There is a problem with some stars that have exploded in supernovae, and it might point to new, exotic physics. They do not seem to have been as bright as they should have been, and particles called axions might have dimmed them before they blew up.
Type II supernovae come from the collapse and explosion of a massive star called a red giant. Our best models for these stars predict a strict relationship between these stars鈥� brightness and mass, but Oscar Straniero at the National Institute for Astrophysics in Italy and his colleagues found that some of them do not follow it.
The mass of a star that goes supernova can be estimated in two ways. The amount of oxygen produced in a supernova depends on the mass of the star, so you can look at the explosion, measure the oxygen produced, and estimate the mass that way. You can also look at archival images of the star before it exploded and estimate its mass from its brightness based on an established model of how the two should be related. Straniero and his team used both methods to estimate the masses of eight red giants 鈥� they found that the two did not match.
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Most of the stars were fainter than we鈥檇 expect from the estimates of their mass based on the oxygen. 鈥淭he luminosity pre-explosion depends on the balance between the processes that produce energy, which are nuclear reactions; and the processes that bring out energy from the stars, which are photons and neutrinos,鈥� says Straniero. The models only take into account photons and neutrinos, so something else must be carrying energy away. 鈥淲e think there is something missing, some other mechanism that helps photons and neutrinos do this job.鈥�
The researchers considered several possibilities related to the high uncertainties in the calculations. For example, the models do not take into account that most stars rotate. We are also not entirely sure how convection inside stars could change the energy balance. But accounting for these things made the discrepancy worse, not better.
So, Straniero says, we need some sort of unfamiliar physics leeching energy from the star. The most expedient way to do this is with a particle similar to a neutrino that does not interact much with other matter, so it can escape the core of the star without bouncing around too much.
One potential fit is the axion, a possible dark matter particle that was hypothesised to solve a problem in physics called the strong CP problem, which is related to the mystery of why there is more matter than antimatter in the universe. Axions are predicted to be fairly low-mass, so it doesn鈥檛 take unfeasibly large amounts of energy to produce them. They could be made in highly energetic environments like the cores of big stars.
Axion spotting
By comparing models of axions to how much energy they would need to filch from red supergiants, the researchers showed that axions or axion-like particles could account for the discrepancy between the stars鈥� brightness and mass. What is more, the predicted axions are within an energy range that will be probed by experiments that are already being planned, like the International Axion Observatory at the CERN particle physics laboratory near Geneva, Switzerland.
鈥淚f this discrepancy that we find originates from axions, they will be found in the next decade,鈥� says Straniero. That would explain not only the discrepancy with type II supernovae, but the much bigger strong CP problem as well.
Unfortunately, even though axions are a dark matter candidate, finding them in red giant stars might not solve the question of dark matter,聽says Straniero. 鈥淭here are different ways to make axions, but the highly energetic environment one won鈥檛 produce enough axions to explain the dark matter density,鈥� says Chanda Prescod-Weinstein at the University of New Hampshire. 鈥淭his is not how we expect a dark matter population to arise.鈥� That had to happen in the early universe, well before stars and supernovae.
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