An international team of astronomers used powerful supercomputers from the Lawrence Berkeley National Laboratory in the USA and the National Astronomical Observatory of Japan. After years of dedicated research and the consumption of more than five million supercomputer computing hours, they have finally created the world’s first high-resolution 3D hydrodynamic radiation simulations for exotic supernovae! This finding will appear in the latest issue The Astrophysical Journal.
Supernova explosions are the most spectacular terminations of massive stars, as they close their life cycles in a self-destructive manner, instantly releasing the luminosity of billions of suns and illuminating the entire universe. During this explosion, the heavy elements formed in the star are also ejected, laying the foundation for the birth of new stars and planets and playing a key role in the origin of life. Therefore, supernovae have become one of the leading topics in modern astrophysics, involving numerous important astronomical and physical problems in both theory and observation and of significant research value.
Over the past half century, research has given us a fairly comprehensive understanding of supernovae. However, recent large-scale observations have begun to reveal many unusual stellar explosions (exotic supernovae) that challenge and overturn previously established knowledge of supernova physics.
Mysteries of exotic supernovae
Among the exotic supernovae are the most tangled supernovae and the ever-luminous supernovae. The brightness of superluminous supernovae is about 100 times higher than that of ordinary supernovae, which usually retain their brightness for only a few weeks to 2-3 months. In contrast, the recently discovered ever-luminous supernovae retain their brightness for several years or longer.
Even more amazing, several exotic supernovae show irregular and intermittent changes in brightness, reminiscent of fountain-like eruptions. These strange supernovae may hold the key to understanding the evolution of the most massive stars in the universe.
Origins and evolutionary structures
The origin of these exotic supernovae is still not fully understood, but astronomers believe they may come from unusual massive stars. For stars between 80 and 140 solar masses, as they near the end of their lives, their cores undergo carbon fusion reactions. During this process, high-energy photons can create electron-positron pairs, triggering pulsations in the nucleus and leading to several violent contractions.
These contractions release huge amounts of fusion energy and trigger explosions, resulting in large eruptions in stars. These eruptions themselves can be similar to normal supernova explosions. Additionally, when materials from different eruptive periods collide, it is possible to create phenomena similar to superluminous supernovae.
Currently, the number of such massive stars in the universe is relatively rare, which is consistent with the lack of special supernovae. Therefore, scientists believe that stars with masses in the range of 80 to 140 solar masses are highly likely to be progenitors of special supernovae. However, the unstable evolutionary structures of these stars make their modeling quite challenging, and current models remain mostly limited to one-dimensional simulations.
Limitations of previous models
However, previous one-dimensional models were found to have serious shortcomings. Supernova explosions create significant turbulence, and turbulence plays a key role in the explosion and brightness of supernovae. However, one-dimensional models are unable to simulate turbulence from first principles. These challenges have made gaining a deep understanding of the physical mechanisms behind exotic supernovae still a major problem in current theoretical astrophysics.
A leap in simulation capabilities
This high-resolution simulation of supernova explosions presented immense challenges. As the scale of the simulation increased, maintaining high resolution became increasingly difficult, significantly increasing the complexity and computational demands, while requiring many physical processes to be taken into account. Ke-Jung Chen emphasized that their team’s simulation code has advantages over other competing groups in Europe and America.
Previous relevant simulations have been mainly limited to one-dimensional and a few two-dimensional fluid models, whereas in exotic supernovae, multidimensional effects and radiation play a crucial role, affecting the light emission and overall explosion dynamics.
The power of radiative hydrodynamic simulations
Radiation hydrodynamic simulations take into account the propagation of radiation and its interaction with matter. This complex radiative transfer process makes the calculations exceptionally challenging, with computational requirements and difficulties much higher than fluid simulations. Given the team’s extensive experience in modeling supernova explosions and performing large-scale simulations; they finally managed to create the world’s first three-dimensional simulations of the radiation hydrodynamics of exotic supernovae.
Findings and Implications
The research team’s findings suggest that the phenomenon of intermittent eruptions in massive stars may exhibit properties similar to multiple fainter supernovae. When materials from different eruptive periods collide, approximately 20-30% of the gas’s kinetic energy can be converted into radiation, which explains the superluminescent supernova phenomenon.
In addition, the radiative cooling effect causes the gas eruption to form a dense but uneven three-dimensional sheet structure, and this sheet layer becomes the primary source of light emission in the supernova. The results of their simulation effectively explain the observational features of the aforementioned exotic supernovae.
Through state-of-the-art supercomputer simulations, this study has significantly advanced insights into the physics of exotic supernovae. As the next-generation supernova survey projects begin, astronomers will detect more exotic supernovae, further shaping our understanding of the final stages of common massive stars and their explosion mechanisms.
Reference: “Multidimensional radiative hydrodynamic simulations of pulsating paired instability supernovae” by Ke-Jung Chen, Daniel J. Whalen, SE Woosley and Weiqun Zhang, 14 Sep 2023, The Astrophysical Journal.