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      Souped-up supernovas may produce much of the universe’s heavy elements

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      Violent explosions of massive, magnetized stars may forge most of the universe’s heavy elements, such as silver and uranium.

      These r-process elements, which include half of all elements heavier than iron, are also produced when neutron stars merge (SN: 10/16/17). But collisions of those dead stars alone can’t form all of the r-process elements seen in the universe. Now, scientists have pinpointed a type of energetic supernova called a magnetorotational hypernova as another potential birthplace of these elements.

      The results, described July 7 in Nature, stem from the discovery of an elderly red giant star — possibly 13 billion years old — in the outer regions of the Milky Way. By analyzing the star’s elemental makeup, which is like a star’s genetic instruction book, astronomers peered back into the star’s family history. Forty-four different elements seen in the star suggest that it was formed from material left over “by a special explosion of one massive star soon after the Big Bang,” says astronomer David Yong of the Australian National University in Canberra.

      The ancient star’s elements aren’t from the remnants of a neutron star merger, Yong and his colleagues say. Its abundances of certain heavy elements such as thorium and uranium were higher than would be expected from a neutron star merger. Additionally, the star also contains lighter elements such as zinc and nitrogen, which can’t be produced by those mergers. And since the star is extremely deficient in iron — an element that builds up over many stellar births and deaths — the scientists think that the red giant is a second-generation star whose heavy elements all came from one predecessor supernova-type event.

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      Simulations suggest that the event was a magnetorotational hypernova, created in the death of a rapidly spinning, highly magnetized star at least 25 times the mass of the sun. When these stars explode at the end of their lives as a souped-up type of supernova, they may have the energetic, neutron-rich environments needed to forge heavy elements.

      Magnetorotational hypernovas might be similar to collapsars — massive, spinning stars that collapse into black holes instead of exploding. Collapsars have previously been proposed as birthplaces of r-process elements, too (SN: 5/8/19).

      The researchers think that magnetorotational hypernovas are rare, composing only 1 in 1,000 supernovas. Even so, such explosions would be 10 times as common as neutron star mergers today, and would produce similar amounts of heavy elements per event. Along with their less energetic counterparts, called magnetorotational supernovas, these hypernovas could be responsible for creating 90 percent of all r-process elements, the researchers calculate. In the early universe, when massive, rapidly rotating stars were more common, such explosions could have been even more influential.

      The observations are impressive, says Stan Woosley, an astrophysicist at the University of California, Santa Cruz, who was not involved in the new study. But “there is no proof that the [elemental] abundances in this metal-deficient star were made in a single event. It could have been one. It could have been 10.” One of those events might even have been a neutron star merger, he says.

      The scientists hope to find more stars like the elderly red giant, which could reveal how frequent magnetorotational hypernovas are. For now, the newly analyzed star remains “incredibly rare and demonstrates the need for … large surveys to find such objects,” Yong says. More

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      A shadowy birthplace may explain Jupiter’s strange chemistry

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      Jupiter may have formed in a shadow that kept the planet’s birthplace colder than Pluto. The frigid temperature could explain the giant world’s unusual abundance of certain gases, a new study suggests.

      Jupiter consists mostly of hydrogen and helium, which were the most common elements in the planet-spawning disk that spun around the newborn sun. Other elements that were gases near Jupiter’s birthplace became part of the planet, too, but in only the same proportions as they existed in the protoplanetary disk (SN: 6/12/17).

      Astronomers think the sun’s composition of elements largely reflects that of the protoplanetary disk, so Jupiter’s should resemble that solar makeup — at least for elements that were gases. But nitrogen, argon, krypton and xenon are about three times as common on Jupiter, relative to hydrogen, as they are on the sun.

      “This is the main puzzle of Jupiter’s atmosphere,” says Kazumasa Ohno, a planetary scientist at the University of California, Santa Cruz. Where did those extra elements come from?

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      If Jupiter was born at its current distance from the sun, the temperature of the planet’s birthplace would have been around 60 kelvins, or –213˚ Celsius. In the protoplanetary disk, those elements should be gases at that temperature. But they would freeze solid below about 30 kelvins, or –243˚ C. It’s easier for a planet to accrete solids than gases. So if Jupiter somehow arose in a much colder environment than its current home, the planet could have acquired solid objects laden with those extra elements as ice.

      For this reason, in 2019 two different research teams independently made the radical suggestion that Jupiter had originated in the deep freeze beyond the current orbits of Neptune and Pluto, then spiraled inward toward the sun.

      Now Ohno and astronomer Takahiro Ueda of the National Astronomical Observatory of Japan propose a different idea: Jupiter formed where it is, but a pileup of dust in between the planet’s orbit and the sun blocked sunlight, casting a long shadow that cooled Jupiter’s birthplace. The frosty temperature made nitrogen, argon, krypton and xenon freeze solid and become a greater part of the planet, the scientists suggest in a study in the July Astronomy & Astrophysics.

      The dust that cast the shadow came from rocky objects closer to the sun that collided and shattered. Farther from the sun, where the protoplanetary disk was colder, water froze, giving rise to objects that resembled snowballs. When those snowballs collided, they were more likely to stick together than shatter and thus didn’t cast much of a shadow, the researchers say.

      “I think it’s a clever fix of something that might have been difficult to rectify otherwise,” says Alex Cridland, an astrophysicist at the Max Planck Institute for Extraterrestrial Physics in Garching, Germany.

      Cridland was one of the scientists who had suggested that Jupiter formed beyond Neptune and Pluto. But that theory, he says, means Jupiter had to move much closer to the sun after birth. The new scenario avoids that complication.

      Measuring the atmospheric composition of Saturn may pinpoint the birthplace of Jupiter.NASA, ESA, A. Simon/GSFC, M.H. Wong/UCB, the OPAL Team

      How to test the new idea? “Saturn might hold the key,” Ohno says. Saturn is nearly twice as far from the sun as Jupiter is, and the scientists calculate that the dust shadow that chilled Jupiter’s birthplace barely reached Saturn’s. If so, that means Saturn arose in a warmer region and so should not have acquired nitrogen, argon, krypton or xenon ice. In contrast, if the two gas giants really formed in the cold beyond the present orbits of Neptune and Pluto, then Saturn should have lots of those elements, like Jupiter.

      Thanks to the Galileo probe, which dove into the Jovian atmosphere in 1995, astronomers know these abundances for Jupiter. What’s needed, the researchers say, is a similar mission to Saturn. Unfortunately, while orbiting Saturn, the Cassini spacecraft (SN: 8/23/17) measured only an uncertain level of nitrogen in the Ringed Planet’s atmosphere and detected no argon, krypton or xenon, so Saturn doesn’t yet constrain where the two gas giants arose.     More

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      Scientists spotted an electron-capture supernova for the first time

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      A long-predicted type of cosmic explosion has finally burst onto the scene.

      Researchers have found convincing evidence for an electron-capture supernova, a stellar explosion ignited when atomic nuclei sop up electrons within a star’s core. The phenomenon was first predicted in 1980, but scientists have never been sure that they have seen one. A flare that appeared in the sky in 2018, called supernova 2018zd, matches several expected hallmarks of the blasts, scientists report June 28 in Nature Astronomy.

      “These have been theorized for so long, and it’s really nice that we’ve actually seen one now,” says astrophysicist Carolyn Doherty of Konkoly Observatory in Budapest, who was not involved with the research.

      Electron-capture supernovas result from stars that sit right on the precipice of exploding. Stars with more than about 10 times the sun’s mass go supernova after nuclear fusion reactions within the core cease, and the star can no longer support itself against gravity. The core collapses inward and then rebounds, causing the star’s outer layers to explode outward (SN: 2/8/17). Smaller stars, with less than about eight solar masses, are able to resist collapse, instead forming a dense object called a white dwarf (SN: 6/30/21). But between about eight and 10 solar masses, there’s a poorly understood middle ground for stars. For some stars that fall in that range, scientists have long suspected that electron-capture supernovas should occur.

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      During this type of explosion, neon and magnesium nuclei within a star’s core capture electrons. In this reaction, an electron vanishes as a proton converts to a neutron, and the nucleus morphs into another element. That electron capture spells bad news for the star in its war against gravity because those electrons are helping the star fight collapse.

      According to quantum physics, when electrons are packed closely together, they start moving faster. Those zippy electrons exert a pressure that opposes the inward pull of gravity. But if reactions within a star chip away at the number of electrons, that support weakens. If the star’s core gives way — boom — that sets off an electron-capture supernova.

      But without an observation of such a blast, it remained theoretical. “The big question here was, ‘Does this kind of supernova even exist?’” says astrophysicist Daichi Hiramatsu of the University of California, Santa Barbara and Las Cumbres Observatory in Goleta, Calif. Potential electron-capture supernovas have been reported before, but the evidence wasn’t definitive.

      So Hiramatsu and colleagues created a list of six criteria that an electron-capture supernova should meet. For example, the explosions should be less energetic, and should forge different varieties of chemical elements, than more typical supernovas. Supernova 2018zd checked all the boxes.

      A stroke of luck helped the team clinch the case. Most of the time, when scientists spot a supernova, they have little information about the star that produced it — by time they see the explosion, the star has already been blown to bits. But in this case, the star showed up in previous images taken by NASA’s Hubble Space Telescope and Spitzer Space Telescope. Its properties matched those expected for the type of star that would produce an electron-capture supernova.

      “All together, it really is very promising,” says astrophysicist Pilar Gil-Pons of Universitat Politècnica de Catalunya in Barcelona. Reading the researchers’ results, she says, “I got pretty excited, especially about the identification of the progenitor.” 

      Finding more of these supernovas could help unveil their progenitors, misfit stars in that odd mass middle ground. It could also help scientists better nail down the divide between stars that will and won’t explode. And the observations could reveal how often these unusual supernovas occur, an important bit of information for better understanding how supernovas seed the cosmos with chemical elements. More

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      This moon-sized white dwarf is the smallest ever found

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      Only a smidge bigger than the moon, a newfound white dwarf is the smallest of its kind known. 

      The white dwarf, a type of remnant left behind when certain stars peter out, has a radius of about 2,100 kilometers, researchers report June 30 in Nature. That’s remarkably close to the moon’s approximately 1,700-kilometer radius. Most white dwarfs are closer to the size of Earth, which has a radius of about 6,300 kilometers.

      The white dwarf’s small girth means, counterintuitively, that it is also one of the most massive known objects of its kind, at about 1.3 times the sun’s mass. That’s because white dwarfs shrink as they gain mass (SN: 8/12/20).

      “That’s not the only very amazing characteristic of this white dwarf,” astrophysicist Ilaria Caiazzo of Caltech said June 28 in an online news conference. “It is also rapidly rotating.”

      The white dwarf spins around approximately once every seven minutes. And it has a powerful magnetic field, more than a billion times the strength of Earth’s. Caiazzo and colleagues discovered the unusual stellar remnant, dubbed ZTF J1901+1458 and located about 130 light-years from Earth, using the Zwicky Transient Facility at Palomar Observatory in California, which searches for objects in the sky that change in brightness.

      The white dwarf probably formed when two white dwarfs orbited one another and merged to create a single white dwarf with an extra-large mass and extra-small size, the team says. That convergence would also have spun up the white dwarf and given it a strong magnetic field.

      This white dwarf is living on the edge: If it were much more massive, it wouldn’t be able to support its own weight, causing it to explode. Studying such objects can help scientists understand the limits of what’s possible for these dead stars. More

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      Gravitational waves reveal the first known mergers of a black hole and neutron star

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      Caught in a fatal inward spiral, a neutron star met its end when a black hole swallowed it whole. Gravitational ripples from that collision spread outward through the cosmos, eventually reaching Earth. The detection of those waves marks the first reported sighting of a black hole engulfing the dense remnant of dead star. And in a surprise twist, scientists spotted a second such merger just days after the first.

      Until now, all identified sources of gravitational waves were twos of a kind: either two black holes or two neutron stars, spiraling around one another before colliding and coalescing (SN: 1/21/21). The violent cosmic collisions create waves that stretch and squeeze the fabric of spacetime, undulations that can be sussed out by sensitive detectors.

      The mismatched pairing of a black hole and neutron star was the final type of merger that scientists expected to find with current gravitational wave observatories. By pure coincidence, researchers spotted two of these events within 10 days of one another, the LIGO, Virgo and KAGRA collaborations report in the July 1 Astrophysical Journal Letters.

      Not only have unions between black holes and neutron stars not been seen before via gravitational waves, the smashups have also never been spotted at all by any other means.

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      “This is an absolute first look,” says theoretical physicist Susan Scott of the Australian National University in Canberra, a member of the LIGO collaboration.

      The result adds another tick mark to the tally of new discoveries made with gravitational waves. “That’s worth celebration,” says astrophysicist Cole Miller of the University of Maryland in College Park, who was not involved with the research. Since the first gravitational waves were detected in 2015, the observatories keep revealing new secrets. “It’s fantastic new things; it’s not just the same old, same old,” he says.

      Signs of the black hole-neutron star collisions registered in the LIGO and Virgo gravitational wave observatories in 2020, on January 5 and January 15. The first merger consisted of a black hole about 8.9 times the mass of the sun and a neutron star about 1.9 times the sun’s mass. The second merger had a 5.7 solar mass black hole and a 1.5 solar mass neutron star. Both collisions occurred more than 900 million light-years from Earth, the scientists estimate.

      To form detectable gravitational waves, the objects that coalesce must be extremely dense, with identities that can be pinned down by their masses. Anything with a mass above five solar masses could only be a black hole, scientists think. Anything less than about three solar masses must be a neutron star.

      One earlier gravitational wave detection involved a black hole merging with an object that couldn’t be identified, as its mass seemed to fall in between the cutoffs that separate black holes and neutron stars (SN: 6/23/20). Another previous merger may have resulted from a black hole melding with a neutron star, but the signal from that event wasn’t strong enough for scientists to be certain that the detection was the real deal. The two new detections clinch the case for black hole and neutron star meetups.

      One of the new events is more convincing than the other. The Jan. 5 merger was seen in just one of LIGO’s two gravitational wave detectors, and the signal has a relatively high probability of being a false alarm, Miller says. “If this were the only event, then you would not be as confident.” The Jan. 15 event, however, “seems pretty solid,” he says.

      Epic rendezvous between neutron stars and black holes happen regularly throughout the cosmos, the detections suggest. Based on the pace of detections, the researchers estimate that these events take place about once a month within 1 billion light-years of Earth.

      [embedded content]
      In a newly reported class of cosmic smashup, a neutron star (apparent in orange in this computer simulation, after the video zooms in) and black hole (dark gray) spiral inward, producing gravitational waves (blue) in a dance that ends when the black hole swallows the neutron star.

      Scientists don’t yet know how neutron stars and black holes come to meet up. They might form together, as two stars that orbit one another until both run out of fuel and die, with one collapsing into a black hole and the other forming a neutron star. Or the two objects might have formed separately and met up in a crowded region packed with many neutron stars and black holes.

      As a black hole and neutron star spiral inward and merge, scientists expect that the black hole could rip the neutron star to shreds, producing a light show that could be observed with telescopes. But astronomers found no fireworks in the aftermath of the two newly reported encounters, nor any evidence that the black holes deformed the neutron stars.

      That could be because in both cases the black hole was significantly larger than the neutron star, suggesting that the black hole gulped down the neutron star whole in a meal worthy of Pac-Man, Scott says.

      If scientists could spot a black hole shredding a neutron star in the future, that could help researchers pin down the properties of the ultradense, neutron-rich material that makes up the dead stars (SN: 4/20/21).

      In past detections of gravitational waves, the Advanced Laser Interferometer Gravitational-Wave Observatory, or LIGO, based in the United States, has teamed up with Virgo, in Italy. The new observations are the first to include members of a third observatory, KAGRA, in Japan (SN: 1/18/19). But the KAGRA detector itself didn’t contribute to the results, as scientists were still preparing it to detect gravitational waves at the time. LIGO, Virgo and KAGRA are all currently offline while scientists tinker with the detectors, and will resume their communal search for cosmic collisions in 2022. More

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      Dark matter may slow the rotation of the Milky Way’s central bar of stars

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      Dark matter can be a real drag. The pull of that unidentified, invisible matter in the Milky Way may be slowing down the rotating bar of stars at the galaxy’s heart.

      Based on a technique that re-creates the history of the slowdown in a manner akin to analyzing a tree’s rings, the bar’s speed has decreased by at least 24 percent since it formed billions of years ago, researchers report in the August Monthly Notices of the Royal Astronomical Society.

      That slowdown is “another indirect but important piece of evidence that dark matter is a thing, not just a conjecture, because this can’t happen without it,” says astrophysicist Martin Weinberg of the University of Massachusetts Amherst, who was not involved with the study.

      Many spiral galaxies, including the Milky Way, contain a central bar-shaped region densely packed with stars and surrounded by the galaxy’s pinwheeling arms. The bar also has some groupies: a crew of stars trapped by the bar’s gravitational influence. Those stars orbit a gravitationally stable point located alongside the bar and farther from the galaxy’s center, known as a Lagrange point (SN: 2/26/21). 

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      If the bar’s rotation slows, it will grow in length, and the bar’s tagalongs will also move outward. As that happens, that cohort of hangers-on will gather additional stars. According to computer simulations of the process, those additional stars should arrange themselves in layers on the outside of the group, says astrophysicist Ralph Schönrich of University College London. The layers of stars imprint a record of the group’s growth. “It’s actually like a tree that you can cut up in your own galaxy,” he says.

      Schönrich and astrophysicist Rimpei Chiba of the University of Oxford studied how the composition of stars in the group changed from its outer edge to its deeper layers. Data from the European Space Agency’s Gaia spacecraft revealed that stars in the outer layers of the bar tended to be less enriched in elements heavier than helium than were stars in the inner layers. That’s evidence for the group of stars moving outward, as a result of the bar slowing, the researchers say. That’s because stars in the center of the galaxy — which would have glommed on to the group in the more distant past — tend to be more enriched in heavier elements than those farther out.

      The bar’s slowdown hints that a gravitational force is acting on it, namely, the pull of dark matter in the galaxy. Normal matter alone wouldn’t be enough to reduce the bar’s speed. “If there is no dark matter, the bar will not slow down,” Chiba says.

      But the results have drawn some skepticism. “Unfortunately, this is not yet convincing to me,” says astrophysicist Isaac Shlosman of the University of Kentucky in Lexington. For example, he doubts that the tree ring layering would really occur. It is “hard to believe that this is the case in a realistic system” as opposed to in a simplified computer simulation, he says.

      Weinberg, on the other hand, says that although the study relies on a variety of assumptions, he suspects it’s correct. “It’s got the right smell.” More

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      Any aliens orbiting these 2,000 stars could spot Earth crossing the sun

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      Astronomers look for distant planets by watching for the shadow the worlds cast when passing between their star and Earth. If any aliens are searching for other intelligent life, they could spot us using the same trick.

      Now, scientists have identified 1,715 star systems whose hypothetical inhabitants could have seen Earth cross in front of the sun sometime in the last 5,000 years. Another 319 stars will come into the right positions for spotting Earth in the next 5,000 years, astrophysicist Jackie Faherty and astronomer Lisa Kaltenegger report in the June 23 Nature.

      Those 2,034 stars had or will have “the front row seat to finding Earth as a transiting planet,” says Faherty, of the American Museum of Natural History in New York City.

      Seventy-five of the stars are close enough that human-made radio waves have already reached them, and seven of those stars have potentially habitable planets.

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      Faherty and Kaltenegger, of Cornell University, used maps of more than 1 billion stars from the European Space Agency’s Gaia spacecraft, which measures stars’ movements and distances from Earth. The researchers identified the Earth transit zone, the region of space from which stars can see Earth cross the sun, and ran the clock backward and forward to see stars move in and out of that zone. “The way I think about this is, ‘Where are we the aliens?’” Kaltenegger says.

      Previous research identified stars that can currently see Earth silhouetted against the sun (SN: 4/20/16). But those studies did not take into account stellar movements through space and time. The new work shows that most star systems with a good look at Earth will keep that view for thousands of years.

      The newly described stellar collection includes some of the nearest and most well-known stars with planets, including Ross 128 and TRAPPIST-1, with its septet of rocky worlds (SN: 2/22/17). More

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      Cosmic filaments may be the biggest spinning objects in space

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      Moons do it, stars do it, even whole galaxies do it. Now, two teams of scientists say cosmic filaments do it, too. These tendrils stretching hundreds of millions of light-years spin, twirling like giant corkscrews.

      Cosmic filaments are the universe’s largest known structures and contain most of the universe’s mass (SN: 1/20/14). These dense, slender strands of dark matter and galaxies connect the cosmic web, channeling matter toward galaxy clusters at each strand’s end (SN: 7/5/12).

      At the instant of the Big Bang, matter didn’t rotate; then, as stars and galaxies formed, they began to spin. Until now, galaxy clusters were the largest structures known to rotate. “Conventional thinking on the subject said that’s where spin ends. You can’t really generate torques on larger scales,” says Noam Libeskind, cosmologist at the Leibniz Institute for Astrophysics Potsdam in Germany.

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      So the discovery that filaments spin — at a scale that makes galaxies look like specks of dust — presents a puzzle. “We don’t have a full theory of how every galaxy comes to rotate, or every filament comes to rotate,” says Mark Neyrinck, cosmologist at University of the Basque Country in Bilbao, Spain.

      To test for rotation, Neyrinck and colleagues used a 3-D cosmological simulation to measure the velocities of dark matter clumps as the clumps moved around a filament. He and his colleagues describe their results in a paper posted in 2020 at arXiv.org and now in press with the Monthly Notices of the Royal Astronomical Society. Meanwhile, Libeskind and colleagues searched for rotation in the real universe, they report June 14 in Nature Astronomy. Using the Sloan Digital Sky Survey, the team mapped galaxies’ motions and measured their velocities perpendicular to filaments’ axes.

      [embedded content]
      A computer simulation shows how a cosmic filament twists galaxies and dark matter into a strand of the cosmic web. Filaments pull matter into rotation and toward clusters at their ends, visualized here with “test particles” shaped like comets.  

      The two teams detected similar rotational velocities for filaments despite differing approaches, Neyrinck says, an “encouraging [indication] that we’re looking at the same thing.”

      Next, researchers want to tackle what makes these giant space structures spin, and how they get started. “What is that process?” Libeskind says. “Can we figure it out?” More