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From Quanta Magazine:
A New Blast May Have Forged Cosmic Gold
For decades, researchers believed that violent supernovas forged gold and other heavy elements. But many now argue for a different cosmic quarry.
Ashley Mackenzie for Quanta Magazine
Across history and folklore, the question of where Earth’s gold came from — and maybe how to get more of it — has invited fantastical explanation. The Inca believed gold fell from the sky as either the tears or the sweat of the sun god Inti. Aristotle held that gold was hardened water, transformed when the sun’s rays penetrated deep underground. Isaac Newton transcribed a recipe for making it with a philosopher’s stone. Rumpelstiltskin, of course, could spin it from straw.
Modern astrophysicists have their own story. The coda, at least, is relatively clear: About four billion years ago, during a period called the “late veneer,” meteorites flecked with small amounts of precious metals — gold included — hammered the nascent Earth. But the more fundamental question of where gold was forged in the cosmos is still contentious.
For decades, the prevailing account has been that supernova explosions make gold, along with dozens of other heavy elements on the bottom few rows of the periodic table. But as computer models of supernovas have improved, they suggest that most of these explosions do just about as well at making gold as history’s alchemists. Perhaps a new kind of event — one that has traditionally been difficult, if not impossible, to study — is responsible.
In the past few years, a debate has erupted. Many astronomers now believe that the space-quaking merger of two neutron stars can forge the universe’s supply of heavy elements. Others hold that even if garden-variety supernovas can’t do the trick, more exotic examples might still be able to. To settle the argument, astrophysicists are searching for clues everywhere, from alchemical computer simulations to gamma-ray telescopes to the manganese crust of the deep ocean. And the race is on to make an observation that would seal the deal — catching one of the cosmos’s rarest mints with its assembly line still running.
Supernova Trouble
In 1957, the physicists Margaret and Geoffrey Burbidge, William Fowler and Fred Hoyle laid out a set of recipes for how the lives and deaths of stars could fill in almost every slot in the periodic table. That implied that humans, or at least the elements making up our bodies, were once stardust. So was gold — somehow.
“The problem itself is rather old, and now for a long time has been the last stardust secret,” said Anna Frebel, an astronomer at the Massachusetts Institute of Technology.
The Big Bang left behind hydrogen, helium and lithium. Stars then fused these elements into progressively heavier elements. But the process stops at iron, which is among the most stable elements. Nuclei bigger than iron are so positively charged, and so difficult to bring together, that fusion no longer returns more energy than you have to put in.
To make heavy elements more reliably, you can bombard iron nuclei with charge-free neutrons. The new neutrons often make the nucleus unstable. In this case, a neutron will decay into a proton (popping out both an electron and an antineutrino). The net increase of a proton leads to a new, heavier element.
When additional neutrons are thrown into a nucleus more slowly than it can decay, the process is called slow neutron capture, or the s process. This makes elements such as strontium, barium and lead. But when neutrons land on a nucleus faster than they decay, rapid neutron capture — the r process — occurs, beefing up nuclei to form heavy elements including uranium and gold.
In order to coax out the r-process elements, the Burbidges and their colleagues recognized, you would need a few things. First, you have to have a relatively pure, unadulterated source of neutrons. You also need heavy “seed” nuclei (such as iron) to capture those neutrons. You need to bring them together in a hot, dense (but not too dense) environment. And you want all this to happen during an explosive event that will scatter the products out into space.
To many astronomers, those requirements implicate one specific kind of object: a supernova.
A supernova erupts when a massive star, having fused its core into progressively heavier elements, reaches iron. Then fusion stops paying off, and the star’s atmosphere crashes down. A sun’s worth of mass collapses into a sphere only about a dozen kilometers in radius. Then, when the core reaches the density of nuclear matter, it holds firm. Energy rebounds outward, ripping apart the star in a supernova explosion visible from billions of light years away....MORE
A supernova seems to tick the necessary boxes. During the star’s collapse, protons and electrons in the core are forced together, making neutrons and converting the core into an infant neutron star. Iron is abundant. So is heat. And the glowing ejecta keep expanding out into space for millennia, dispersing the products.
By the 1990s, a specific picture had begun to emerge in computational models. Half a second after the core of a massive star collapses, a gale of neutrinos streams out, continuing for up to a minute. Some of that wind would blow off iron nuclei that could serve as seeds, along with lots and lots of neutrons.
“That was the hope,” said Thomas Janka of the Max Planck Institute for Astrophysics in Garching, Germany. “This was, I would say, the most interesting and the most promising site for forming the r-process elements for almost 20 years.” And the explanation still has its adherents. “If you open a textbook, it will tell you that the r process is made by supernova explosions,” said Enrico Ramirez-Ruiz, an astrophysicist at the University of California, Santa Cruz.
But as supernova models got more and more sophisticated, the situation got worse, not better. Temperatures in the neutrino-driven wind didn’t seem to be high enough. The wind might also be too slow, allowing seed nuclei to form so abundantly that they wouldn’t find enough neutrons to build up heavy elements all the way up to uranium. And the neutrinos could also convert neutrons back into protons — meaning there might not even be a lot of neutrons to work with.
That left theorists circling back to one of the strongest points of the supernova model. Supernovas make neutron stars, which seem indispensable to the process.
“They are fantastic for this type of nucleosynthesis,” said Stephan Rosswog at Stockholm University. “You start with this gigantic amount of neutrons that you don’t have anywhere else in the universe.” But a neutron star also has a strong gravitational field, he said. “The question is just, well, how can you convince the neutron star to eject something?”
One way to crack open a neutron star would be to use the same explosion that birthed it. That didn’t seem to work. But what if you came back later, and tore one open again?
The Neutron Star Story...
