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Dwarf merger. Frames from a simulation of two white dwarfs merging.

The Birth of Extreme Helium Stars

Among the many oddities in the cosmic zoo, some of the strangest are "extreme helium stars." These rare and enigmatic stars seem to consist primarily of helium, rather than the ubiquitous hydrogen that makes up the bulk of typical stars such as the sun. For astronomers, it's about as unsettling as a biologist finding an elephant without a trunk. Now a theoretical study has provided the first detailed explanation of how such unusual stars could originate.

Nearly 20 years ago, astronomers floated the idea that a merger of white dwarfs could yield an extreme helium star. White dwarfs are the dim, condensed remnants of stars such as the sun. They have exhausted most of their hydrogen supply, and, depending on their mass and evolution, consist mainly of helium or a mixture of carbon and oxygen. This would be a simple way to create a luminous star consisting mainly of helium. But there were other possible explanations, and until now there hasn't been a convincing theory to back up the merger idea.

Now astronomers Hideyuki Saio of Tohoku University in Japan and Simon Jeffery of Armagh Observatory in Armagh, Northern Ireland, have done computer simulations that suggest the white dwarf merger hypothesis is probably right on the mark. The simulations reveal that a merger of a helium white dwarf and a carbon-oxygen white dwarf first results in a yellow giant star whose outer layers are almost pure helium--an extreme helium star. Then, the star begins to contract and heat up--a process that matches observations of four extreme helium stars. "This is the best, if not only, viable model for the creation of extreme helium stars," the authors argue in a paper to be published in an upcoming issue of the Monthly Notices of the Royal Astronomical Society.

Astronomers would love to see one of these mergers in action. Unfortunately, although they occur every few centuries in a galaxy such as the Milky Way, it's not completely clear how a white dwarf merger would look to astronomers on Earth. For instance, the simulations by Saio and Jeffery do not take the rotation of the white dwarfs into account, which could affect the merging process, notes Gijs Nelemans, an astronomer at Cambridge University in the United Kingdom. "This is certainly not the last word on white dwarf mergers," he says.

Related sites
Paper by Saio and Jeffery
Jeffery's page on extreme helium stars