When a star goes supernova, it experiences a large increase in brightness and sheds much of its mass through a gigantic explosion. A hypernova is much the same – except with a massive star, a particularly strong explosion and about a hundred times more energy in play. These giant cosmic bombs are quite rare, making observation and ensuing research very difficult. Now, a team of researchers from the Academia Sinica’s Institute of Astronomy and Astrophysics (ASIAA) in Taipei has used supercomputer power to fill in the gaps using high-performance simulations.

“A star must be 140 to 260 times the mass of the Sun to die in such a manner,” Ke-Jung Chen, head of the ASIAA team, said of hypernovae. The team set out to simulate these monstrous explosions to discover what they looked like hundreds of days after their explosions, basing their numerical models of hypernovae on the “pair-instability” model of supernovae. The team opted for this model over the “core-collapse” model (or “black hole” model) because hypernovae don’t leave anything behind – not even a black hole.

To run these simulations, Chen and his team turned to the supercomputer at the Central for Computational Astrophysics (CfCA) of the National Astronomical Observatory of Japan (NAOJ). The CfCA is equipped with a 1005-node Cray XC50 system called ATERUI II. The system’s 40,200 cores (provided by Intel Xeon Gold 6148 CPUs) deliver around 2.1 Linpack petaflops of performance, placing the system 181st on the most recent Top500 list of the world’s most powerful supercomputers. 

Chen said this computational firepower was necessary to run the “extremely challenging” code, which — at 300 days of simulated time — aimed for timescales an order of magnitude longer than previous pair-instability simulations (around 30 days of simulated time). “The larger the simulation scale, to keep the resolution high, the entire calculation will become very difficult and demand much more computational power,” he said. “Not to mention that the physics involved [are] also complicated.”

This longer timescale allowed them to assess the full decay of nickel-56 (Ni-56), which Chen described as “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 researchers found that during this decay, the gas expanded and formed thin-shell structures.

“The temperature inside the gas shell is extremely high,” Chen said. “From calculation we understand that there should be [around] 30 percent energy used in gas movement, then the remaining … 70 percent [of the] energy likely becomes the supernova’s luminosity. Earlier models have ignored the gas dynamic effects, so the supernova luminosity results were all overestimated.”

The researchers are confident that their results will contribute to further understanding of not just hypernovae, but supernovae as well.