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Home , James Lattimer, Neutron star, Neutron star merger, Reticulum II

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Tracing ’scosmicoriginstory NEWS FEATURE Astronomers thought they had finally figured out where the gold, platinum, and other heavy elements in the universe came from. In of recent results, they’re not so sure.

Ken Croswell, Writer

Desperate phone calls made in the dead of night amounts of europium, an element that’s even rarer in rarely convey good news, much less first word of a the cosmos than gold. Frebel told Ji to observe an- major scientific discovery. Alex Ji made such a call in other in the same small . That star also 2015 from atop a mountain in Chile, where he was turned out to be rich in europium, as did a third. In using one of the world’s largest telescopes. “This was the end, seven of the nine Ji observed had ex- actually the first time that I had taken data on a tele- treme levels of the rare element. It would take some scope ever,” says Ji, then a graduate student at the time for the astronomers to interpret their surprising Massachusetts Institute of Technology in Cambridge. result, but when they did, it bore on one of the most “I was absolutely convinced that I had done pressing questions in : How does the uni- something wrong.” verse make its heaviest elements? So, just after midnight, Ji called his advisor Anna It’s a question astronomers have wrestled with for Frebel to describe what he had just seen and could more than half a century. Elements such as gold, silver, barely believe: A faint star in a dim nearby galaxy and platinum don’t normally form in the nuclear reactions named II seemed to have extraordinary that power stars. So most researchers had long thought

How does the universe make heavy elements such as gold? It’s a question astronomers have wrestled with for more than half a century—and the answers are only starting to emerge. Image credit: Science Source/Tom McHugh.

Published under the PNAS license. Published January 20, 2021.

PNAS 2021 Vol. 118 No. 4 e2026110118 https://doi.org/10.1073/pnas.2026110118 | 1of4 Downloaded by guest on September 30, 2021 only up to . Iron is a nuclear dead end because it is the most stable element: Fusing it to create heavier ones actually requires , which means stars don’t normally make them. Elements slightly heavier than iron, such as copper and zinc, can be forged in the turmoil of a explosion. But what about even heavier elements? In the 1950s, astronomers and physicists identified two processes as the source of these elements (1, 2). One, called the s-process (“s” for slow), involves nuclear reactions near the end of a star’s life that release . Unlike , neutrons can sail into the positively charged nucleus of an without getting repelled. As a result, the iron nuclei the star inherited at birth occasionally capture these neutrons; because the flux is low, the newly formed atomic nuclei have plenty of time to decay if they are radioactive. During this process, neutrons in the newly formed nuclei can turn into protons, thereby creating ele- ments with greater atomic numbers. Astronomers have long observed s-process elements such as technetium on the surfaces of the aging stars that are making them. But the s-process didn’t explain everything. In particular, most gold, silver, and platinum, as well as all and , are synthesized when a rapid Although supernova explosions, such as the one that created the Crab flux of neutrons bombards iron nuclei. New neutrons (shown here based on 24 exposures from the Hubble Telescope), were once accumulate in nuclei before those captured earlier can thought to forge gold and other heavy elements, astronomers now know that decay, leading to even heavier elements. This is the ’ this isn t so for most of these cataclysmic events. Image credit: NASA, ESA, and r-process, as in “rapid.” It occurs in nuclear bombs, Allison Loll/Jeff Hester (Arizona State University). which is why for decades astronomers thought that supernova explosions drove the r-process and that these elements were created when massive stars accounted for the universe’s gold and platinum. explode at the ends of their lives as fiery supernovae. Whereas an aging star can spend millions of years Ji’s 2015 discovery helped overturn that conven- churning out s-process elements, a supernova rams tional wisdom. Two years later, a dramatic observation neutrons into iron nuclei and forges r-process material seemed to confirm the new thinking. Astronomers in mere seconds. saw heavy elements actually forming when two The rapidity of the process, however, means that dense neutron stars spiraled into each other, which it’s much more difficult to study the r-process than the implied that merging binaries and not s-process. Until recently, no one had ever seen the supernovae were the main source of the heaviest r-process actually operate in space, and no one has elements. ever seen a supernova create r-process elements. Now, however, doubts have arisen, suggesting To study the r-process, astronomers have long fo- that the original solution—supernova explosions— cused on europium. Unlike gold, whose spectral lines may be an important part of the answer after all. lie in the region of the and are blocked by ’s atmosphere, Rare Finds europium’s spectral lines appear in the visible part of These heavy elements barely exist. If you add up every the spectrum. The high europium abundance that Ji atom in the universe from gallium (atomic number 31, found in the galaxy Reticulum II yields a key clue to the which is the number of protons in the nucleus) r-process (3): “Whatever produced this r-process ele- to uranium (atomic number 92), you’dhaveonly ment is very rare,” Ji says. In particular, it must be 1/2,300th of the total number of iron (atomic much rarer than an ordinary supernova. number 26). Europium (atomic number 63) and gold This conclusion follows from the of Reticu- (atomic number 79) belong in this same category. lum II. It is an ultra-faint , with just a few The heaviest elements are rare because stars tens of thousands of stars, and it is so dim that as- hardly make them. To survive, every star must gener- tronomers spotted it only a few years ago—even ate energy so as not to collapse under its own . though it’s in our backyard, just 100,000 light-years This energy comes from reactions ini- from Earth. The nine stars Ji observed told the tiated by intense heat and . The reactions history of the numerous supernovae that had occurred start as fuses to form , which later in the galaxy. Two stars are extremely iron-poor, be- gets transformed into heavier elements, such as car- cause they formed earliest, after only a few stars had bon and oxygen. But these reactions produce energy exploded to supply the iron. These two iron-poor stars

2of4 | PNAS Croswell https://doi.org/10.1073/pnas.2026110118 News Feature: Tracing gold’s cosmic origin story Downloaded by guest on September 30, 2021 also lack europium. Then additional supernova ex- Flash of Inspiration plosions occurred, raising the level of iron in the gal- All that changed on August 17, 2017. Astronomers axy and in stars that formed later. Somewhere along detected signals from the merger of two neutron stars the way, a rare r-process event showered the galaxy in a galaxy 130 million light-years away. The resulting with europium. The seven europium-rich stars Ji ob- gravitational radiation reached Earth (6), along with a served formed from this material, which also had burst of gamma rays (7) and—crucially—a flash of higher levels of iron. Thus, whatever drove the visible light (8): “The holy trinity all at once,” says Stan r-process was a lot rarer than a typical supernova—so Woosley at the University of California, Santa Cruz. rare it had happened only once in Reticulum II and not Woosley, a theorist, had long struggled to get su- at all in the previously studied ultra-faint dwarf . pernova models to make r-process material, so he had The rare r-process event might have been an exotic already concluded that neutron star mergers were supernova. But Ji and his colleagues favored a dif- probably the solution. By observing the visible light of ferent idea, one that another graduate student had the 2017 merger, which came from the radioactive explored four decades earlier. decay of the many r-process elements produced during the event, astronomers deduced the amount of Star Turns material made: roughly 5 percent of a solar , in In 1973, was pursuing his doctorate in line with the predictions from Schramm and his stu- astronomy at the University of Texas, Austin. Although dents. It was the first time, and still the only time, that supernovae were then thought to be the source of anyone had seen the r-process in action beyond Earth. r-process material, Lattimer’s advisor, David Schramm, Unfortunately, Schramm never lived to see the historic suspected that these explosions might not actually discovery; he died piloting a small plane in Colorado produce the heaviest elements. So Schramm sug- in 1997. gested that Lattimer look at something else: a neutron The 2017 that confirmed ’ star, the tiny but super-dense core of a dead massive Schramm s idea convinced many astronomers that star. “Since I was a student at the time, I was more than these events, once considered exotic and far-fetched, made nearly all of the gold, platinum, and other happy to get paid to work on something that was r-process elements in the universe. Ji, now at the exciting and kind of outlandish,” Lattimer says. “He Carnegie Observatories in Pasadena, CA, says he too suggested we try to find ways we could get neutron thought that neutron star mergers were the answer, stars to blow up.” After all, if you want lots of neutrons and he still leans that way, but he and others are also to drive the r-process, a neutron star seems like a considering alternate theories. “I think most astrono- good bet. mers outside the field said, ‘Oh, great. This looks like Another graduate student—Jocelyn Bell, in Eng- the thing is all solved. Now it’s neutron star mergers,’” land—had discovered the first neutron star a few years says John Cowan at the University of Oklahoma in earlier. Lattimer and Schramm calculated what would Norman. “But I think the people working in the field happen if a neutron star spiraled into a . The realize there’s still some complications to solve” (9). black hole’s tidal forces would tear the neutron star For one thing, ever since the 1980s astronomers apart, ramming neutrons into nearby iron nuclei and have seen europium in some of the ’s oldest producing a profusion of gold, platinum, and other stars, which formed soon after the birth of our galaxy. r-process elements. The amount made, their study If neutron star mergers really made this europium, showed, would be about 5 percent of a , how did two neutron stars in a spiral into “ plus or minus 5 percent of a solar mass. Looking each other so fast? The coalescence, which is thought back, I laugh, because that kind of covers all the bases, to take roughly 100 million years or more, occurs be- ” including zero, Lattimer says. Their article, published cause the system radiates gravitational waves, causing in 1974, argued that mergers between neutron stars the neutron stars to slowly draw closer together. and black holes could make all of the gold, platinum, Astronomers are therefore taking another look at and other r-process elements in the universe, elimi- supernovae, because massive stars can explode just a nating the need for their creation in supernovae (4). few million years after their birth. If some of these It was also in 1974 that astronomers discovered the supernovae managed to forge gold, europium, and first neutron star binary: two neutron stars orbiting other r-process elements, then stellar explosions each other. Even before this discovery, Lattimer and could explain the presence of this material in the Schramm recognized that the merger of two neutron galaxy’s senior citizens. stars could also make r-process material. That calcu- In fact, supernova explosions are a necessary part lation was more complicated, however, and Lattimer of the solution, says Chiaki Kobayashi at the University never pursued it. Schramm later enlisted another stu- of Hertfordshire in England. Neutron star mergers “are dent, Eugene Symbalisty, who confirmed the concept not enough” to explain all the r-process material in the in the early 1980s (5). universe, Kobayashi says. She and her colleagues But the idea had little initial impact. Lattimer recalls recently set out to understand the origin of every one of his superiors saying that he “should have element from carbon to uranium. They calculated how worked on a real problem rather than something so stars with different lifetimes endow the galaxy with speculative.” After all, no one had ever seen two dead different elements, then compared the observed stars coalesce and create r-process material. compositions of stars from young to old. For example,

Croswell PNAS | 3of4 News Feature: Tracing gold’s cosmic origin story https://doi.org/10.1073/pnas.2026110118 Downloaded by guest on September 30, 2021 short-lived high-mass stars make lots of oxygen, so If, as Kobayashi says, supernovae also create r-process this element appeared early in the Milky Way’s history, elements, it’s only a rare breed—less than 1% of all whereas iron took longer to form, because most of supernovae—that do so. that element comes from the explosions of long-lived Future discoveries could change the thinking stars. By comparing their models of the galaxy’s again, especially the observation of a second neutron chemical with actual observations of euro- star merger that spawns r-process material. “It’s never pium and other r-process elements in stars of different good to base a whole paradigm on one observation,” ages, Kobayashi’s team concluded that neutron star Woosley notes. Seeing a second such merger would ’ mergers made only a fraction of the universe s help pin down the of these rare events and r-process material; some rare type of supernova made also how much r-process material each one produces. the rest (10). In addition, astronomers may someday find a pair ’ Ironically, Kobayashi can t explain the best-known of neutron stars so close together that they will merge “ ” r-process element: gold. This is a really big mystery, in only a few million years. If that neutron star binary is she says. Her calculations indicate that neither neutron young—for example, if it resides in a star-forming re- star mergers nor supernova explosions make nearly gion—then it will mean that such binaries can form enough gold. quickly and merge quickly, a sign that neutron star It’s possible that researchers have greatly over- mergers might have occurred soon after the Milky estimated the amount of gold in the universe. But the Way’s birth. That would explain the presence of eu- two different ways they determine the cosmic gold ropium in the galaxy’s oldest stars and eliminate the abundance—by observing the and measuring need for supernovae to create the element. meteorites—agree with each other. So Kobayashi says On the other hand, astronomers may succeed in that the estimates of the nuclear reaction rates that seeing r-process material emerge from a supernova. make gold may be incorrect. Such a discovery would demonstrate that these stellar Star Witness explosions did forge some of the most precious met- Despite current uncertainties, some things now seem als on Earth. But those observations will be difficult to clear about the r-process. First, it’s rare. Whether pull off; the faint glow of supernova-born r-process neutron star mergers or some exotic supernovae mint elements would be hard to discern amidst the gold, silver, and other r-process elements, these events brighter light sparked by the creation of iron. happen infrequently, even in a galaxy as large as ours. The quest for a comprehensive understanding of The Milky Way has about two supernovae a century, the r-process sites in space will continue for some but r-process events occur roughly a thousand times time. “I literally thought that three years ago we were less often. basically done with this question,” says Ji, who now Second, neutron star mergers made some of the realizes things are more complicated than they had gold and platinum on Earth and throughout the uni- seemed in 2017. “We’re not done with this question, verse. Third, most supernova explosions make none. but I hope we figure it out in the next five years.”

1 E. M. Burbidge, G. R. Burbidge, W. A. Fowler, F. Hoyle, Synthesis of the elements in stars. Rev. Mod. Phys. 29, 547–650 (1957). 2 A. G. W. Cameron, Nuclear reactions in stars and nucleogenesis. Publ. Astron. Soc. Pac. 69, 201–222 (1957). 3 A. P. Ji, A. Frebel, A. Chiti, J. D. Simon, R-process enrichment from a single event in an ancient dwarf galaxy. Nature 531,610–613 (2016). 4 J. M. Lattimer, D. N. Schramm, Black-hole-neutron-star collisions. Astrophys. J. Lett. 192, L145–L147 (1974). 5 E. Symbalisty, D. N. Schramm, Neutron star collisions and the r-process. Astrophys. Lett. 22, 143–145 (1982). 6 B. P. Abbott et al., GW170817: Observation of gravitational waves from a binary neutron star inspiral. Phys. Rev. Lett. 119, 161101 (2017). 7 B. P. Abbott et al., Gravitational waves and gamma-rays from a binary neutron star merger: GW170817 and GRB 170817A. Astrophys. J. Lett. 848, L13 (2017). 8 B. P. Abbott et al., Multi-messenger observations of a binary neutron star merger. Astrophys. J. Lett. 848, L12 (2017). 9 J. J. Cowan et al., Origin of the heaviest elements: The rapid neutron-capture process. Rev. Mod. Phys., in press. 10 C. Kobayashi, A. I. Karakas, M. Lugaro, The origin of elements from carbon to uranium. Astrophys. J. 900, 179 (2020).

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