May 2, 2008 -- Astronomers have spotted a new type of stripped-down white dwarf star with a pulsating carbon surface. The new and so far unique white dwarf was predicted to exist somewhere in the cosmos, but was found only because of some massive surveys of the sky.
"They're extremely rare," said astronomer Kurtis Williams of the University of Texas at Austin's McDonald Observatory, where the discovery was made. "It's a real needle in a haystack to find one."
The white dwarf star they discovered is 800 light-years away in the constellation Ursa Major -- a.k.a. the Big Dipper. Its light wavers by almost 2 percent every eight minutes.
Like other white dwarfs, this new star is the remnant of a star which, in its youth, was probably a bright shining star no more than nine times the mass of the sun. In other words, it was neither exceptionally large nor small as stars go.
Today what remains is a glowing sphere smaller than Earth but with a mass equal to our sun and a brightness that's only one six-hundredth of old Sol.
Unlike other white dwarfs, however, this one has been stripped of its outer layers of hydrogen and helium and is left with carbon on its surface. That carbon appears to be at a very toasty 35,100 degrees F (19,500 C), which allows its carbon to shift en masse between a higher and a lower energy state, driven by just one electron per atom. That shifting carbon is visible as the star's pulsation.
Other white dwarfs have been found pulsating, but they are doing it with hydrogen and helium -- not carbon, explained Williams. His colleague Michael Montgomery had, in fact, predicted this sort of carbon pulsation.
That's what sent them, along with graduate student Steven DeGennaro, to McDonald Observatory's 2.1-meter Otto Struve Telescope to look at the dozen of known carbon white dwarfs which had been previously discovered by the Sloan Digital Sky Survey. The team's discovery appears in the May 1 issue of Astrophysical Journal Letters.
"Of 20,000 known (white dwarfs) there were only 10 candidates," said Williams describing their hunt for hot carbon white dwarfs. Of those, only the one in Ursa Major proved to be pulsating. But that's enough for now, he said.
"We know that Mike Montgomery's model works," Williams said. "That makes us very happy."
As for how this and other carbon white dwarfs became so stripped down, that is a bit of a mystery, Williams said. One possibility it that the star was very close to the mass at which stars blow up as supernova at the end of their lives instead of becoming white dwarfs. So its corpse is, likewise, a borderline, strange dwarf.
Luckily, the pulsations themselves may contain clues to the mystery in the form of seismic information from the interior of the white dwarf, said astronomer Michael Briley of the National Science Foundation.
"This will allow us to probe the white dwarf's interior," said Briley, "which in turn should help us solve the riddle of where the carbon white dwarfs come from and what happens to their hydrogen and helium."
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