Neutron star collisions are a ‘gold mine’ of heavy elements, study finds | MIT News

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Most items lighter than iron are forged from the core of stars. The white-hot center of a star fuels the fusion of protons, squeezing them together to build heavier and heavier elements. But beyond iron, scientists have wondered what could give birth to gold, platinum and the rest of the heavy elements in the universe, the formation of which requires more energy than a star can. to assemble.

A new study by researchers at MIT and the University of New Hampshire finds that of two long-suspected sources of heavy metals, one is more of a gold mine than the other.

The study, published today in Letters from astrophysical journals, reports that over the past 2.5 billion years, more heavy metals have been produced in mergers of binary neutron stars, or collisions between two neutron stars, than in mergers between a neutron star and a black hole.

The study is the first to compare the two types of fusion in terms of heavy metal production and suggests that binary neutron stars are a likely cosmic source for the gold, platinum, and other heavy metals we see today. ‘hui. The results could also help scientists determine the rate at which heavy metals are produced in the universe.

“What we find exciting about our result is that at a certain level of confidence we can say that binary neutron stars are probably more of a gold mine than star neutron-black hole mergers,” says lead author Hsin-Yu Chen, post-doctoral fellow at MIT’s Kavli Institute. for astrophysics and space research.

Chen’s co-authors are Salvatore Vitale, assistant professor of physics at MIT, and François Foucart from UNH.

An efficient flash

When stars undergo nuclear fusion, they need energy to fuse protons and form heavier elements. Stars are efficient at producing lighter elements, from hydrogen to iron. However, the fusion of more than 26 protons in iron becomes energetically inefficient.

“If you want to overtake iron and build heavier elements like gold and platinum, you need another way to throw protons together,” says Vitale.

Scientists suspected that supernovae might be an answer. When a massive star collapses into a supernova, the iron at its center could eventually combine with lighter elements in the extreme fallout to generate heavier elements.

In 2017, however, a promising candidate was confirmed, in the form of a fusion of binary neutron stars, first detected by LIGO and Virgo, the gravitational wave observatories in the United States and Italy, respectively. . The detectors have picked up gravitational waves, or ripples in space-time, originating 130 million light years from Earth, following a collision between two neutron stars – massive star nuclei collapsed, which are teeming with neutrons and are among the densest objects in the world. universe.

The cosmic fusion emitted a flash of light, which contained heavy metal signatures.

“The magnitude of the gold produced in the fusion was several times the mass of the Earth,” Chen explains. “It completely changed the picture. Calculations showed that binary neutron stars were a more efficient way to create heavy elements, compared to supernovae. “

A binary gold mine

Chen and his colleagues wondered: How could neutron star mergers compare to collisions between a neutron star and a black hole? This is another type of fusion that has been detected by LIGO and Virgo and could potentially be a heavy metal factory. Under certain conditions, scientists suspect that a black hole could disrupt a neutron star so that it sparks and spits out heavy metals before the black hole completely swallows the star.

The team set out to determine how much gold and other heavy metals each type of smelting could typically produce. For their analysis, they focused on LIGO and Virgo’s detections to date of two binary neutron star mergers and two neutron star and black hole mergers.

The researchers first estimated the mass of each object in each fusion, as well as the rotational speed of each black hole, estimating that if a black hole is too massive or slow, it would swallow a neutron star before it did. ‘had the chance to produce heavy elements. They also determined the resistance of each neutron star to be disturbed. The more resistant a star, the less likely it is to produce heavy elements. They also estimated how often one merger occurs relative to the other, based on observations from LIGO, Virgo and other observatories.

Finally, the team used numerical simulations developed by Foucart to calculate the average amount of gold and other heavy metals that each fusion would produce, given various combinations of mass, rotation, degree of disturbance, and rate. occurrence of objects.

On average, the researchers found that mergers of binary neutron stars could generate two to 100 times more heavy metals than mergers between neutron stars and black holes. The four mergers on which they based their analysis are estimated to have occurred in the past 2.5 billion years. They then conclude that during this period, at least, more heavy elements were produced by mergers of binary neutron stars than by collisions between neutron stars and black holes.

The balance could tip in favor of star-neutron-black hole fusions if black holes had high spins and low masses. However, scientists have yet to observe such black holes in the two mergers detected to date.

Chen and his colleagues hope that as LIGO and Virgo resume their observations next year, more detections will improve the team’s estimates of how quickly each merger produces heavy elements. These rates, in turn, can help scientists determine the age of distant galaxies, based on the abundance of their various elements.

“You can use heavy metals the same way we use carbon to date dinosaur remains,” says Vitale. “Because all of these phenomena have different intrinsic rates and different yields of heavy elements, it will affect how you attach a time stamp to a galaxy. So this kind of study can improve those analyzes.

This research was funded, in part, by NASA, the National Science Foundation, and the LIGO lab.

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