Primordial black holes may have helped
to forge heavy elements
Date: August 4, 2017
Source: University of California - San Diego
Summary: Astronomers like to say we are the byproducts
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? Astronomers believe most of these "r-process
elements" -- elements much heavier than iron -- 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.
FULL STORY
Artist’s depiction of
a neutron star. Credit
Credit: NASA
Astronomers like to
say we are the byproducts of stars, stellar furnaces that long ago fused
hydrogen and helium into the elements needed for life through the process of
stellar nucleosynthesis.
As the late Carl Sagan
once put it: "The nitrogen in our DNA, the calcium in our teeth, the iron
in our blood, the carbon in our apple pies were made in the interiors of
collapsing stars. We are made of star stuff."
But what about the
heavier elements in the periodic chart, elements such as gold, platinum and
uranium?
Astronomers believe
most of these "r-process elements" -- elements much heavier than iron
-- 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," explained George Fuller, a theoretical astrophysicist
and professor of physics who directs UC San Diego's Center for Astrophysics and
Space Sciences. "These elements most likely formed in an environment rich
with neutrons."
In a paper published
August 7 in the journal Physical Review Letters, he and two other
theoretical astrophysicists at UCLA -- Alex Kusenko and Volodymyr Takhistov --
offer another means by which stars could have produced these heavy elements:
tiny black holes that came into contact with and are captured by neutron stars,
and then destroy them.
Neutron stars are the
smallest and densest stars known to exist, so dense that a spoonful of their
surface has an equivalent mass of three billion tons.
Tiny black holes are
more speculative, but many astronomers believe they could be a byproduct of the
Big Bang and that they could now make up some fraction of the "dark
matter" -- the unseen, nearly non-interacting stuff that observations
reveal exists in the universe.
If these tiny black
holes follow the distribution of dark matter in space and co-exist with neutron
stars, Fuller and his colleagues contend in their paper that some interesting
physics would occur.
They calculate that,
in rare instances, a neutron star will capture such a black hole and then
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," Fuller explained. "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."
"As the neutron
stars are devoured," he added, "they spin up and eject cold neutron
matter, which decompresses, heats up and make these elements."
This process of
creating the periodic table's heaviest elements would also provide explanations
for a number of other unresolved puzzles in the universe and within our own
Milky Way galaxy.
"Since these
events happen rarely, one can understand why only one in ten dwarf galaxies is
enriched with heavy elements," said Fuller. "The systematic
destruction of neutron stars by primordial black holes is consistent with the
paucity of neutron stars in the galactic center and in dwarf galaxies, where
the density of black holes should be very high."
In addition, the
scientists calculated that ejection of nuclear matter from the tiny 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," said Fuller. "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."
Funding for this
project was provided by the National Science Foundation (PHY-1614864) at UC San
Diego and by the U.S. Department of Energy (DE-SC0009937) at UCLA. Alex Kusenko
was also supported, in part, by the World Premier International Research Center
Initiative (WPI), MEXT, Japan.
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