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    A rare glimpse of a star before it went supernova defies expectations

    A rare glimpse of a star before it exploded in a fiery supernova looks nothing like astronomers expected, a new study suggests.

    Images from the Hubble Space Telescope reveal that a relatively cool, puffy star ended its life in a hydrogen-free supernova. Until now, supernovas without hydrogen were thought to originate only from extremely hot, compact stars.

    The discovery “is a very important test case for stellar evolution,” says Sung-Chul Yoon, an astrophysicist at Seoul National University in South Korea, who was not involved in the work. Theorists have some ideas about how massive stars behave right before they blow up, but such hefty stars are scant in the local universe and many are nowhere near ready to go supernova, Yoon says. Retroactively identifying the star responsible for a supernova provides an opportunity to test scenarios of how stars evolve right before exploding.

    Finding those stars, however, is difficult, explains Charlie Kilpatrick, an astronomer at Northwestern University in Evanston, Ill. A telescope must have looked at that exact region of the sky in the years leading up to the supernova. And the explosion must have happened close enough for light from its much fainter source star to have reached a telescope.

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    Although both conditions are tricky to meet, Kilpatrick is undaunted by the hunt. After scientists discovered a supernova in December 2019, in a galaxy called NGC 4666 about 46 million light-years away, he and colleagues rushed to check old Hubble observations from the same region of the sky. They wanted to find the star behind the explosion, dubbed SN 2019yvr.

    After pouring over images and cross-checking observations with those from ground-based telescopes, the team found their quarry: a star at the same spot as the supernova, observed about 2.6 years before the explosion. It appeared to be a yellow star about 6,500° Celsius and about 320 times wider than the sun.

    “I was kind of puzzled by all that,” Kilpatrick says. The supernova SN 2019yvr lacked hydrogen, so its progenitor was expected to be hydrogen-deficient, too. But “if a star lacks a hydrogen envelope, then you expect to be seeing deeper inside of the star to the hotter layers,” Kilpatrick says. That is, the star should have looked extremely hot and blue and compact — maybe 10,0000 to 50,000° C, and no more than 50 times wider than the sun. The cool, large, yellow progenitor of SN 2019yvr, on the other hand, appeared to be padded with lots of hydrogen. The researchers report the results May 5 in the Monthly Notices of the Royal Astronomical Society.

    For this kind of star to have produced a supernova like SN 2019yvr, it must have shed much of its hydrogen before blowing up, Kilpatrick says. But how?

    He and colleagues have come up with a couple scenarios. The star could have expelled much of its hydrogen into space through violent eruptions, possibly caused by some instability in the star’s core or interference from another star nearby. Or perhaps the star’s hydrogen could have been stripped off by another star that was in orbit around it.

    To whittle these possibilities down, Jan Eldridge, an astrophysicist at the University of Auckland in New Zealand, suggests turning the Hubble telescope back on that area of the sky. Astronomers should first make sure that the star seen 2.6 years before SN 2019yvr really is gone now, says Eldridge, who was not involved in the work. Researchers could also check whether a star that once orbited SN 2019yvr’s progenitor still remains.

    “They’ve found a mystery, and they’ve got some solutions,” Eldridge notes. Trying to figure out how such an unlikely star pulled off this particular supernova, she says, “is going to be fun.” More

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    Stars made of antimatter could lurk in the Milky Way

    Fourteen pinpricks of light on a gamma-ray map of the sky could fit the bill for antistars, stars made of antimatter, a new study suggests.

    These antistar candidates seem to give off the kind of gamma rays that are produced when antimatter — matter’s oppositely charged counterpart — meets normal matter and annihilates. This could happen on the surfaces of antistars as their gravity draws in normal matter from interstellar space, researchers report online April 20 in Physical Review D.

    “If, by any chance, one can prove the existence of the antistars … that would be a major blow for the standard cosmological model,” says Pierre Salati, a theoretical astrophysicist at the Annecy-le-Vieux Laboratory of Theoretical Physics in France not involved in the work. It “would really imply a significant change in our understanding of what happened in the early universe.”

    It’s generally thought that although the universe was born with equal amounts of matter and antimatter, the modern universe contains almost no antimatter (SN: 3/24/20). Physicists typically think that as the universe evolved, some process led to matter particles vastly outnumbering their antimatter alter egos (SN: 11/25/19). But an instrument on the International Space Station recently cast doubt on this assumption by detecting hints of a few antihelium nuclei. If those observations are confirmed, such stray antimatter could have been shed by antistars.

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    Intrigued by the possibility that some of the universe’s antimatter may have survived in the form of stars, a team of researchers examined 10 years of observations from the Fermi Gamma-ray Space Telescope. Among nearly 5,800 gamma-ray sources in the catalog, 14 points of light gave off gamma rays with energies expected of matter-antimatter annihilation, but did not look like any other known type of gamma-ray source, such as a pulsar or black hole.

    Based on the number of observed candidates and the sensitivity of the Fermi telescope, the team calculated how many antistars could exist in the solar neighborhood. If antistars existed within the plane of the Milky Way, where they could accrete lots of gas and dust made of ordinary matter, they could emit lots of gamma rays and be easy to spot. As a result, the handful of detected candidates would imply that only one antistar exists for every 400,000 normal stars.

    If, on the other hand, antistars tended to exist outside the plane of the galaxy, they would have much less opportunity to accrete normal matter and be much harder to find. In that scenario, there could be up to one antistar lurking among every 10 normal stars.

    But proving that any celestial object is an antistar would be extremely difficult, because besides the gamma rays that could arise from matter-antimatter annihilation, the light given off by antistars is expected to look just like the light from normal stars. “It would be practically impossible to say that [the candidates] are actually antistars,” says study coauthor Simon Dupourqué, an astrophysicist at the Institute of Research in Astrophysics and Planetology in Toulouse, France. “It would be much easier to disprove.”

    Astronomers could watch how gamma rays or radio signals from the candidates change over time to double-check that these objects aren’t really pulsars. Researchers could also look for optical or infrared signals that might indicate the candidates are actually black holes.

    “Obviously this is still preliminary … but it’s interesting,” says Julian Heeck, a physicist at the University of Virginia in Charlottesville not involved in the work.

    The existence of antistars would imply that substantial amounts of antimatter somehow managed to survive in isolated pockets of space. But Heeck doubts that antistars, if they exist, would be abundant enough to account for all the universe’s missing antimatter. “You would still need an explanation for why matter overall dominates over antimatter.” More

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    Fast radio bursts could help solve the mystery of the universe’s expansion

    Astronomers have been arguing about the rate of the universe’s expansion for nearly a century. A new independent method to measure that rate could help cast the deciding vote.

    For the first time, astronomers calculated the Hubble constant — the rate at which the universe is expanding — from observations of cosmic flashes called fast radio bursts, or FRBs. While the results are preliminary and the uncertainties are large, the technique could mature into a powerful tool for nailing down the elusive Hubble constant, researchers report April 12 at

    Ultimately, if the uncertainties in the new method can be reduced, it could help settle the longstanding debate that holds our understanding of the universe’s physics in the balance (SN: 7/30/19).

    “I see great promises in this measurement in the future, especially with the growing number of detected repeated FRBs,” says Stanford University astronomer Simon Birrer, who was not involved with the new work.

    Astronomers typically measure the Hubble constant in two ways. One uses the cosmic microwave background, the light released shortly after the Big Bang, in the distant universe. The other uses supernovas and other stars in the nearby universe. These approaches currently disagree by a few percent. The new value from FRBs comes in at an expansion rate of about 62.3 kilometers per second for every megaparsec (about 3.3 million light-years). While lower than the other methods, it’s tentatively closer to the value from the cosmic microwave background, or CMB.

    “Our data agrees a little bit more with the CMB side of things compared to the supernova side, but the error bar is really big, so you can’t really say anything,” says Steffen Hagstotz, an astronomer at Stockholm University. Nonetheless, he says, “I think fast radio bursts have the potential to be as accurate as the other methods.”

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    No one knows exactly what causes FRBs, though eruptions from highly magnetic neutron stars are one possible explanation (SN: 6/4/20). During the few milliseconds when FRBs blast out radio waves, their extreme brightness makes them visible across large cosmic distances, giving astronomers a way to probe the space between galaxies (SN: 5/27/20).

    As an FRB signal travels through the dust and gas separating galaxies, it becomes scattered in a predictable way that causes some frequencies to arrive slightly later than others. The farther away the FRB, the more dispersed the signal. Comparing this delay with distance estimates to nine known FRBs, Hagstotz and colleagues measured the Hubble constant.

    The largest error in the new method comes from not knowing precisely how the FRB signal disperses as it exits its home galaxy before entering intergalactic space, where the gas and dust content is better understood. With a few hundred FRBs, the team estimates that it could reduce the uncertainties and match the accuracy of other methods such as supernovas.

    “It’s a first measurement, so not too surprising that the current results are not as constraining as other more matured probes,” says Birrer.

    New FRB data might be coming soon. Many new radio observatories are coming online and larger surveys, such as ones proposed for the Square Kilometer Array, could discover tens to thousands of FRBs every night. Hagstotz expects there will sufficient FRBs with distance estimates in the next year or two to accurately determine the Hubble constant. Such FRB data could also help astronomers understand what’s causing the bright outbursts.

    “I am very excited about the new possibilities that we will have soon,” Hagstotz says. “It’s really just beginning.” More

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    50 years ago, experiments hinted at the possibility of life on Mars

    Organics on Mars — Science News, March 27, 1971

    [Researchers] have exposed a mixture of gases simulating conditions believed to exist on the surface of Mars to ultraviolet radiation. The reaction produced organic compounds. They conclude that the ultraviolet radiation bombarding the surface of Mars could be producing organic matter on that planet.… The fact that such organic compounds may be produced on the Martian surface increases the possibility of life on Mars.


    In 1976, a few years after those experiments, NASA took its search for organic molecules to the Red Planet’s surface. That year, the Viking landers became the first U.S. mission to land on Mars. Though the landers failed to turn up evidence in the soil, NASA has continued the hunt. In 2018, the Curiosity rover found hints of life: organic molecules in rocks and seasonal shifts in atmospheric methane. A new phase of the hunt began in February when the Perseverance rover landed on Mars (SN Online: 2/17/21). It will find and store rocks that might preserve signs of past life for eventual return to Earth. More

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    ‘Oumuamua may be a chip knocked off an icy, Pluto-like exoplanet

    Since its discovery, the interstellar object known as ‘Oumuamua has defied explanation. First spotted in 2017, it has been called an asteroid, a comet and an alien spaceship (SN: 10/27/17). But researchers think they finally have the mystery object pegged: It could be a shard of nitrogen ice broken off a Pluto-like planet orbiting another star.

    “The idea is pretty compelling,” says Garrett Levine, an astronomer at Yale University not involved in the work. “It does a really good job of matching the observations.”

    ‘Oumuamua’s origin has been a mystery because it looks sort of like a comet, but not quite (SN: 12/18/17). After whipping by the sun, ‘Oumuamua zoomed away slightly faster than gravity alone would allow. That happens when ices on the sunlit sides of comets vaporize, giving them a little rocketlike boost in speed. But unlike comets, ‘Oumuamua didn’t appear to have a tail from typical cometary ices, such as carbon monoxide or carbon dioxide, streaming off it.

    Alan Jackson and Steven Desch, planetary scientists at Arizona State University in Tempe, set out to discover what other kind of evaporating ice could give ‘Oumuamua a big enough nudge to explain its movement. The pair reported their results March 17 at the virtual Lunar and Planetary Science Conference and in two studies published online March 16 in the Journal of Geophysical Research: Planets.

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    The amount of force that a vaporizing ice exerts on a comet depends on factors such as how much the ice heats up when it absorbs energy, the mass of its molecules and even the ice’s crystal structure. By calculating the rocketlike push on ‘Oumuamua if it were made of ices such as nitrogen, hydrogen and water, “we learned that nitrogen ice would work perfectly well,” Desch says.

    Because nitrogen ice covers outer solar system bodies such as Pluto and Neptune’s moon Triton, but not smaller objects like comets, ‘Oumuamua is probably a chip off a Pluto-like exoplanet, the researchers report.

    To determine how realistic that scenario is, Jackson and Desch calculated how many chunks of nitrogen ice could have been knocked off Pluto-like bodies in the early solar system. Back then, the Kuiper Belt of objects beyond Neptune was much more crowded than it is today, including thousands of Pluto-like bodies iced with nitrogen. But some 4 billion years ago, Neptune’s orbit expanded. That disruption caused many Kuiper Belt objects to collide with each other, and most sailed out of the solar system altogether.

    Under such chaotic conditions, collisions could have broken trillions of nitrogen ice fragments off Pluto-like bodies, Jackson and Desch estimate. If other planetary systems throw out as many shards of ice, those objects could make up about 4 percent of the bodies in interstellar space. That would make seeing an object like ‘Oumuamua mildly unusual but not exceptional, the researchers say.

    “When I first started reading it, I was skeptical … but it does tick a lot of the necessary boxes,” says Scott Sheppard, an astronomer at the Carnegie Institution for Science in Washington, D.C. not involved in the work. “It’s definitely plausible that this could be a fragment of an icy dwarf planet.” But plausible, he notes, does not necessarily mean correct.

    ‘Oumuamua is now too far away to confirm this idea with more observations. But the upcoming Vera Rubin Observatory and European Space Agency’s Comet Interceptor mission could detect more interstellar objects, says Yun Zhang, a planetary scientist at Côte d’Azur Observatory in Nice, France not involved in the research. The Vera Rubin Observatory is expected to spot, on average, one interstellar visitor per year, and the Comet Interceptor spacecraft may actually visit one.

    Getting a closer look at more of these objects could narrow down which possible explanations for ‘Oumuamua are most reasonable, she says (SN: 2/27/19). More

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    Most of Mars’ missing water may lurk in its crust

    An ocean’s worth of water may be lurking in minerals below Mars’ surface, which could help explain why the Red Planet dried up.

    Once home to lakes and rivers, Mars is now a frigid desert (SN: 12/8/14). Scientists have typically blamed that on Mars’ water wafting out of the planet’s atmosphere into space (SN: 11/12/20). But measurements of atmospheric water loss made by spacecraft like NASA’s MAVEN orbiter are not enough to account for all of Mars’ missing water — which was once so abundant it could have covered the whole planet in a sea up to 1,500 meters deep. That’s more than half the volume of the Atlantic Ocean.

    Computer simulations of water moving through Mars’ interior, surface and atmosphere now suggest that most of the Red Planet’s water molecules may have gotten lodged inside the crystal structures of minerals in the planet’s crust, researchers report online March 16 in Science. 

    The finding “helps bring focus to a really important mechanism for water loss on Mars,” says Kirsten Siebach, a planetary geologist at Rice University in Houston who was not involved in the work. “Water getting locked up in crustal minerals may be equally important as water loss to space and could potentially be more important.”

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    Planetary scientist Eva Scheller of Caltech and colleagues simulated possible scenarios for water loss on Mars, based on observations of the Red Planet made by rovers and orbiting spacecraft, and lab analyses of Martian meteorites. These simulations accounted for possible water loss to space and into the planet’s crust through bodies of water or groundwater interacting with rock.

    In order for the simulations to match how much water was on Mars 4 billion years ago, how much is left in polar ice caps today and the observed abundance of hydrogen in Mars’ atmosphere, 30 to 99 percent of Mars’ ancient water must be stashed away inside its crust. The rest was lost to space.

    Judging by modern Martian landscapes, like this image taken by the Curiosity rover at the base of Mount Sharp, the Red Planet appears bone dry. But an entire ocean’s worth of water may be lurking underground, in the minerals of the planet’s crust.MSSS/JPL-Caltech/NASA

    Water gets locked inside minerals on Earth, too, says Scheller, who presented the results March 16 in a news conference at the virtual Lunar and Planetary Science Conference. But unlike on Mars, that underground water is eventually belched back out into the atmosphere by volcanoes. That difference is important for understanding why one rocky planet may be lush and wet and habitable, while another is an arid wasteland. 

    Mars’ underground water could be mined by future explorers, says Jack Mustard, a planetary geologist at Brown University in Providence, R.I., not involved in the work. The most easily accessible water on Mars may be at its polar ice caps (SN: 9/28/20). But “to get the ice, you’ve got to go up to [high latitudes] — kind of cold, harder to live there,” Mustard says. If water can be extracted from minerals, it could support human colonies at warmer climes closer to the equator.  More

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    Crushed space rocks hint at exoplanets’ early atmospheric makeup

    Burning bits of ground-up meteorites may tell scientists what exoplanets’ early atmospheres are made of.
    A set of experiments baking the pulverized space rocks suggests that rocky planets had early atmospheres full of water, astrophysicist Maggie Thompson of the University of California, Santa Cruz reported January 15 at the virtual meeting of the American Astronomical Society. The air could also have had carbon monoxide and carbon dioxide, with smaller amounts of hydrogen gas and hydrogen sulfide.
    Astronomers have discovered thousands of planets orbiting other stars. Like the terrestrial planets in the solar system, many could have rocky surfaces beneath thin atmospheres. Existing and future space telescopes can peek at starlight filtering through those exoplanets’ atmospheres to figure out what chemicals they contain, and if any are hospitable to life (SN: 4/19/16).
    Thompson and her colleagues are taking a different approach, working from the ground up. Instead of looking at the atmospheres themselves, she’s examining the rocky building blocks of planets to see what kind of atmospheres they can create (SN: 5/11/18).

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    The researchers collected small samples, about three milligrams per experiment, of three different carbonaceous chondrite meteorites (SN: 8/27/20). These rocks are the first solids that condensed out of the disk of dust and gas that surrounded the young sun and ultimately formed the planets, scientists say. The meteorites form “a record of the original components that formed planetesimals and planets in our solar system,” Thompson said in a talk at the AAS meeting. Exoplanets probably formed from similar stuff.
    The researchers ground the meteorites to powder, then heated the powder in a special furnace hooked up to a mass spectrometer that can detect trace amounts of different gases. As the powder warmed, the researchers could measure how much of each gas escaped.
    That setup is analogous to how rocky planets formed their initial atmospheres after they solidified billions of years ago. Planets heated their original rocks with the decay of radioactive elements, collisions with asteroids or other planets, and with the leftover heat of their own formation. The warmed rocks let off gas. “Measuring the outgassing composition from meteorites can provide a range of atmospheric compositions for rocky exoplanets,” Thompson said.
    All three meteorites mostly let off water vapor, which accounted for 62 percent of the gas emitted on average. The next most common gases were carbon monoxide and carbon dioxide, followed by hydrogen, hydrogen sulfide and some more complex gases that this early version of the experiment didn’t identify. Thompson says she hopes to identify those gases in future experimental runs.
    The results indicate astronomers should expect water-rich steam atmospheres around young rocky exoplanets, at least as a first approximation. “In reality, the situation will be far more complicated,” Thompson said. Planets can be made of other kinds of rocks that would contribute other gases to their atmospheres, and geologic activity changes a planet’s atmosphere over time. After all, Earth’s breathable atmosphere is very different from Mars’ thin, carbon dioxide-rich air or Venus’s thick, hot, sulfurous soup (SN: 9/14/20).
    Still, “this experimental framework takes an important step forward to connect rocky planet interiors and their early atmospheres,” she said.
    This sort of basic research is useful because it “has put a quantitative compositional framework on what those planets might have looked like as they evolved,” says planetary scientist Kat Gardner-Vandy of Oklahoma State University in Stillwater, who was not involved in this new work. She studies meteorites too and is often asked whether experiments that crush the ancient, rare rocks are worth it.
    “People inevitably will ask me, ‘Why would you take a piece of a meteorite and then ruin it?’” she says. “New knowledge from the study of meteorites is just as priceless as the meteorite itself.” More