Supernova Simulations Reveal Mysteries of Dying Stars

An international team of astronomers have created the longest consistent 3D model of a neutrino-driven supernova explosion to date, helping scientists to better understand the violent deaths of massive stars.

The research, conducted using supercomputers in Australia, Germany, and the DiRAC facility in the UK, is published in the journal Monthly Notices of the Royal Astronomical Society.

The largest explosions in the universe, so-called 'supernovae,' occur when stars many times larger than our own Sun reach the end of their lives and exhaust the nuclear fuel at their centres. At this point the innermost part of the star, an iron core itself about 1.5 times as massive as the Sun, succumbs to gravity and collapses to an ultra-dense neutron star within a fraction of a second.

In the process, the outer layers of the star are expelled in a gigantic supernova explosion, which ejects material at velocities of thousands of kilometres per second. Such supernovae are regularly observed in distant galaxies, and within the Milky Way we can still see the debris of many of them thousands of years later.

But a puzzle remains: how is the collapse of the star turned into an explosion? The team, from Monash University, Queen's University Belfast, and the Max Planck Institute for Astrophysics, have worked on a solution to this problem, and the most promising theory suggests that extremely light and weakly interacting particles called neutrinos are the key to this process.

Vast numbers of neutrinos are emitted from the surface of the young neutron star, and if the heating caused by the initial collapse is sufficiently strong, the neutrino-heated matter drives an expanding shock wave through the star and the collapse is reversed. Scientists have long attempted to show that this idea works with the help of computer simulations, but the computer models often still fail to explode, and can't be run long enough to reproduce observed supernovae.

"What is crucial for success in three dimensions is the violent churning of hot and cold material behind the shock wave, which develops naturally due to the neutrino heating," explains Dr. Tobias Melson, a co-author of the study at the Max Planck Institute for Astrophysics in Germany. "But it often seems we need to stir these churning motions a bit more to obtain an explosion."

To explore this possibility, the team simulated the fusion of oxygen to silicon in a star 18 times the size of our Sun, for the last 6 minutes before the supernova. They found that they could obtain a successful explosion because the collapsing silicon-oxygen shell was strongly stirred already.

They then followed the explosion for more than 2 seconds. Although it still takes about a day for the shock to reach the surface, they could tell that the explosion and the left-over neutron star were starting to look like the ones that we observe in nature.

"It's reassuring that we now get plausible explosion models without having to tweak them by hand," comments Dr. Bernhard Mueller of the Monash Centre for Astrophysics in Australia, the lead author of the study.

Now that the team have shown that longer duration simulations are feasible, they plan to systematically explore how different supernovae can be produced from different parent stars. Ultimately the goal is to understand why there is so much variation among supernovae and their remnants: for example, why are some neutron stars kicked out with velocities of many hundreds of kilometres per second, while some, like the famous Crab pulsar, move much more slowly? This is only one among many questions to be addressed with the next computer models that the team are planning.

Reference: "Supernova Simulations from a 3D Progenitor Model -- Impact of Perturbations and Evolution of Explosion Properties," B. Müller, T. Melson, A. Heger, H.-T. Janka, 2017 Aug. 2, Monthly Notices of the Royal Astronomical Society [https://doi.org/10.1093/mnras/stx1962, preprint: https://arxiv.org/abs/1705.00620].

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