Astronomers suspect that a massive white dwarf with a peculiar atmosphere is the end product of a merger between two white dwarfs. This unique object, known as WDJ0551+4135, is believed to have only narrowly avoided complete annihilation. The finding is reported in Nature Astronomy.
White dwarfs are the stellar core of smaller stars like the Sun. Having run out of fuel and with its outer layers dispersed, what remains is about the mass of the Sun in a volume smaller than Earth. White dwarfs are usually made of mostly carbon and oxygen or oxygen and neon, depending on their progenitor star’s mass.
These cores are within a shell of helium that is then enveloped in hydrogen, which often stops the measurements of other elements. But WDJ0551+4135 is not like other white dwarfs – its atmosphere is surprisingly rich in carbon and hydrogen.
“This star stood out as something we had never seen before. You might expect to see an outer layer of hydrogen, sometimes mixed with helium, or just a mix of helium and carbon. You don’t expect to see this combination of hydrogen and carbon at the same time as there should be a thick layer of helium in between that prohibits that,” lead author Dr Mark Hollands, from the University of Warwick, said in a statement. “When we looked at it, it didn’t make any sense.”
The object was discovered thanks to the European Space Agency’s Gaia spacecraft, which is compiling the largest 3D map of the stars in the Milky Way. The team followed up with the William Herschel Telescope and discovered that the white dwarf is egregiously heavy, 1.14 times the mass of the Sun compared to the average 0.6 times for white dwarfs.
“We have a composition that we can’t explain through normal stellar evolution, a mass twice the average for a white dwarf, and a kinematic age older than that inferred from cooling," said Dr Hollands. "We’re pretty sure of how one star forms one white dwarf and it shouldn’t do this. The only way you can explain it is if it was formed through a merger of two white dwarfs.”
While the team considers the atmosphere a clear hallmark of the merging event, its mass is equally important. And it could be a case that many of the heaviest white dwarfs are the end product of a merger. White dwarfs can’t be heavier than 1.4 times the mass of the Sun; at that point, they just turn into a supernova.
The team has plans to confirm whether or not the merger was the true formation channel for this object by using a technique called asteroseismology to study the pulsation of the star and understand its composition.
“Indeed, asteroseismology is the most successful methodology for investigating the interior physics of white dwarfs, including core composition,” wrote the researchers. “Therefore, the future detection of additional pulsation frequencies may allow the core composition to be established via detailed asteroseismology.”