It’s superb. We’ve witnessed them enough that we’re pretty sure of that. Why it’s happening was a whole other matter. As we worked on understanding the physics that cause these huge explosions, we occasionally went through awkward periods when the stars in our models stopped exploding. The addition of more realistic physics has made the models booming again, and now we are in a time when the latest models seem to be happily self-destructing.
The challenge is trying to find evidence that the physics we use in our successful models accurately reflect what is happening to a dying star – it is not an easy task with an event that instantly destroys much of the evidence.
Now, data from the Chandra X-ray Observatory provides an indication that a mechanism used in recent supernova models is probably correct. The results are published in the weekly issue of the journal Nature.
Boom goes (especially)
The supernovae in question appear here when a massive star runs out of fuel, causing its core to collapse. Right here, you can see a potential problem: how can a collapse lead to an explosion?
The general idea is that once the fuel runs out and the fusion stops, the inner core of the star collapses into a neutron star. The layers above the core, deprived of the energy that pushed them out, drop down to the core, hit the neutron star and return. This recoil is then what spoils the most visible outer layers of the star.
Unfortunately, this does not work exactly. The outer layers of the star are also removed by the energy that counteracted the attraction of gravity and begin to throw towards the core. Somewhere inside the star, the twisting layers that throw outward will roll into the more distant layers that still collapse inward. The result is a shock front that stops before it reaches the star’s surface. Nothing grows.
The equilibrium point is reached close enough to the surface of the star, however, that an additional input of energy would be enough to turn things back in an explosive way. And physicists came up with a rather unlikely source for this energy: neutrinos. These particles are distinguished by the fact that they rarely interact with other matter, so they appear to be a terrible candidate for transferring energy to the material that shakes in the outer layers of the star. But so many of them are produced during the collapse of the core that neutrino-driven heating is one thing, even if it’s not something you want to reheat the debris.
And fortunately, in this context, it is something that has consequences. The material that is heated by neutrinos continues to try to expand and get rid of the star. Material that has not been baked by neutrinos does everything possible to collapse. The result is a dramatic convection in the outer layers of the star, as the collapsing and exploding materials pass side by side. This has the potential to create asymmetric explosions, which we have seen happen. And it also has consequences for the material that is discharged.
Freezing
Neutrino-powered heating may seem a little strange, but one of its consequences is just as strange. The heated material forms what physicists call a “high entropy panel.” In this case, high entropy simply refers to a combination of low density and extremely high energies. It is large enough that some of the newly formed atoms end up being disassembled into protons, neutrons and alpha particles, a combination of two neutrons / two protons. (An alpha particle is the same as the nucleus of a typical helium atom.)
As the material cools, however, the energy and density decrease to where all this material begins to form larger atomic nuclei in a process called alpha-rich freezing. This process has a distinct atomic signature, because the physics of freezing can form a number of specific elements and isotopes. So, looking at the remnants of the exploded star, we can potentially find evidence that an alpha-rich freeze has occurred.
And that’s exactly what was done in this new study. One of the isotopes produced in alpha-rich freezing is 56Ni, which decomposes rapidly at 56Fe. And the previous imaging of the supernova remains at Cassiopeia A showed that there are areas of the expelled material that are rich in iron. So, a collaboration between American and Japanese researchers sought in these iron-rich regions the presence of chromium and titanium, which are also produced during alpha-rich freezing.
Obviously, the researchers found them, or this article should not have been written. Equally critical, chromium and titanium were present in quantities that were consistent with their formation in a panel of high-energy, proton-rich material.
Equally significant, supernova models suggest that short stories of neutrino-induced heating material should travel around 4,000 to 5,000 kilometers per second. And the iron-rich material travels at over 4,000 kilometers per second, placing it in the right neighborhood.
All of this suggests that our current models of exploding stars seem to be on the right track. Not only do model stars actually explode – they do so in a way that seems to be consistent with an existing supernova remnant. Obviously, this will be something we will want to look at other supernova remnants to confirm. But for now, at least for now, model makers can enjoy the ease of having good reason to believe they’re not bad.
Nature, 2021. DOI: 10.1038 / s41586-021-03391-9 (About DOI).