Illustration of a hot, young white dwarf. Such objects could develop an atmosphere of carbon as they cool down. (C) Sliwinski, MS; Sliwinska, LI
Read aloud Among the immensely dense white dwarves, there are scattered specimens with an atmosphere of pure carbon. Eight of these hitherto unknown exotics discovered a team led by Patrick Dufour of the University of Arizona as part of the Sloan Digital Sky Survey, a kind of census under the stars. White dwarfs are the remnants of ordinary stars, such as the Sun, which emit hydrogen and helium fuels. They are extremely dense, but only as big as the earth. So far, researchers have assumed that a white dwarf consists mainly of oxygen and carbon, so to speak, the ashes of nuclear fusion. A direct view of the naked core is usually not possible because white dwarfs have an atmosphere of hydrogen or helium.

Dufour studied a special class of white dwarfs with an atmosphere of helium and carbon during his dissertation. He found that the spectrum of some of them, with relatively warm surface temperatures, could only be explained by an atmosphere of pure carbon. "Nobody believed that such stars exist, " says Dufour, "we were excited and excited."

The researchers assume that the carbon dwarfs arise from particularly heavy precursor stars, with a mass of nine to eleven times the mass of the sun. That's about the limit at which a star is heavy enough to explode as a supernova. The researchers around Dufour have already found a model star that could be an earlier stage of development on the way to the carbon dwarf: The object H1504 + 65 is the hottest white dwarf known so far. Astronomers assume that it consists only of oxygen and carbon. "We think this star could get a carbon atmosphere when it cools, " says Dufour. The researchers want to take a closer look at their eight carbon dwarfs in December with the MMT Observatory on Mount Hopkins to determine their mass. With the help of these data, they hope to delimit the hitherto inaccurately known mass limit, at which a star becomes a supernova.

Patrick Dufour (University of Arizona, Tucson) et al .: Nature, vol. 450, p. 522 Ute Kehse advertisement

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