A team of astronomers led by Dr. Souza Oliveira Kepler from the Federal University of Rio Grande do Sul, Brazil, is reporting the discovery of a white dwarf with an atmosphere completely dominated by oxygen, SDSS J124043.01+671034.68 (SDSS J1240+6710 for short).
SDSS J1240+6710 (arrow). Image credit: S.O. Kepler et al / Sloan Digital Sky Survey.
Stars born with masses below 8 to 11 solar masses end their lives as small, hot, and extremely dense objects known as white dwarf stars.
Their atmospheres are dominated by the lightest elements, such as hydrogen or helium, because gravitational diffusion brings the lightest element to the surface.
While combing through data from the Sloan Digital Sky Survey (SDSS), Dr. Kepler and his colleagues identified SDSS J1240+6710, a white dwarf with its outer layer of light elements stripped away, revealing a nearly pure layer of oxygen.
“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 astronomers said.
“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.”
According to the team, SDSS J1240+6710 has a mass of 0.56 solar masses and is located 1,174 light-years away.
“This star, a possible oxygen-neon white dwarf, will provide a rare observational test of the evolutionary paths toward white dwarfs,” Dr. Kepler and co-authors said.
Several different theories have predicted that the outer layer of a white dwarf can be stripped, but identification of SDSS J1240+6710 provides the first evidence of this phenomenon.
One possibility is that interactions with a nearby companion in a binary star caused this unique star to bare its oxygen envelope.
Another possibility is that a massive pulse of burning carbon from the center of the star eliminated the lighter elements.
The discovery is reported today in the journal Science.
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S.O. Kepler et al. 2016. A white dwarf with an oxygen atmosphere. Science, vol. 352, no. 6281, pp. 67-69; doi: 10.1126/science.aad6705
