The heaviest elements in the periodic table are created in something called the r-process – a series of nuclear reactions that happen in the most energetic events in the Universe. Gold, platinum, uranium, together with half of the atoms heavier than iron are formed this way. A new paper argues that the prime site of the r-process is collisions between neutron stars.
For the r-process to take place, an obscene amount of energy is required. Some supernovae might do it, but we have observational evidence of neutron stars colliding doing it, and the possibility that a collision between a neutron star and a black hole could also produce such elements.
The paper estimates the production rate in these two scenarios and found that binary neutron star collisions are the major source of heavy elements. The findings are reported in The Astrophysical Journal Letters.
“What we find exciting about our result is that to some level of confidence we can say binary neutron stars are probably more of a goldmine than neutron star-black hole mergers,” lead author Hsin-Yu Chen, a postdoc in MIT’s Kavli Institute for Astrophysics and Space Research, said in a statement.
Stars are powered by nuclear fusion and they start by fusing hydrogen into helium. Once they run out of hydrogen in their core, they undergo changes and begin fusing helium, which releases less energy and creates heavier elements such as carbon and oxygen. Once helium is spent, the heavier stars might start fusing carbon and oxygen, releasing less energy than fusing helium, and so on until the star has a core of iron. No matter what you do, you can’t get energy by fusing iron.
But iron is only the 26th element of the periodic table? How are the rest of the elements produced? When massive stars run out of fuel, they just collapse on themselves and become supernovae, which creates a lot of heavy elements. Exploding white dwarfs create others, and dying low-mass stars create others still. But for the heavier elements, more energetic processes are needed.
The observation of GW170817, the first historic detection of neutron stars colliding, brought new understanding. The event was first “seen” using gravitational waves and then spotted by electromagnetic telescopes. And in the light observed, there were signatures of heavy elements.
“The magnitude of gold produced in the merger was equivalent to several times the mass of the Earth,” Chen added. “That entirely changed the picture. The math showed that binary neutron stars were a more efficient way to create heavy elements, compared to supernovae.”
The work uses simulations to show what is the ideal configuration in the interaction between a neutron star and a black hole to produce heavy elements. The data show that it is far from easy or common. In the best-case scenario, binary neutron stars produce twice as many heavy elements as a mixed collision. But the work suggests that they might be responsible for up to 100 times more heavy metals.
This work will be refined with future observations of collisions that provide better constraints on the types of events and their properties.