Hubble Discovers White Dwarf Merger Remnant with Unusual Carbon Signature

An international team of astronomies just announced the discovery of a cosmic oddity: A white dwarf star measuring up to 2.27 solar masses as a result of a white dwarf merging with another star, rather than from the thermonuclear fusion which underpins the majority of white dwarfs. This finding, made using sensitive ultraviolet observations with NASA's Hubble Space Telescope, indicates it is possible these rare white dwarfs are far more widespread than initially thought. The star, known as WD 0525+526, is approximately 128 light-years from Earth. This research is described in a paper published Aug. 6 in the journal Nature Astronomy.

Star with an Unusual Past

According to the paper, WD 0525+526 is what is known as a white dwarf, the superdense core of a star of a type similar to our Sun. In visible light it looked totally unremarkable at first, but its chemical makeup was a different story.

The observatory's powerful ultraviolet tools discovered in the star's atmosphere an even higher surge of carbon than astronomers had expected, suggesting that some previous violent disruption of the star had nearly stripped away its hydrogen-helium layers, exposing the secretly carbon-enriched core.

The aftermath left WD 0525+526 unusually hot and massive for a white dwarf: it now has roughly 1.2 times the mass of the Sun and a surface temperature of around 21,000 K, all crammed into what's basically an Earth-sized ball. These characteristics make WD 0525+526 a rare merger remnant as opposed to a more standard white dwarf.

Understanding stellar evolution

WD 0525+526 appeared completely normal in visible light until it was observed by Hubble's powerful ultraviolet instruments. This has led the astronomers to think that many more white dwarfs could be hiding similar explosive origins.

"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," Antoine Bedrad, a researcher at the University of Warwick who co-led the study, said in a statement. "That will be an important contribution to our understanding of white dwarf binaries, and the pathways to supernova explosions."