Supernovae happen. We’ve witnessed it enough that we’re pretty sure of that. Why they happen has been a completely different problem. While we’ve worked to understand the physics driving these immense explosions, we have occasionally gone through awkward times when the stars in our models stop exploding. By adding more realistic physics, the models have generally boomed again, and right now we are in a period when the latest models thankfully seem to be self-destructing.
The challenge is trying 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.
Now data from the Chandra X-ray Observatory indicates that a mechanism used in recent supernova models is likely correct. The results are published this week in Nature.
It goes boom (most of the time)
The supernovae in question here occurs when a massive star runs out of fuel, causing its core to collapse. Here you see a potential problem: how does 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. Layers above the nucleus, devoid of the energy pushing them out, plummeted to the nucleus, hitting the neutron star and bouncing back. This rebound is what blows the outer visible layers of the star to pieces.
Unfortunately, this doesn’t really work. The outer layers of the star are also being cut off from the energy that opposes gravity, and they are also beginning to pour towards the core. Somewhere in the star, the reflective layers that shoot out will run into the more distant layers that are still collapsing inward. The result is a shock front that explodes before reaching the star’s surface. Nothing goes boom.
However, the equilibrium point is reached close enough to the star’s surface that an additional input of energy would be enough to turn things around again in an exploding mode. And physicists came up with a rather unlikely source for this energy: neutrinos. These particles are notable for rarely interacting with other matter, so they appear to be a terrible candidate for transferring energy to the material churning in the star’s outer layers. But so many are produced during the core collapse that neutrino-powered heating is a thing, even if it’s not something you’d like to have to reheat your leftovers.
And luckily it has consequences in this context. The material heated by neutrinos keeps trying to expand and escape the star. The material that has not been baked by neutrinos is still trying its best to collapse. The result is dramatic convection in the star’s outer layers, as collapsing and exploding materials squeeze past each other. This has the potential to create asymmetric explosions, which we have seen happen. And it also affects the material that is ejected.
The freeze
Neutrino-controlled heating may seem a bit 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 simply refers to a combination of low density and extremely high energies. It is high enough that some of the recently formed atoms are eventually disassembled into protons, neutrons and alpha particles, a combination of two neutrons and two protons. (An alpha particle is the same as the nucleus of a typical helium atom.)
As the material cools, the energy and density drop to where all of this material begins to form larger atomic nuclei in a process called an alpha-rich freeze. This process has a distinct atomic signature, as the physics of the freeze is likely to form a number of specific elements and isotopes. So by looking at the remains of the exploded star, we may be able to find evidence that an alpha-rich freeze has occurred.
And that’s exactly what has been done in this new study. One of the isotopes produced by alpha-rich freezes is 56Ni, which quickly decays to 56Fe. And previous images of the supernova remnants in Cassiopeia A have shown that there are areas in the ejected material that are rich in iron. So a collaboration between American and Japanese researchers looked for the presence of chromium and titanium, which are also produced during an alpha-rich freeze, in these iron-rich regions.
Obviously, the researchers found them, otherwise this article wouldn’t have had to be written. Equally critically, chromium and titanium were present in amounts consistent with their formation in a proton-rich, high-energy plume of material.
Equally important, the supernova models suggest that the plumes of material powered by neutrino heating should travel in the neighborhood of 4,000 to 5,000 kilometers per second. And the iron-rich material moves at more than 4,000 kilometers per second, getting 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 the model stars actually explode – they do so in a way that seems to fit an existing supernova remnant. Obviously, this is something we want to look to other supernova remnants to confirm. But for now, at least, the modellers can enjoy the relief that they have good reason to believe they are not on bad track.
Nature, 2021. DOI: 10.1038 / s41586-021-03391-9 (about DOIs).