AsianScientist (July 3, 2017) – Elaborate computer simulations strongly suggest that Cassiopeia A is the remnant of a supernova explosion powered by tiny particles known as neutrinos. These findings, by a team of researchers from RIKEN in Japan and the Max Planck Institute for Astrophysics (MPA), have been published in The Astrophysical Journal.
The physical processes that trigger and drive supernovae explosion have been an unsolved puzzle for more than 50 years. However, radioactive atomic nuclei are synthesized in the hot, innermost regions during the explosion and can thus serve as probes of the unobservable physical processes that initiate the blast.
One of the theoretical mechanisms proposed invokes nearly massless particles called neutrinos. According to the theory, neutrinos leaking out from the hot interior of the neutron star causes the surrounding gas to move violently, triggering an asymmetric explosion.
The initial asymmetry of the explosion has two immediate consequences. On the one hand, the neutron star receives a recoil momentum opposite to the direction of the stronger explosion, where the supernova gas is expelled with more violence. This effect is similar to the kick a rowing boat receives when a passenger jumps off.
On the other hand, the production of heavy elements from silicon to iron, in particular of 44Ti and 56Ni, is more efficient in directions where the explosion is stronger and where more matter is heated to high temperatures.
“We have predicted both effects some years ago by our three-dimensional simulations of neutrino-driven supernova explosions,” said Dr. Annop Wongwathanarat, researcher at the RIKEN Astrophysical Big Bang Laboratory. “The asymmetry of the radioactive ejecta is more pronounced the larger the neutron star kick is.”
To confirm these predictions, the researchers observed Cassiopeia A, the gaseous remnant of a supernova whose light reached the Earth around the year 1680. Since the neutron star propagates with an estimated speed of at least 350 kilometers per second, the asymmetry in the spatial distribution of the radioactive elements is expected to be very pronounced in Cassiopeia A.
This is exactly what was observed experimentally. While the compact remnant speeds toward the lower hemisphere, the biggest and brightest clumps with most of the radioactive titanium are found in the upper half of the gas remnant.
The computer simulation exhibited a striking similarity to the observational image. The simulation not only reproduced the spatial distributions of titanium and iron observed in Cassiopeia A, but also the total amounts of these elements, their expansion velocities and the velocity of the neutron star.
“This ability to reproduce basic properties of the observations impressively confirms that Cas A may be the remnant of a neutrino-driven supernova with its violent gas motions around the nascent neutron star,” said study co-author Dr. Hans-Thomas Janka of MPA.
More work is needed to finally prove that the explosions of massive stars are powered by energy input from neutrinos, the authors said.
The article can be found at: Wongwathanarat et al. (2017) Production and Distribution of 44Ti and 56Ni in a Three-dimensional Supernova Model Resembling Cassiopeia A.
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Source: RIKEN; Photo: NASA JPL.
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