Supernovae happen. We’ve seen enough of it that we’re pretty sure of it. The reason why this happens is a completely different matter. As we worked to understand the physics that drive these tremendous explosions, we sometimes went through awkward periods when the stars in our models stopped exploding. The addition of more realistic physics has allowed the models to flourish again in general, and currently we are in a period in which the latest models seem fortunately self-destructive.
The challenge is to find evidence that the physics we use in our successful models accurately reflects what’s going on in a dying star – not an easy task with an event that immediately destroys much of the evidence.
The data from the Chandra X-ray Observatory give an indication that a mechanism used in recent supernova models is probably correct. The results are published in this week’s issue of Nature.
It’s going high (mostly)
The supernovae in question occur when the fuel of a massive star runs out, causing its core to collapse. Here’s you see a potential problem: how does a collapse lead to an explosion?
The general idea is that as soon as fuel runs out and fusion stops, the inner core of the star collapses into a neutron star. Low above the nucleus, deprived of the energy they pushed outward, falls in the direction of the nucleus, hits the neutron star and bounces back. This setback is then what blows the outer visible layers of the star into pieces.
Unfortunately, this does not work. The outer layers of the star are also cut off from the energy that counteracted gravity, and they also begin to dip to the core. Somewhere in the star, the backward layers that shoot outward will run into the farthest layers that still fall inward. The result is a shock front that stands still before it reaches the surface of the star. Nothing is going to boom.
The balancing point is reached close enough to the star surface, but an extra supply of energy will be sufficient to turn things back into an explosion mode. And physicists have devised a rather unlikely source for this energy: neutrinos. These particles are remarkable because they rarely interact with other matter, so they appear to be a terrible candidate for transferring energy to the material that lies in the outer layers of the star. But so much of it is produced during the collapse of the nucleus that neutrino-powered heating is a thing, even if it’s not something you want to reheat your leftovers.
And fortunately, in this context, it is something that has consequences. The material heated by neutrinos tries to continue to expand and escape the star. The material not baked by neutrinos still does its best to collapse. The result is a dramatic convection in the outer layers of the star, while collapsing and exploding materials pass by each other. It has the potential to cause asymmetric explosions, which we have seen happen. And it also has consequences for the material that is thrown out.
The freezing
Neutrino-powered heating may seem a little strange, but one of the consequences is equally strange. The heated material forms what physicists call a ‘high-entropy plume’. In this case, the high entropy merely refers to a combination of low density and extremely high energies. It is high enough that some of the newly formed atoms are eventually disassembled into protons, neutrons and alpha particles, a two-neutron / two-proton combination. (An alpha particle is the same as the nucleus of a typical helium atom.)
However, as the material cools, the energy and density decrease to where all this material begins to form larger atomic nuclei in a process called an alpha-rich freezing. This process has a clear atomic signature, as the physics of freezing is likely to form a number of specific elements and isotopes. So if we look at the remains of the exploding star, we can find evidence that an alpha-rich freezing occurred.
And that is exactly what was done in this new study. One of the isotopes produced in alpha-rich freezes is 56Ni, which quickly expires to 56Fe. And previous survey of the supernova at Cassiopeia A showed that there are areas within the ejected material that are iron-rich. Thus, a collaboration between American and Japanese researchers searched these iron-rich regions for the presence of chromium and titanium, which are also produced during an alpha-rich freeze.
It is clear that the researchers found it, otherwise this article would not have had to be written. Equally critical was chromium and titanium present in amounts corresponding to its formation in a proton-rich high energy plume.
Equally significant, the supernova models suggest that the plumes of neutrino-heated material move at around 4,000 to 5,000 kilometers per second. And the iron-rich material moves at more than 4000 kilometers per second and places it in the right environment.
All of this suggests that our current models of exploding stars appear to be on the right track. Not only do the model stars actually explode – they do so in a way that looks like it corresponds to an existing supernova remnant. Clearly, this is something we want to look at other supernova remnants to confirm. But now the model builders can at least enjoy the relief of having good reason to believe that they are not bad off the track.
Nature, 2021. DOI: 10.1038 / s41586-021-03391-9 (About DOIs).