rationalskepticism.org

Discovery of an oxygen white dwarf

The vast majority of stars will eventually evolve into a white dwarf, a small, hot, and extremely dense object made of leftover material from the star's core. Stellar evolution theory suggests that white dwarfs should be mostly made of helium, carbon, or oxygen, but even a tiny amount of hydrogen or helium floats to the surface and hides the underlying composition. Kepler et al. searched through thousands of white dwarf spectra and discovered one that has an atmosphere dominated by oxygen, with no contamination by hydrogen or helium (see the Perspective by Gänsicke). This pristine object confirms the long-postulated theory and will be an important test case for stellar evolution.

A white dwarf with an oxygen atmosphere
S. O. Kepler1,
Detlev Koester2,
Gustavo Ourique1

Abstract
Stars born with masses below around 10 solar masses end their lives as white dwarf stars. Their atmospheres are dominated by the lightest elements because gravitational diffusion brings the lightest element to the surface. We report the discovery of a white dwarf with an atmosphere completely dominated by oxygen, SDSS J124043.01+671034.68. After oxygen, the next most abundant elements in its atmosphere are neon and magnesium, but these are lower by a factor of ≥25 by number. The fact that no hydrogen or helium are observed is surprising. Oxygen, neon, and magnesium are the products of carbon burning, which occurs in stars at the high-mass end of pre–white dwarf formation. This star, a possible oxygen-neon white dwarf, will provide a rare observational test of the evolutionary paths toward white dwarfs.