Science1 publisher2 min readPublished
Switching neutrino flavor flipped some dying stars from explosion to collapse in study of 195 simulations
Niels Bohr Institute astrophysicists ran 195 collapsing-star models and found neutrino flavor conversion flips the outcome for stars of 16 to 30 solar masses. The density at which conversion begins is still an input, so the effect on supernova counts is unmeasured.
The Scientist · Science desk
What happened
- The Copenhagen pair ran each collapsing-star model twice, with and without neutrino flavor conversion, and repeated the exercise with the conversion triggered at different densities inside the star.
- Across the grid, neutrino behavior significantly altered whether a star exploded as a supernova or collapsed into a black hole, with the change concentrated in stars of 16 to 30 solar masses.
- Flavor conversion had been left out of supernova simulations as too computationally demanding, so Gogilashvili and Tamborra built a simplified model in order to sweep a grid of progenitors.
- The paper appears in Physical Review D under the title "Neutrino flavor conversion shapes the rate of failed core-collapse supernovae".
Compiled by The ScientistSomething wrong?How this is made
Why it matters
- constraint Because the onset density was imposed by hand, the study bounds where flavor physics can change a star's fate, but it does not say how often that happens.
- decision Groups rationing time on core-collapse codes now have a specific place to spend it, on progenitors of 16 to 30 solar masses, where the reduced model says the outcome is unstable.
- capability A cheap model lets two people scan hundreds of progenitors, a survey that full radiation-hydrodynamic codes cannot afford to attempt.
- exposure Predicted supernova rates computed without flavor conversion inherit a bias of unmeasured size. That bias matters most for anyone comparing those predictions with survey counts.
The density at which flavor conversion switches on is an input the authors imposed, not a number their model produced [3]. A sweep like that measures how sensitive a star's fate is to the assumption. Which density real stars actually pick is a separate question.
A reduced model is what makes a grid this size possible at all. "Simulating the death of a massive star is something that is pretty much at the frontier of what we can do computationally at the moment," said Irene Tamborra, a professor at the Niels Bohr Institute and the study's second author [7][13]. Her co-author, the postdoctoral researcher Mariam Gogilashvili, led the work [13].
Spread evenly, 195 progenitors across 9 to 120 solar masses come out at one model every 0.57 solar masses [15]. The band where outcomes changed, 16 to 30 solar masses, is 14 solar masses wide, about an eighth of the range covered [14]. How much of the supernova rate that eighth accounts for depends on how many collapsing stars are born inside it. The phys.org account gives neither that figure nor the number of the 195 runs that changed outcome [16].
What the sweep does undercut is the old default. "We have long known that neutrinos can switch between different flavors. But we generally assumed that this had no effect on the outcome of the explosion itself," Gogilashvili said [8]. On seeing the grid assembled, she said: "It was a really exciting moment when we put all 195 simulations side by side and saw a whole range of stars flip from exploding to failing" [9].
The link the authors draw is to the supernova rate problem, where significantly fewer supernovae are observed than theoretical models predict [10]. "But if a star collapses directly into a black hole without a visible explosion, or is obscured by dust, it can effectively 'disappear' from our counts. Our results therefore suggest that there is a mechanism that could make such 'failed supernovae' more likely," Gogilashvili said [11]. Tamborra said the work "could therefore not only give us better tools to predict a dying star's fate, but it may also help explain why observations do not always match theoretical predictions" [17].
What to watch
- Whether a full three-dimensional core-collapse simulation that includes flavor transport reproduces the flips in the 16-to-30-solar-mass band, or washes them out.
- The Physical Review D paper's own tally: how many of the 195 runs changed outcome, and at which trigger densities.
- Survey counts of massive stars that vanish without a visible explosion: those counts would put a number on the failed-supernova fraction the authors' mechanism implies.