Biggest Cosmic Nuclear Bombs -- See the First Supernovae in Cutting-edge Supercomputer Simulations

Released: July 15th., 2020, Academia Sinica, Institute of Astronomy & Astrophysics, Taiwan

A hypernova is a type of supernovae that are 100 times more energetic. Astronomers think these biggest cosmic bombs hold the key to peek into genesis moment of first supernovae birth. However, hypernovae are extremely rare in observation. Therefore, the ASIAA team led by Ke-Jung (Ken) Chen, using the NAOJ’s CfCA supercomputer, has completed high-resolution simulations to tackle this issue. By exploring deeply into the core, they discovered what hypernovae would look like after 300 days of their explosion. The highly innovative work offers an unprecedented conclusion: the effect which gas movement has on the luminosity estimation, has long been overlooked in previous theoretical models. This result boosts our understanding of hypernova formation and may prove to be instrumental in the future hypernova observations.

Right after the big bang, the only elements produced in the universe were hydrogen and helium, all the other natural elements did not come about until after the first stars born and evolved. To understand how the first stars and the elements formed in the first place necessitates the research on supernova. Nearly 50 years of supernova research has simply proven to us: it is not an easy task, many mysteries still open. Thinking that hypernova plays a key role in the breaking-through, the ASIAA Ken Chen team took a deep look into the heart of hypernova by numerical simulations. Because, despite hypernova ejects 100 times more energy than supernovae, observationally, it is in fact extremely rare. And that’s where astronomers start looking for help from good theoretical models and supercomputer simulations.

There are currently two theoretical models of how hypernovae formed, and Chen's team chose to build their simulations on the pair-instability supernovae model -- the one that is highly anticipated and relatively more robust (the other is called the core-collapse supernovae model, or, “the black hole model”). The difference between them is that one (the later) leaves a black hole, and the other (the former) doesn't even leave a black hole when it completely blows itself up. Usually when massive stars explode, they leave something behind – either a dense core called a neutron star or a black hole. But for the massive stars - the first stars in the universe - there was only hydrogen and helium, no traces of other elements yet. These very massive first stars can begin making pairs of electron-positrons in the end of their evolution, causing a runaway effect where the pressure drops in the star’s core, triggering a collapse, leading to an enormous explosion that completely disrupts the star, leaving nothing behind, not even a black hole.

"A star must be 140-260 times the mass of the Sun to die in such a manner" Chen said. Astronomers call stars that explode in this way the “pair-instability supernovae” (where the “pair” means the electron – positron pair.)

Such an explosion produces a large amount of radioactive isotope Ni 56, which according to Chen, is “the most important element in a supernova, because its decay energy accounted for most of the visible light of a supernova, and without it, many supernovae would have been too dark to observe".

The international team led by Ken Chen has used the NAOJ's CfCA supercomputer to run their high resolution hydrodynamical simulation for hypernova. Describing the code and the running “extremely challenging”, Chen explains, “larger the simulation scale, to keep the resolution high, the entire calculation will become very difficult and demand much more computational power, not to mention that the physics involved is also complicated.” To combat these, Chen said, their best advantage is their “well-craft code and a robust program structure.”

While previous simulations run for pair-instability supernovae model have only done 30 days after the explosion, Chen's team has run the simulation up to 300 days -- which allow them to study the entire decay process of Ni 56 (which has a half-life of 70 days, so the simulation had to be long enough). They are the first team who has done this. With extensive experience in simulating large scale supernovae, the team probed the relationship between the gas movement and energy radiation inside the supernova. What they found is that during the initial decay of Nickel 56, the heated gas expanded and formed thin-shell structures.

A 2-D snapshot of a pair-instability supernovae as the explosion waves is about to break through the star's surface. The tiny disturbs represent fluid instability - in a region where different elements interact and mix.

Image Credit: ASIAA/Ken Chen

A 3-D profile of a pair-instability supernovae. The blue cube shows the entire simulated space. Orange region is where nickel 56 decays.

Image Credit: ASIAA/Ken Chen

Chen said, "the temperature inside the gas shell is extremely high, from calculation we understand that there should be ~ 30% energy used in gas movement, then the remaining ~ 70% energy can likely become the supernova luminosity. Earlier models have ignored the gas dynamic effects, so the supernova luminosity results were all overestimated.”

Therefore, in the field of pair instability supernovae study, these results will certainly contribute to the further understanding of its radiation mechanism and observational characteristics.

Several studies showed that the mass of first stars in the universe would be 100 to 300 solar masses, somewhat hint that the chances for first supernovae to be pair-instability supernovae could really be high. On the other hand, the first stars may be detectable by the James Webb Space Telescope (JWST) - the successor to the Hubble Space Telescope - making the observation and theoretical work of pair-instability supernovae an important subject in the near future.

Terminology explained:

Hypernova: A hypernova is a type of stellar explosion which ejects material with an unusually high kinetic energy, an order of magnitude higher than most supernovae.

Massive Star: A massive star is a star that is larger than eight solar masses during its regular main sequence lifetime.

Pair-instability supernovae model: Massive stars, between about 130 and 250 solar masses, are thought to lead to a pair-instability supernovae (PISN). In these stars, electron-positron pairs are created in the core. This leads the star to become dynamically unstable and leads to the collapse then explosion of these stars.

Core-collapse supernovae model: Core-collapse supernovae are dramatic explosions of giant stars at the end of their thermonuclear evolution giving birth to neutron stars and black holes.

More Information:

Paper: Gas Dynamics of the Nickel-56 Decay Heating in Pair-Instability Supernovae published on July 14th. by The Astrophysical Journal


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Article written by: Ken Chen & Lauren Huang

Webpage Editor: Lauren Huang