Computer Simulations Shed New Light On Neutron Star Formation
June 28, 2013

SASI Behavior In The Formation Of Supernovae And Neutron Stars

redOrbit Staff & Wire Reports - Your Universe Online

Researchers from the Max Planck Institute for Astrophysics (MPA) have, for the first time, created three-dimensional computer models in order to study the formation of neutron stars at the center of collapsing stars, officials from the German research center announced earlier this week.

By creating what they call the most expensive and elaborate computer simulations of the process to date, the team of investigators confirmed, "extremely violent, hugely asymmetric sloshing and spiral motions occur when the stellar matter falls towards the center," MPA said. "The results of the simulations thus lend support to basic perceptions of the dynamical processes that are involved when a star explodes as supernova."

After stars with at least eight times the mass of our Sun end their lives in a massive explosion, the stellar gas is forcefully expelled into the surrounding space. These supernovae are among the most energetic and brightest phenomena in the entire universe, the researchers said, and can shine brighter than an entire galaxy for a period of several weeks.

"Supernovae are also the birth places of neutron stars, those extraordinarily exotic, compact stellar remnants, in which about 1.5 times the mass of our Sun is compressed to a sphere with the diameter of Munich," the Institute explained. "This happens within fractions of a second when the stellar core implodes due to the strong gravity of its own mass. The catastrophic collapse is stopped only when the density of atomic nuclei - gargantuan 300 million tons in a sugar cube - is exceeded."

The exact processes that cause the disruption of the star, and how the implosion of a stellar core can be reversed to an explosion, are still up for debate, they said. In many preferred scenarios, a tremendous amount of the electrically neutral subatomic particles, known as neutrinos, are produced at the extreme temperatures and densities of the collapsing stellar core and nascent neutron star.

Under this scenario, experts believe the neutrinos heat the gas surrounding the hot neutron star, essentially igniting the explosion. The particles pump energy into the stellar gas, building up pressure until a shock wave is accelerated to disrupt the star in a supernova, the researchers said. Since the processes involved cannot be replicated under laboratory conditions, the MPI researchers turned to computer simulations to investigate.

However, since exceptionally complex mathematical equations are required to describe the motion of the stellar gas and the physical processes that occur within the collapsing stellar core, they needed the aid of some of the planet´s most powerful supercomputers. They gained access to those machines thanks to the Rechenzentrum Garching (RZG), the computing center of the Max Planck Institute for Plasmaphysics (IPP) and the Max Planck Society.

With access to these supercomputers, the MPA researchers "could now for the first time simulate the processes in collapsing stars in three dimensions and with a sophisticated description of all relevant physics," the Institute said. According to Florian Hanke, a PhD student who performed the simulations, the researchers used "nearly 16,000 processor cores in parallel mode, but still a single model run took about 4.5 months of continuous computing."

After analyzing several terabytes worth of data, the researchers discovered the stellar gas exhibited "the violent bubbling and seething with the characteristic rising mushroom-like plumes driven by neutrino heating in close similarity to what can be observed in boiling water." In addition, they discovered "powerful, large sloshing motions, which temporarily switch over to rapid, strong rotational motions" - a behavior which had been observed previously and dubbed Standing Accretion Shock Instability (SASI).

When SASI occurs, the initial spherical nature of the supernova shock is suddenly broken because the shock develops "large-amplitude, pulsating asymmetries by the oscillatory growth of initially small, random seed perturbations," the Institute researchers said.

Previously, the phenomenon had only been observed in less detailed, incomplete computer models, but it was also demonstrated in this most recent research, proving SASI plays an important role in the processes behind neutron star formation - even in more realistic computer simulations.

"It does not only govern the mass motions in the supernova core but it also imposes characteristic signatures on the neutrino and gravitational-wave emission, which will be measurable for a future Galactic supernova," research team member Bernhard Mueller explained. "Moreover, it may lead to strong asymmetries of the stellar explosion, in course of which the newly formed neutron star will receive a large kick and spin."

The Institute investigators now intend to take a closer look at the measurable effects connected to SASI, and to enhance their predictions of associated signals, they said. Furthermore, they also plan to perform additional and longer-term simulations in order to understand how the instability acts with neutrino heating, making the later process more efficient. Ultimately, they hope to determine whether or not this process is the long-sought after mechanism which triggers a supernova explosion and results in a neutron star.

Their research is detailed in the study "SASI Activity in Three-Dimensional Neutrino-Hydrodynamics Simulations of Supernova Cores", published in the Astrophysical Journal. A related study, "Shallow Water Analogue of the Standing Accretion Shock Instability: Experimental Demonstration and a Two-Dimensional Model," was published in 2011 by the journal Physical Review Letters.