Hubble spots a white dwarf created in a violent star collision

Sometimes, a white dwarf isn’t just a white dwarf. What seems ordinary at first glance can have a much more dramatic origin. That’s what astronomers discovered when they took a closer look at a star called WD 0525+526.

This compact, Earth-sized star appeared to be typical – but it wasn’t. Scientists found that it’s actually the aftermath of a stellar collision: two stars that crashed together and merged into one.

White dwarf with a violent history

White dwarfs mark the final stage of life for stars like our Sun. These stellar remnants are incredibly compact. One can pack more mass than the Sun into a body no bigger than our planet. Most white dwarfs are born when a single star burns through its fuel and collapses.

But there’s another, rarer path. Sometimes two stars – often in a close binary system – collide and merge. The result is an ultra-massive white dwarf. That’s what happened with WD 0525+526, and this is the first time scientists have confirmed a merger origin based on ultraviolet evidence.

“It’s a discovery that underlines things may be different from what they appear to us at first glance,” said Boris Gaensicke, principal investigator of the Hubble program at the University of Warwick.

“Until now, this appeared as a normal white dwarf, but Hubble’s ultraviolet vision revealed that it had a very different history from what we would have guessed.”

White dwarf transformed by a merger

Located 128 light-years from Earth, WD 0525+526 is 20% more massive than the Sun. Visually, it looks like any other white dwarf. But scientists used NASA’s Hubble Space Telescope and its ultraviolet capabilities to peer deeper into the star’s composition.

The ultraviolet spectrum revealed something unexpected: carbon in the star’s atmosphere. Normally, white dwarfs formed from a single star have atmospheres made up of hydrogen and helium. Their carbon and oxygen are buried in the core, hidden under thick atmospheric layers.

Mergers, however, change the outcome. When two white dwarfs – or one and another type of star – crash into each other, the resulting explosion can strip away much of that outer hydrogen and helium. What’s left is a thinner atmosphere, where carbon can leak up from the core and become visible.

Detecting carbon in the star’s atmosphere

WD 0525+526 is hotter and more massive than the handful of other known merger-born white dwarfs. At nearly 21,000 kelvins (around 37,000 degrees Fahrenheit), it’s too hot for convection – the usual process that stirs carbon up into a white dwarf’s outer layers.

So how did the carbon get there? The researchers think a more subtle mechanism called semi-convection is at play. It’s a gentler process, but still powerful enough to pull a small trace of carbon to the surface.

What’s truly remarkable is how little carbon is in this star’s atmosphere – about 100,000 times less than what’s been observed in other merger remnants. This made it nearly impossible to spot without the right tools.

“Hubble’s Cosmic Origins Spectrograph is the only instrument that can obtain the superb quality ultraviolet spectroscopy that was required to detect the carbon in the atmosphere of this white dwarf,” said study lead author Snehalata Sahu from the University of Warwick.

Hidden white dwarf mergers

The discovery builds on a 2019 study using ESA’s Gaia mission, which identified a group of unusually blue white dwarfs. Some of those were later confirmed as the products of mergers. WD 0525+526 is now the hottest and most massive member of that group.

Because carbon in ultra-hot white dwarfs is easier to detect in ultraviolet than visible light, it’s possible many more of these cosmic collision survivors are out there – hiding in the data, waiting to be found.

“We would like to extend our research on this topic by exploring how common carbon white dwarfs are among similar white dwarfs, and how many stellar mergers are hiding among the normal white dwarf family,” said study co-author Antoine Bedrad.

“That will be an important contribution to our understanding of white dwarf binaries, and the pathways to supernova explosions.”

The research is published in the journal Nature Astronomy.

Image Credit: NASA, ESA, STScI, Ralf Crawford (STScI)

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