The lightest elements, hydrogen, helium, and lithium, were formed shortly after the Big Bang, and elements up to iron were formed later mostly in the cores of stars. Previous research suggested that atomic nuclei often needed to absorb neutrons rapidly in a phenomenon known as the “r-process;” there lay a key to the formation of heavier elements.
In2017 astronomers detected a collision between neutron stars, superdense neutron-rich remains of large stars that ended their lives as supernovas. This discovery revealed that most r-process elements were forged in the materials blasted off of merging neutron stars. The collision resulted in a black hole, and previous research suggested that the main source of r-process elements from the merger was the accretion disk that formed around the black hole. The same physics could be found around different astrophysical systems.
The researchers created computer simulations of the accretion disks around collapsars, which are “collapsing, rapidly spinning massive stars whose deaths result in supernovas and black holes.” Daniel Siegel, the lead author of the study and a theoretical physicist at the Perimeter Institute for Theoretical Physics in Waterloo, Canada says that much of the material in these accretion disks circularizes around the newly formed black hole. These innermost regions of the disks are incredibly hot and dense, and subatomic particles interact in ways that cause protons to convert into neutrons. According to Siegel, this creates the initial conditions needed for the formation of heavy elements.
Though similar, collapsars are significantly less common than neutron star mergers. However, collapsars fire off more material and thus result in a greater expulsion of r-process elements than neutron star collisions do. Siegel says that collapsars should produce at least 80% of the heavy elements in our galaxy and that almost 20% would result from neutron star mergers.
There is still much to learn about accretion disks and the formation of elements. In the future, scientists would like to explore how elements are created in other kinds of accretion disks aside from those found around neutron stars and collapsars. Siegel hopes to explore the cosmological implications of this work and see what the results suggest for the chemical evolution and assembly of galaxies.