White dwarfs with carbon atmospheres

Nov 22, 2007 08:25 GMT  ·  By

It is a well-known fact that white dwarfs are one of the possible final stages of a star's life. But recently astronomers discovered a new type of white dwarfs which seem to have pure carbon atmospheres that cannot be explained through the current models.

Stars are formed through a process that involves the collapse of large quantities of gas, which eventually light up as the pressure at the core grows bigger. Being mostly composed of molecular hydrogen, the stars 'burn' their fuel, through a process of nuclear fusion, by merging hydrogen atom nuclei together to create heavier elements. Depending on the mass of the original star, they can live as long as a few billion years, after which the hydrogen nuclear fusion process stops and they become unstable and eventually explode as supernovae.

Astronomers now believe that these white dwarfs with carbon rich atmospheres might have evolved from not massive enough stars to explode as supernovae. Calculations show that about three percent of the most massive stars will not trigger a supernova explosion, but will die as white dwarfs.

As the stars deplete their hydrogen fuel, they start burning the resulting element of the previous process: Helium. The helium fusion reaction leaves behind carbon and oxygen 'ashes'. When all the nuclear fuel is exhausted, the star dies as a compact and extremely hot object, which could compress matter the size of the Sun mass into objects as small as the Earth. The previous models regarding the white dwarf formation suggested that most of the resulting carbon and oxygen is composing the star's core, hidden away from our view by a surrounding hydrogen and helium atmosphere.

The fact that no visible traces of hydrogen or helium were present in the atmosphere of the newly discovered white dwarfs might suggest that what is actually seen is the bare core of a remnant white dwarf.

It is now believed that such white dwarfs evolve from unique stars like H1504+65, discovered in 1986, which are extremely massive and can reach temperatures as high as 200.000 degrees Kelvin. These stars might have somehow expelled all the hydrogen and most of the helium, leaving behind the bare core that presents an atmosphere composed of 50 percent carbon and 50 percent oxygen. As the stars cools, the elements composing it will separate with the help of gravity to form a massive carbon nucleus covered by a helium rich atmosphere similar to that of the known white dwarfs. At temperatures lower than 23,000 degrees Kelvin, the oxygen, being heavier than the carbon, will sink to the central core and the carbon will dilute the thin helium atmosphere, thus creating a carbon rich atmosphere.

The current model shows that stars with masses between nine and eleven times that of the Sun will eventually become white dwarfs that might present a peculiar carbon layer and core of oxygen-magnesium-neon, while more massive stars would explode as supernovae. However, astronomers have a hard time estimating the mass that divides the line between a star dying as a white dwarf and a supernova.