Astronomers discover white dwarf stars with pure carbon atmospheres

November 24th, 2007 - 1:31 pm ICT by admin  

Washington, Nov.24 (ANI): Astronomers at the Steward Observatory in the University of Arizona have discovered white dwarf stars with pure carbon atmospheres. The stars were discovered among 10,000 new white dwarf stars found in the Sloan Digital Sky Survey. The survey, known as the SDSS, found about four times as many white dwarf stars previously known.
Claiming that this discovery could offer a unique view into the hearts of dying stars, astronomer Patrick Dufour said these stars possibly evolved in a sequence that were not known before.

They may have evolved from stars that are not quite massive enough to explode as supernovae but are just on the borderline, he added.

“We’ve found stars with no detectable traces of helium and hydrogen in their atmospheres. We might actually be observing directly a bare stellar core. We possibly have a window on what used to be the star’s nuclear furnace and are seeing the ashes of the nuclear reaction that once took place,” said Dufour and his colleagues.

Dufour, UA astronomy Professor James Liebert and their colleagues at the Universite de Montreal and Paris Observatory have published the results in the November 22 issue of Nature.

Astronomers believe that convection in the helium zone dredges up carbon from the star’s carbon-oxygen core.

Dufour developed a model to analyze the atmospheres of DQ stars as part of his doctoral research at the Universite de Montreal. His model simulated cool DQ stars, stars at temperatures between 5,000 degrees and 12,000 degrees Kelvin. For reference, our sun’s surface temperature is around 5,780 degrees Kelvin.

When Dufour joined Steward Observatory in January, he updated his code to analyze hotter stars, stars as hot as 24,000 degrees Kelvin.
“When I first started modeling the atmospheres of these hotter DQ stars, my first thought was that these are helium-rich stars with traces of carbon, just like the cooler ones,” Dufour said.

“But as I started analyzing the stars with the higher temperature model, I realized that even if I increased the carbon abundance, the model still didn’t agree with the SDSS data,” Dufour said.

In May 2007, “out of pure desperation, I decided to try modeling a pure-carbon atmosphere. It worked. I found that if I calculated a pure carbon atmosphere model, it reproduces the spectra exactly as observed. No one had calculated a pure carbon atmosphere model before. No one believed that it existed. We were surprised and excited,” Dufour said.

Dufour and his colleagues have identified eight carbon-dominated atmosphere white dwarf stars among about 200 DQ stars they’ve checked in the Sloan data so far.

The great mystery is why these carbon-atmosphere stars are found only between about 18,000 degrees and 23,000 degrees Kelvin.
Dufour and Liebert say they these stars might have evolved from a star like the unique, much hotter star called H1504+65 that Pennsylvania State University astronomer John A. Nousek, Liebert and others reported in 1986.

If so, carbon-atmosphere stars represent a previously unknown sequence of stellar evolution.

H1504+65 is a very massive star at 200,000 degrees Kelvin.

Astronomers currently believe this star somehow violently expelled all its hydrogen and all but a very small trace of its helium, leaving an essentially bare stellar nucleus with a surface of 50 percent carbon and 50 percent oxygen.

Astronomers had predicted in 1999 that stars nine or 10 times as massive as our sun would become white dwarfs with oxygen-magnesium-neon cores and mostly carbon-oxygen atmospheres. More massive stars explode as supernovae.

But scientists aren’t sure where the dividing line is, whether stars eight, nine, 10 or 11 times as massive as our sun are required to create supernovae.

“We don’t know if these carbon atmosphere stars are the result of nine-or-10 solar mass star evolution, which is a key question,” Liebert said.

The UA astronomers plan making new observations of the carbon atmosphere stars at the 6.5-meter MMT Observatory on Mount Hopkins, Arizona, in December to better pinpoint their masses.

The observations could help define the mass limit for stars dying as white dwarfs or dying as supernovae, Dufour said. (ANI)

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