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Stars may keep spinning fast, long into old age

The idea that older stars continually slow their rotation may be wrong

SLOWDOWN Sunlike stars start as fast-spinning balls of gas (illustrated at left). As this type of star ages, its spin slows and it puffs up, before dying as a nebula (middle and right). But the spin of these aging stars might not slow as much as thought.


CAMBRIDGE, Mass. — Stars may keep some of their youthful vigor as they age. Astronomers have spotted a star in its twilight years that spins much faster than expected. The discovery supports a new idea that, rather than continually slowing with age, some stars may have a magnetic midlife crisis that keeps them on a roll. “This process of slowing rotation … that we assumed happened indefinitely over the lifetime of a star may be interrupted in the middle of a star’s life,” says astronomer Travis Metcalfe of the Space Science Institute in Boulder, Colo. He presented new measurements of the star’s age July 30 at the first TESS Science Conference.

The star, 94 Aquarii Aa, is a member of a triple-star system in the constellation Aquarius about 69 light-years from Earth. Its color and brightness suggest that it’s in the part of a star’s life cycle called the subgiant stage, which happens near the end of a sunlike star’s life as it starts running out of fuel. But it’s difficult to pinpoint a star’s age. Theories of stellar evolution predict that young stars rotate quickly but slow as they age and lose angular momentum, a process called spinning down. So astronomers often use a star’s spin rate to estimate age. Recently, though, data have emerged that raise questions about whether that aging scenario is correct.

NASA’s Kepler space telescope, which watched distant stars for signs of orbiting planets from 2009 to 2018 (SN Online: 10/30/18), tracked how oscillations, or “starquakes,” ripple through a star’s interior, a technique called asteroseismology. Those ripples’ speeds are closely linked to the star’s mass and interior structure. Structure changes over the course of a star’s life, so asteroseismology is a good way to estimate a star’s age. In 2016, Metcalfe and colleagues reported in Nature that Kepler was finding old stars that rotated too fast for their ages. Young stars followed the spin-down trends, but around middle age, stars’ spin speed leveled off. As an aging subgiant, 94 Aquarii Aa made a good test case, Metcalfe said. He used NASA’s Transiting Exoplanet Survey Satellite, or TESS, the successor to Kepler, to estimate the star’s age and mass using asteroseismology. It’s about 6.2 billion years old, he found, and 1.2 times the mass of the sun. (In comparison, the sun is 4.5 billion years old.)

If it had been spinning down its whole life, a star of that mass should now be rotating once every 78 days. But previous measurements made from ground-based telescopes had shown that the star rotates once every 47 days. “The only way to explain a star of that age having that rotation period is that this stalled rotation has to kick in around middle age,” Metcalfe says. “It’s a smoking gun.” He hopes to repeat the experiment with hundreds of more stars over the course of the TESS mission.

Stars might stop slowing their rotation because of a midlife change in their magnetic field. A star’s magnetic field drives its stellar wind, which carries mass and angular momentum away from the star, contributing to its slowdown (SN Online: 7/29/19). But if the magnetic field changes its geometry around the middle of a star’s life, shifting from dominating the entire star to a more small-scale field, that could weaken the magnetic field’s control over the star’s rotation, Metcalfe says. “This is the first time we’ve seen convincing evidence that you have to invoke [the stalled slowdown] to explain the rotation of a subgiant,” says Jason Curtis, an astronomer at Columbia University. Astronomers had a lot of skepticism about Metcalfe and colleagues’ previous work using Kepler data, he says, but “every time they look at it from a different angle, it becomes more convincing.” Unfortunately, the result might mean that astronomers can’t use stars’ spin speeds to guess ages anymore. “If that stops working in old stars, that’s a bummer,” Curtis says.

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