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A cold surprise

The discovery of an ultra-cool white dwarf in Virgo raises more questions than it answers.
Lara Crigger
November 21, 2005
Ultra-cool white dwarfs, or white dwarfs with surface temperatures below 4,000 kelvins, are a rare breed. Only seven of these peculiar stars were known to exist, until now. Christian Wolf of the University of Oxford serendipitously discovered an eighth specimen.

White dwarfs are expired stars that no longer generate energy through nuclear reactions. Therefore, they are cool and dim. Ultra-cool white dwarfs, however, are much cooler and fainter than even the coldest regular white dwarf, making them nearly impossible to detect. Their extremely low temperatures suggest they somehow cooled faster than their normal counterparts.

Which mechanism could cause ultra-cool white dwarfs to cool so quickly is still unknown, but scientists believe it might be key to understanding how early stars in the galaxy formed and died.
M4 white dwarfs
Like these white dwarfs in globular cluster M4, ultra-cool white dwarfs no longer produce nuclear energy. However, ultra-cool white dwarfs, such as the one Wolf found, are much colder than these stars.
NASA and H. Richer (University of British Columbia)
Wolf found the dwarf, named COMBO-17 J114356.08-0144032, as he was comparing infrared magnitudes of stars from the COMBO-17 survey data. COMBO-17 was a deep extragalactic survey designed to pinpoint redshifts of distant galaxies and quasars. All of the magnitudes appeared as expected except for one outlier, which further calculations could not explain. "The unusual colors were real," Wolf says, "and unlike everything I had seen before in COMBO-17."

Research revealed the colors were similar to those of ultra-cool white dwarfs. Also like those stars, this star's spectrum was featureless. It is the faintest ultra-cool white dwarf yet found — and perhaps the most distant.

Unfortunately, the lack of spectral features means little can be determined about the star's physical properties. The composition of the dwarf, the number of companions it has, and even its mass cannot be calculated without spectral features. "At this stage, we are still in the realm of complete speculation," states Wolf. "We can only rule out certain scenarios."

The only easily measurable trait is the star's parallax, which would yield its luminosity. But even that might not help. A parallax is known for the first ultra-cool white dwarf, LHS 3250, a star that shares the same colors as this one. However, the information raised more questions than it answered. "Unless we get more," Wolf says, "We have no idea whether the result is typical for ultra-cool white dwarfs or not."

The results will appear next year in a Letter to Astronomy and Astrophysics.

Lara Crigger is an astronomer and science writer based in Rochester, New York.
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