The Aftermath of Neutron Star Collisions – Where Heavy Elements Are Made

Nuclear physicists used supercomputers to model the extreme state left behind after two ultra-dense neutron stars merged and formed a black holeA black hole is a place in space where the pull of gravity is so strong not even light can escape it. Astronomers classify black holes into three categories by size: miniature, stellar, and supermassive black holes. Miniature black holes could have a mass smaller than our Sun and supermassive black holes could have a mass equivalent to billions of our Sun.”>black hole. Their models showed how this collision expels the left over matter orbiting the resulting black hole. This situation creates the conditions needed to create the heaviest elements in the universe.

For decades, astronomers and nuclear physicists have worked to understand how and where the heavy elements in the universe were originally made. These computer simulations demonstrate how neutron stars colliding can create and expel heavy elements. These models also illustrate the flashes of light these events generate. That information can help astronomers better detect and study these events.

In August 2017, the collision of a pair of neutron stars became the first event ever detected in both gravitational wavesGravitational waves are distortions or ripples in the fabric of space and time. They were first detected in 2015 by the Advanced LIGO detectors and are produced by catastrophic events such as colliding black holes, supernovae, or merging neutron stars.”>gravitational waves and light. This event gave scientists a new window into the behavior of matter and gravity under extreme conditions.

Much of our understanding of what happens in collisions is based on 3-dimensional multi-physics simulations, run on the world’s fastest supercomputers. The modeling is complicated by the extreme gravity (requiring solving Einstein’s equations of general relativity) and the intense magnetic fields that control the matter flow and drive strong turbulence.

The simulations reported here are some of the first to follow in detail the long-term aftermath of a collision, where a disk of residual neutron starA neutron star is the collapsed core of a large (between 10 and 29 solar masses) star. Neutron stars are the smallest and densest stars known to exist. Though neutron stars typically have a radius on the order of just 10 – 20 kilometers (6 – 12 miles), they can have masses of about 1.3 – 2.5 that of the Sun.”>neutron star matter orbits the newly formed black hole. The simulations show how magnetic fields in the disk get twisted and amplified, and ultimately drive powerful relativistic jets and strong winds.

Nearly half of the mass of the disk is found to become unbound in this way, and the matter is neutron-rich enough to provide the requisite conditions for forming heavy elements, like gold and uranium, by a process of repeated rapid neutron capture. The radioactivity of these freshly synthesized isotopes will produce a detectable glow — first observed by astronomers in the 2017 event — which can be better understood by comparison to the new model results.

Reference: “Long-term GRMHD simulations of neutron star merger accretion discs: implications for electromagnetic counterparts” by Rodrigo Fernández, Alexander Tchekhovskoy, Eliot Quataert, Francois Foucart and Daniel Kasen, 30 October 2018, Monthly Notices of the Royal Astronomical Society.
DOI: 10.1093/mnras/sty2932

This work was supported in part by the U.S. Department of Energy, Office of Science, Office of Nuclear Physics. The research used resources of the National Energy Research Scientific Computing Center (NERSC), which is supported by the Office of Science of the U.S. Department of Energy.