One-of-a-Kind Star Was Discovered With an Almost Pure Oxygen Atmosphere

April 5, 2016 | Joanne Kennell

White star
Photo credit: pixabay.com

Astronomers had no idea anything like it could even exist.

Astronomers have discovered a star unlike any other ever found. With an outermost layer of 99.9 percent pure oxygen, compared to Earth’s 21 percent, its atmosphere is the most oxygen-rich in the known universe.

The strange star is a new type of white dwarf, and was discovered by a team of Brazilian astronomers led by Kepler de Souza Oliveira at the Federal University of Rio Grande do Sul in Brazil. They published their findings in a paper in Science.

It is a very unique star because, of the 32,000 known white dwarfs, this is the only one with an almost pure oxygen atmosphere. It is named SDSS J124043.01+671034.68, but has been nicknamed “Dox” (pronounced Dee-Awks) by Kepler’s team.

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White dwarfs like Dox are relatively small and extremely old. This one is only slightly bigger than Earth, but has 60 percent the mass of our sun. In fact, white dwarfs are what’s leftover when stars (all but the largest three percent of them) have mostly run out of hydrogen and helium. After the stars have shed their outer layers, under high gravity, the heaviest elements drop into the star’s dense core while the lighter elements such as hydrogen and helium rise to the surface.

“This white dwarf was incredibly unexpected,” Kepler told Popular Mechanics (PM). “And because we had no idea anything like it could even exist, that made it all the more difficult to find.”

Boris Gänsicke, an astronomer at the University of Warwick, UK, who was not involved in Dox's discovery, confirmed that the “exotic white dwarf... has an almost pure oxygen atmosphere, diluted only by traces of neon, magnesium, and silicon,” he wrote in an essay accompanying the paper. “This chemical composition is unique among known [white dwarfs] and must arise from an extremely rare process.”

Dox was discovered using data collected over 15 years by the Apache Point Observatory in New Mexico observatory in a project called the Sloan Digital Sky Survey. Gustavo Ourique, one of Kepler’s undergraduate students, found Dox while looking through a data pile of 300,000 observations for strange and new types of white dwarfs.

The observations were graphs about what colors of light came from each source, called a spectral graph. And towards the end of the 300,000 graphs, Ourique found Dox.

However, the discovery of Dox has raised more questions than answers. First, almost all other white dwarfs have an atmosphere thick with hydrogen and helium, so for oxygen to reach the surface, all the lighter elements had to have disappeared.

“What happened to all these light elements?” asked Kepler to PM. “How did they all get stripped away?”

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Second, although heavier elements like carbon and oxygen are detected in one out of every five white dwarfs, it’s never purely one element. Even more mysterious, is that when oxygen atoms are found, they occur in far heavier white dwarfs. Smaller white dwarfs evolve from smaller stars, which don’t fuse atoms into oxygen as they collapse — meaning Dox would have had to be double its weight to have produced oxygen atoms in its earlier life.

“You have to wonder where this oxygen even came from,” Kepler said to PM.

Kepler and his colleagues have a couple ideas of how the heavier elements were stripped away. It is possible that Dox is part of a binary star system, and that interactions with another star in the system somehow peeled away the other atmospheric elements, exposing the oxygen below.

However, another idea is that something within the star, such as a pulse of burning carbon at Dox’s core, may have flared outwards, eliminating lighter elements on the surface.

The discovery of Dox is reshaping what astronomers currently know about stellar evolution — particularly how binary systems can impact the development of stars moving within them.  Until more research is done, we will have to wait before we learn more about this exceptionally rare star.

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