The answers to many questions in astronomy are hidden behind the veil of deep time.
One of those questions is about the role that supernovae played in the early Universe. The job of the first supernovae was to forge the heaviest elements that were not forged in the Big Bang. How did that process develop? How did those first stellar explosions develop?
A trio of researchers turned to a supercomputer simulation to find some answers.
Their results are presented in a paper titled ‘Gas Dynamics of Nickel-56 Decay Warming in Peer Instability Supernovae’. The lead author is Ke-Jung Chen of the Academia Sinica, Institute of Astronomy and Astrophysics, Taiwan. The article is published in The Astrophysical Journal.
The work refers to a particular type of supernova called hypernova. They are basically supernovae on steroids. Hypernovae are approximately 100 times more powerful than supernovae of the garden variety, and only occur with stars that have between 130 and 250 solar masses.
Scientists have studied supernovae a lot. Researchers understand how they work and what types there are. And they know how they forge elements heavier than hydrogen and helium and send those elements to the Universe when they explode.
But there are important gaps in our understanding, especially in the early Universe.
The trio of researchers wanted to investigate hypernovae, because they believe it could give them clues about the first supernovae that occurred in the Universe, and how the first elements were produced. In the early Universe, stars tended to be more massive, so there could be more hypernovae.
But hypernovae are extremely rare now, and observing them is problematic. They then resorted to supercomputer simulations.
With their simulation, they thoroughly investigated the core of the simulated hypernovae to see what the exploding star looked like 300 days after the explosion began.
(ASIAA / Ken Chen)
Above: A 2D snapshot of a pair instability supernova when the blast waves are about to traverse the star’s surface.
There are two ways in which hypernovae are formed: from the collapse of the nucleus and from the instability of pairs.
In a core collapse supernova, a massive star has reached the end of its life and is running out of fuel. As the fusion decreases, the external pressure of the fusion decreases. Lacking external pressure, the gravitational energy of the star itself pushes toward the core.
Finally, gravitational energy causes the nucleus to collapse and the star to explode like a supernova. Depending on the mass of the star, it can leave a neutron star remnant or a black hole.
A pair instability supernova occurs in extremely massive stars with around 130 to 250 solar masses. It occurs when electrons and their antimatter counterparts, positrons, are produced in the star.
That creates instability in the star’s core and reduces the internal radiation pressure it takes to support such a massive star against its own enormous gravity. The instability begins a partial collapse, which triggers an out-of-control thermonuclear explosion. Finally, the star is destroyed by a massive explosion, leaving no remnant.
For their simulations, the team focused on pair instability supernovae. One of the reasons for that choice is the large amount of nickel-56 that pair instability supernovae can create.
Nickel-56 is a radioactive isotope of nickel and plays an important role in our observations of supernovae. The decomposition of Ni-56 is what creates the glow of a supernova. Without it, a supernova would be a bright flash, without lingering light.
The team used the supercomputer from the Computational Astrophysics Center (CfCA) of the Japan National Astronomical Observatory (NAOJ) for their simulations.
It’s a Cray XC50, and when it started operating in 2018, it was the world’s fastest supercomputer for astrophysical simulations. Could all of that power help shed some light on the early Universe?
According to lead author Chen, the entire project was extremely challenging.
In a translated press release, Chen said “the higher the simulation scale, to keep the resolution high, the whole calculation will be very difficult and will require much more computational power, not to mention that the physics involved is also complicated.”
To combat this, Chen said, its best advantage is its “well-crafted code and robust program structure.” The trio of researchers have experience in long-term simulations of supernovae, so they were well positioned to do this job.
This is not the first simulation of a hypernova. Other researchers are also interested in understanding them and have performed their own simulations. But while the previous simulations ran for 30 days after the blast, it ran for 300 days.
(ASIAA / Ken Chen)
Above: A 3D profile of an instability pair supernova. The blue cube shows all the simulated space. The orange region is where nickel 56 decomposes.
A key reason for this was Nickel-56. As a result, Ni-56 does more than create the long-lasting glow of a supernova. It plays a continuous role in the explosion. To be thorough, the team ran the simulation of three separate parent stars.
A hypernova needs an extremely massive progenitor star, sometimes more than 200 solar masses. Those hypernovae can create a huge amount of Ni-56.
According to the document, they can synthesize between 0.1 and 30 solar masses of radioactive Ni-56. And in addition to creating all that light, the Ni-56 does other things.
In their article, the authors write that all of that Ni-56 “could also generate significant dynamic effects deep within the ejection that are capable of mixing elements and affecting the observation signatures of these events.”
The team wanted to investigate the “relationship between gas motion and energy radiation within the supernova.” They discovered that in the initial stage of Ni-56 decomposition, the heated gas expanded and formed structures with thin shells.
Explaining one of the simulation results, Chen said that “the temperature inside the gas shell is extremely high. According to calculations, we understand that there must be ~ 30 percent energy used in the movement of the gas, so the ~ The remaining 70 percent energy can become the luminosity of the supernova. Previous models have ignored the effects of gas dynamics, so the results of the luminosity of the supernova were overestimated. “
The article gives more details. “We found that the expansion of the hot 56Ni bubble forms a layer at the base of the ejection silicon layer ~ 200 days after the explosion, but that hydrodynamic instabilities that would mix 56Ni with the 28Si-rich ejection do not develop / 16O. However, while the dynamic effects of 56Ni warming may be weak, they could affect the observation signatures of some PI SNe by diverting the decomposition energy towards the internal expansion of the ejection at the expense of reigniting at later times “
(Chen et al., 2020)
Above: a study figure. The team simulated three types of hypernovae, represented by the three columns. The rows are snapshots of the simulation at 20, 100 and 300 days. The red line in each image represents the shell of the Ni-56 hot bubble. Simulations showed that the expansion of the Ni-56 bubble does not cause any mixing. Mixing in parent star U225, far to the right, is due to instability of reverse shock.
This new understanding of pair instability hypernovae will certainly expand our understanding of the phenomenon. And it could be an aid for future observations.
Although hypernovae are rare in our time, that has not always been the case. Since hypernovae require very massive stars, and those stars were more common in the early Universe, it stands to reason that there were more hypernovae in the past.
But soon, we may have instruments capable of seeing the ancient light of some of those hypernovae.
The authors write that “PI SNe
If these future telescopes can observe these early hypernovae, then studies like this will pave the way for those observations and provide a way to understand something of what we see.
This article was originally published by Universe Today. Read the original article.
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