The origin of many heavy elements, including gold and uranium, is a longstanding astrophysical mystery. While the role of nuclear fusion in forming them from lighter elements – known as the r-process – is understood, it isn’t clear exactly where in the universe the r-process is happening enough to produce the numbers of heavy elements we see around us.
The r-process can only occur in very specific circumstances where there is a high density of neutrons. This is not the case in normal stars, where fusion of heavy elements occurs through a slower chain of reactions called the s-process. Previous proposed sites for the r-process have included merging neutrons stars and certain kinds of supernova, but these would produce tell-tale neutrino and gravitational wave signals that have not been observed. An extra twist is the dwarf galaxy Reticulum II, which has an inexplicably high abundance of r-process elements.
However, an answer may be at hand, according to a new paper published in Physical Review Letters by George Fuller of the University of California, San Diego, and colleagues: neutron stars that swallow tiny black holes and are then devoured from the inside out.
Fuller and co start with the idea of primordial black holes, the hypothetical black holes formed during the Big Bang proposed as a possible answer to the riddle of dark matter. Primordial black holes were not formed by collapsing stars, and can therefore be much smaller than typical ones.
The new paper theorises that if a primordial black hole of the right size (around a billionth the mass of the Sun, packed into a space smaller an atom) crashed into a fast-spinning neutron star, it would set off a chain of events resulting in r-process nucleosynthesis.
As the black hole sucks in matter, the radius of the neutron star would decrease and, like a twirling ice skater drawing her limbs in to her body, it would spin faster. Spinning faster means it would fling out neutron-rich material, providing favourable circumstances for the r-process to occur.
The researchers calculate that – based on some assumptions about the numbers of the primordial black holes around – this scenario could account for all the observed r-process elements in the galaxy.
In addition to explaining the r-process elements, the authors propose that collisions of primordial black holes with neutron stars could also explain a couple of other astrophysical stumpers.
The ejected material would produce antimatter that would produce the signals of electron-positron annihilation measured coming from the centre of the galaxy. The final stage of neutron star implosions might also be responsible for some of the Fast Radio Bursts that perplex astronomers.
This new model is similar to one proposed in 2016 by Joseph Bramante and Tim Linden, which relied on different ideas about how dark matter might collect in a neutron star.
Determining whether either model is correct will require more detailed analysis of exactly how much matter a fast-spinning neutron star would eject, combined with observations of areas that are rich in r-process elements using the next generation of gravitational wave observatories.