Astronomers discover two more double-faced white dwarfs

For many years it was believed that the surface of most, if not all, white dwarfs was made up primarily of hydrogen. But then, two years ago, a white dwarf was spotted by a team of researchers that had a hydrogen surface on one side and a helium surface on the other. Since then, five more of the stars have been found, including two new ones by a team working in Oklahoma.

A trio of astronomers and astrophysicists at the University of Oklahoma, in the U.S., working with a colleague from Université de Montréal, in Canada and another from Harvard & Smithsonian's Center for Astrophysics, also in the U.S., has discovered two more double-faced white dwarfs, bringing the total found to seven. The group's study of the two discoveries, reported in The Astrophysical Journal, uncovered evidence that their nature is influenced by magnetic fields in the stars that impact their convection.

White dwarfs are dense stars with hot cores that have lost their outer layer and have cooled down over billions of years as they burn off their energy source. Prior research has shown that most are comparable in mass to the sun, with a volume roughly equal to that of the Earth's and that in addition to hydrogen, they harbor helium, oxygen nuclei and carbon.

The so-called double-faced white dwarfs make up a new class of white dwarfs. As they spin, the surface that faces the Earth reflects the different gases alternating between hydrogen and helium.

In comparing the two new discoveries with other white dwarfs, the researchers note that four of the seven double-faced white dwarfs discovered thus far are strongly magnetic. They also note that such strong magnetic fields are rare in white dwarfs that are not double-faced, suggesting that such fields play a role in the double-facedness of the unique stars.

They suggest that the magnetic field on double-faced stars is stronger at the poles than at the equator, resulting in convection pulling different materials to different parts of the surface.

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