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The Milky Way's fastest stars could solve a 50-year-old mystery



Ken Shen ran against the sun. It was 3 A.M. On April 25, Shen – an astronomer at the University of California, Berkeley – sat in his pajamas at his kitchen table. At this very moment, scientists behind the Gaia probe of the European Space Agency published the mission's second series of data. And Shen was on a mission to search that data to find the Milky Way's fastest stars, and then verify their identity through independent observations on ground-based telescopes. Thirty minutes before the sun hit the West Coast, Shen managed to find his first destination and send his coordinates to a staff member at the Lick Observatory near San Jose.

But he could not crawl back to bed. For the next 24 hours, Shen and his colleagues searched the Gaia data for a handful of candidates who could observe them using telescopes in South Africa, the Canary Islands, Arizona, and California. After a week of careful analysis, the researchers were certain that they had found three of the fastest stars in our galaxy. Not that they were only interested in speed: such fast-moving stars were proof that Shen was looking for his novel theory on how certain stars explode. On April 30, the team released its results on the preprint server arXiv and submitted The Astrophysical Journal for a peer-reviewed publication.

Shen's theory concerns type Ia supernovae that are so uniform and evenly bright. Astronomers used them as a measure to measure the width of the cosmos. Whether here in the Milky Way or in a galaxy on the other side of the observable universe, these catastrophic explosions always show almost exactly the same luminosity and allow precise calculations of their distances. This work has shown that the expansion of the universe is accelerating ̵

1; an advance that claimed the 2011 Nobel Prize for Physics – and has helped confirm the existence of dark energy. Type Ia explosions also create and distribute heavy elements (including the iron in the hemoglobin that flows through our veins) and therefore play a critical role in galactic evolution studies.

But astronomers do not understand the exact process that sets off these brilliants explosions. "It's kind of embarrassing," says Anthony Piro, an astronomer at Carnegie Observatories who was not involved in the new study. "Type Ia supernovae are fundamental to a number of different areas of astrophysics."

Astronomers remain convinced that white dwarfs – dense, earth-like remnants of sun-like stars – are the astrophysical culprits behind Type Ia supernovae. But because these objects are too stable to explode, a companion star – be it another white dwarf, a star like our sun, or even a huge star – has to push it over the edge. A classic model suggests that some type Ia blasts occur when two white dwarfs spirally converge and merge. But for almost a decade, Shen has come up with a slightly different idea: he thinks that in these scenarios, the two White Dwarves never really collide. Instead, as they approach each other, one star's gravity pulls material (specifically hydrogen and helium gas) from the other star. The gas hits its neighbor like a hammer and triggers an explosion on the surface of the star, sending a shockwave deep underground and igniting a second – the Type Ia detonation – in and around the core of the star. This explosion would be strong enough to push the remaining star away at a tremendous speed of more than a thousand miles per second. But that was just a model for almost a decade. If Shen could find these fast stars, he could only gain some evidence to support his hypothesis and get a better understanding of these vital cosmic lighthouses.

Gaia is uniquely suited to discover fast stars and thus could create candidates for Shen to investigate. To create its galactic map, the spacecraft tracks the positions, motions, and magnitudes of more than a billion stars, providing full three-dimensional trajectories to many of them. Such accurate tracking helps determine how fast a particular object moves relative to our position in the solar system.

From the Gaia data, Shen's team has tagged seven candidates for Hypervelocity White Dwarf Stars. Four turned out to be more ordinary, slower stars whose movements in the data were probably inaccurate. But the remaining three were real Speedsters. In addition, they had bizarre properties that were thought to suggest the formation mechanics proposed by Shen: each is about 10 times larger than a typical white dwarf, presumably because their outer layers were inflated by the extra energy of the Type Ia explosion , The three stars also lack hydrogen and helium, which would be expected if they had donated these items to an accompanying White Dwarf to trigger the detonation. Eventually, Shen and his colleagues traced the stars back in time and found that at least one of them came from a weak supernova remnant three years ago in the Pegasus constellation.

"This observation is extremely exciting," says Piro. " I think that will bring a lot of interesting research lines to the road in the next few years, and it's always exciting to be able to determine this crucial moment when you know that many people will jump into it." Piro says the three Hypervelocity white dwarfs from Gaia's data increase the chances that Shen's proposed mechanism is correct, taking notes more final evidence is still needed. For example, an explosion of type 1a would likely spray the surface of the surviving white dwarf with detectable amounts of heavy elements. If astronomers were to see such stellar ashes on these white dwarfs, this would further support Shen's hypothesis.

Keith Hawkins, an astronomer at Columbia University who studies hyper-speed stars but did not participate in research, agrees to do work. The first step will be to review the data supporting the wild find of Shen's team. " Whenever you look at an outlier – in that case, these things are moving at incredibly high speeds, so they are outliers – you have to be very careful about whether or not something is wrong with the data," he says. For example, two of the White Dwarves only seemed to move across the sky, with no movement toward or away from us – a strange alignment with Gaia's line of sight that most celestial objects would probably not possess.

For now, Shen welcomes further study of these stars to see if their origins are as bizarre as he suggests. He and his team are simply thrilled to even find candidates. He had estimated the chances of success at around 25 percent and feared that Hypervelocity White Dwarfs might prove too weak to discover. But as the evidence grows over the years that these ultraspeedy stars are the bona fide white dwarfs of his hypothesis, the implications would be far-reaching – allowing astronomers to better understand how some Type Ia supernovae explode, thereby providing a better grip on the measures which support their search for the large-scale structure and evolution of the universe.


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