Cosmic bumper stars forge rare, hefty white dwarf

A massive, smoldering stellar remnant located in Earth’s cosmic neighborhood really stretches the definition of a “white dwarf.” But after reviewing data collected by the Hubble Space Telescope, astronomers at the University of Warwick believe they now know why: it wasn’t birthed out of a single star’s demise. Instead, it likely came from two celestial fusion reactors smacking into each other. Their evidence is laid out in a paper published on August 6 in Nature Astronomy.

White dwarfs are common across the universe. The remnants of collapsed stars, these Earth-sized objects are composed of carbon-oxygen cores enveloped in layers of hydrogen and helium. Despite their diminutive size, white dwarfs are still extremely dense. Their mass is usually about half that of the sun, but in rare cases they can possess a much higher mass. For example, the white dwarf located 130 light-years away called WD 0525+526 is 20 percent larger than our sun—so large that astronomers classify it as an “ultra-massive” white dwarf. While it’s entirely possible that WD 0525+526 simply formed out of a massive star’s death, ultraviolet data recently collected by the Hubble telescope suggests otherwise.

“In optical light (the kind of light we see with our eyes), WD 0525+526 looks like a heavy but otherwise ordinary white dwarf,” explained University of Warwick research fellow and study first author Snehalata Sahu. “However, through ultraviolet observations obtained with Hubble, we were able to detect faint carbon signatures that were not visible to optical telescopes.”

A typical white dwarf’s core elements are obscured by thick layers of hydrogen and helium—but that covering almost entirely burns away when stars smash together. What’s left behind is a massive white dwarf with a hydrogen-helium layer that’s 10 billion times thinner than usual, allowing detectable carbon to reach the surface.

But WD 0525+526 is even weirder. It’s not only the hydrogen-helium layer that’s in short supply. It also features roughly 100,000 times less carbon on its surface compared to similar cosmic mergers.

“The low carbon level, together with the star’s high temperature (nearly four times hotter than the Sun), tells us WD 0525+526 is much earlier in its post-merger evolution than those previously found,” said study co-first author and astrophysicist Antoine Bédard.

Detecting and analyzing these oddities can help Bédard, Sahu, and other researchers gain a more comprehensive understanding of the ultimate fate of many binary star systems. These relationships are often a major component in supernova explosions. But a major reason why experts have difficulty finding binary star white dwarfs is because of their trademark ultraviolet emissions. Earth’s atmosphere shields the planet from the brunt of stellar UV light, so the only way to search for objects like WD 0525+526 is to use space telescopes like Hubble.

“[The study] tells us there may be many more merger remnants like this masquerading as common pure-hydrogen atmosphere white dwarfs,” said Sahu, stressing the continuing importance of space telescope projects.

“Only ultraviolet observations would be able to reveal them to us,” Sahu added.