In a paper to be published in the journal Physical Review Letters (arXiv.org preprint), Professor George Fuller of the University of California, San Diego, and co-authors show that disruptions of neutron stars by primordial black holes in dark matter-rich environments, such as the Galactic center and dwarf spheroidal galaxies, provide a viable site for the rapid neutron capture (r-process) nucleosynthesis, a set of reactions in nuclear astrophysics that are responsible for the creation of half the elements heavier than iron in the Universe.
This artist’s conception illustrates one of the most primitive supermassive black holes known at the core of a young, star-rich galaxy. Image credit: NASA / JPL-Caltech.
Astrophysicists like to say we are the by-products of stars, stellar furnaces that long ago fused hydrogen and helium into the elements needed for life through the process of stellar nucleosynthesis. But what about the heavier elements in the periodic chart, elements such as gold, platinum and uranium?
Scientists believe most of these ‘r-process’ elements were created, either in the aftermath of the collapse of massive stars and the associated supernova explosions, or in the merging of binary neutron star systems.
“A different kind of furnace was needed to forge gold, platinum, uranium and most other elements heavier than iron. These elements most likely formed in an environment rich with neutrons,” Professor Fuller said.
Professor Fuller and his colleagues — Alex Kusenko and Volodymyr Takhistov, theoretical astrophysicists at the University of California, Los Angeles — offer another means by which stars could have produced these heavy elements: tiny primordial black holes that came into contact with and are captured by neutron stars, and then destroy them.
Many astrophysicists believe primordial black holes could be a byproduct of the Big Bang and that they could now make up some fraction of dark matter.
If these tiny black holes follow the distribution of dark matter in space and co-exist with neutron stars, Professor Fuller and colleagues contend in the paper that some interesting physics would occur.
The team calculates that, in rare instances, a neutron star will capture such a black hole and then be devoured from the inside out by it. This violent process can lead to the ejection of some of the dense neutron star matter into space.
“Small black holes produced in the Big Bang can invade a neutron star and eat it from the inside. In the last milliseconds of the neutron star’s demise, the amount of ejected neutron-rich material is sufficient to explain the observed abundances of heavy elements,” Professor Fuller said.
“As the neutron stars are devoured, they spin up and eject cold neutron matter, which decompresses, heats up and make these elements.”
In addition, the team calculated that ejection of nuclear matter from primordial black holes devouring neutron stars would produce three other unexplained phenomenon observed by astronomers.
“They are a distinctive display of infrared light (sometimes termed a kilonova), a radio emission that may explain the mysterious fast radio bursts from unknown sources deep in the cosmos, and the positrons detected in the Galactic center by X-ray observations,” Professor Fuller said.
“Each of these represent long-standing mysteries. It is indeed surprising that the solutions of these seemingly unrelated phenomena may be connected with the violent end of neutron stars at the hands of tiny black holes.”
_____
George M. Fuller et al. 2017. Primordial Black Holes and r-Process Nucleosynthesis. Physical Review Letters, in press; arXiv: 1704.01129
