A giant white dwarf star may be the offspring of a collision between two other white dwarfs, a new study finds.
The new finding suggests that other ultramassive white dwarfs with similar origins could exist; this may help shed light on how massive a white dwarf must be to explode in a supernova, the scientists behind the new research said.
White dwarfs are the dim, fading, Earth-size cores of dead stars that are left behind after average-size stars have exhausted their fuel and shed their outer layers. Our sun will one day become a white dwarf, as will more than 90% of the stars in our galaxy.
Scientists studied a white dwarf known as WDJ0551+4135, which is located about 150 light-years from Earth. Recent findings from the European Space Agency's Gaia telescope suggested that this object might be an exceptionally massive white dwarf. To learn more about this unusual dead star, the scientists analyzed it using data from the William Herschel Telescope in the Canary Islands.
By examining the spectrum of colors in light emitted by the white dwarf, the astronomers were able to identify the chemicals within the star. Unexpectedly, the researchers found a high level of carbon in the object's atmosphere, which scientists had never seen before in white dwarfs.
"A white dwarf has an internal structure kind of like an onion, in that it's in layers," study lead author Mark Hollands, an astrophysicist at the University of Warwick in England, told Space.com. "In the core of the white dwarf, most of its mass is made of carbon and oxygen, and then you usually have a layer of helium on top of that, and then a layer of hydrogen. So when you point a telescope at a white dwarf, you're just seeing the outer layers."
That structure means that scientists studying white dwarfs typically see just hydrogen or just helium, or perhaps a mix of helium and carbon. But that isn't what Hollands and his colleagues saw at this white dwarf.
"You never expect to see the hydrogen and carbon mixed together in a white dwarf — there should be a layer of helium keeping them separated," Hollands said. "That was really strange and very surprising and raised the big question of how that might have happened."
Hollands and his colleagues said they have a solution to this puzzle: The strange structure originated when two other white dwarfs smashed together and their innards mixed, the scientists wrote. But the new massive white dwarf wasn't quite large enough to trigger a supernova, leaving the strangely large star intact.
For the researchers, a key clue to this mystery was the mass of this white dwarf. Most white dwarfs are about 0.6 times the mass of the sun, but WDJ0551+4135 is nearly twice the average mass of white dwarfs, at 1.14 solar masses, all compacted into a space just two-thirds of Earth's diameter.
Another clue is this white dwarf's apparent age. Older white dwarfs typically are cooler and orbit the center of the Milky Way faster than their younger counterparts, because they have lost more heat over time and have experienced more gravitational interactions with other stars that speed them up. However, WDJ0551+4135 moves faster than 99% of other nearby white dwarfs that have about the same temperature, suggesting that one of these two age measurements was skewed.
The researchers knew WDJ0551+4135 had a strange composition, a mass twice that of the average white dwarf and an age that seemed relatively young if one looked at the star's heat but relatively old if one looked at its motions. The astronomers realized that one event could explain all of these mysteries: WDJ0551+4135 formed when two white dwarfs collided, heating the resulting white dwarf and restarting its cooling process.
"Although there is lots of evidence that such mergers occur, including examples in distant galaxies, actually picking out a white dwarf and saying this one here is from a merger is quite hard," Hollands said. "This is only one of a handful of cases where it's been possible to do that and the only one identified through its composition."
The scientists said that the merger occurred about 1.3 billion years ago. But that event reset WDJ0551+4135's cooling age, making it difficult to determine the ages of the two original white dwarfs, which could have been many billions of years old.
There are few white dwarfs as massive as WDJ0551+4135, but there are more such massive white dwarfs than current models of white dwarf formation might predict, Hollands said, suggesting that some of these might have formed via mergers. The researchers said they are particularly excited by the insight this white dwarf and others like it could provide on the boundary line between massive white dwarf and supernova.
The astronomers could also learn more about the innards of WDJ0551+4135 by examining the subtle pulsations in brightness caused by waves traveling through the star's interior, Hollands said. If scientists can detect multiple waves of different frequencies traveling through this white dwarf, they can learn more about the star's interior structure. "That could provide independent insights on whether this star formed from a merger," Hollands said.
The scientists detailed their findings online Monday (March 2) in the journal Nature Astronomy.
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