Double trouble: Feuding white dwarfs

U. WARWICK (UK) — Researchers have found an unusual double star system made up of feuding white dwarfs. Each star appears to be stripped down to just its helium.

Only about 50 close double white dwarfs are known to exist, but this system is only the second eclipsing close white dwarf pair to be found.

Knowing that the stars eclipse each other when seen from Earth allowed University of Warwick astronomers Steven Parsons and Tom Marsh to make particularly detailed observations of the system.

Their work revealed that uniquely both the white dwarf stars in this pairing are composed largely of helium. The findings are scheduled for publication in the Astrophysical Journal.

Most white dwarfs tend to have largely inert cores of carbon and oxygen that have formed over the star’s long life due to loss of hydrogen and helium. Helium white dwarfs are stars that have undergone extreme mass loss. To find two such helium white dwarfs stars is a clear sign to astronomers that both stars have had an exotic and mutually destructive past.

What was originally the most massive star of the pair had begun to expand and would have eventually become a red giant, but its outer hydrogen envelope was ripped off by its companion.

As a result, the star never got an opportunity to start fusing its helium and was left as a helium white dwarf.

When the companion star began to expand, it also had its expanding layer torn off by the first star—but as the first star was already reduced to a white dwarf, it could not use that new material. That hydrogen was therefore simply lost to the star system, leaving behind helium white dwarfs.

In about 1 billion years, the feud between the two stars will end as they spiral together and merge, finally igniting each other’s helium to become an object known as a hot subdwarf, which should last for 100 million years

The researchers discovered this star system known as CSS 41177—which is more than 351 parsecs, or 1140 light years, away in the constellation Leo—using a combination of data from the robotic 2m Liverpool Telescope in the Canary Islands and the 8m Gemini Telescope on Hawaii.

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