino heating and trigger the explosion sooner. And an earlier explosion means less time for the proto-neutron star to accrete matter, so it ends up less massive.

Jocelyn: Right. And the numbers bear this out. Without flavor conversion, the baryonic masses of the remnants from successful explosions span a range up to about one point nine solar masses. With flavor conversion, they tend to cluster lower, around one point two to one point four solar masses after you account for the gravitational mass correction. And that's actually a big deal, because the observed neutron star mass distribution peaks right around one point two to one point four solar masses.

Vera: So the standard simulations without flavor conversion tend to produce neutron stars that are too heavy compared to what we actually observe. And this paper shows that flavor conversion naturally brings the theoretical predictions into better agreement with the observed low-mass tail of the neutron star population. That's a really elegant result — it's not just about the rate of explosions, but about the properties of the objects that survive.

Jocelyn: For the failed explosions, the effect is a bit different. The remnant masses tend to be slightly larger when flavor conversion is included, because the shock never revives and accretion continues for longer. But the paper notes that these masses shouldn't be interpreted as the final black hole masses, because there could be significant fallback of the stellar envelope later on.

Vera: So the picture that emerges is that flavor conversion isn't just a detail — it's a fundamental ingredient that shapes both which stars explode and what they leave behind. And this has implications for gravitational wave observations, for the neutron star mass distribution, and for our understanding of how black holes form. But I should also mention that the authors are careful to note the limitations of their approach. They're using a schematic treatment of flavor conversion, and the results are sensitive to things like the nuclear equation of state and the details of the progenitor models.

Jocelyn: And that's the perfect setup for our final segment, where we'll wrap up what this means for the field and what questions remain open.

Conclusion: Vera: So let's bring it all together. "Neutrino Flavor Conversion Shapes the Rate of Failed Core-collapse Supernovae" has shown us that the humble neutrino, changing flavor as it streams out of a collapsing star, can dramatically alter the fate of that star. More failed explosions, especially in that problematic sixteen to thirty solar mass range, and lighter neutron stars when explosions do happen.

Jocelyn: And the implications ripple outward. This could help explain the red supergiant problem, the supernova rate problem, and the fact that observed neutron stars tend to be lighter than many simulations predict. It's a reminder that the microphysics of neutrinos — particles that barely interact with anything — can have macroscopic consequences for the populations of neutron stars and black holes across the universe.

Vera: Of course, the authors are the first to admit that this is a schematic treatment. The real physics of neutrino flavor conversion is enormously complex, involving quantum mechanical effects that are still not fully understood. But papers like this are crucial because they show us what's at stake — they tell us that getting the neutrino physics right isn't just an academic exercise, it's essential for interpreting observations from gravitational wave detectors, X-ray telescopes, and neutrino observatories.

Jocelyn: And that's the takeaway for today. This paper identifies a key missing ingredient in our models of stellar death, and it gives us a roadmap for what needs to be done next — embedding more realistic treatments of flavor conversion into full three-dimensional simulations of core collapse.

Vera: Well said. So we'll be watching for those follow-up papers. For now, we've said goodbye to "Neutrino Flavor Conversion Shapes the Rate of Failed Core-collapse Supernovae" — thanks to Gogilashvili and Tamborra for this thought-provoking work. Join us next time when we'll be looking at a new paper on a completely different corner of the cosmos. Until then, keep looking up.

Jocelyn: And keep asking questions. The universe is full of surprises, and the neutrinos are always up to something. See you next time.