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October 21, 2004

Mystery object isn’t a star or a brown dwarf

News and Information

A team of astronomers using telescopes at two Hawaiian observatories has found that one of the interacting stars in a binary star system has lost so much mass to its partner that it has deteriorated to a strange, inactive body that doesn’t resemble any known star type.

The team, which includes Paula Szkody, a UW astronomy professor, reports that the dead star, unable to sustain nuclear fusion at its core and doomed to orbit with its much more energetic white dwarf partner for millions of years, is essentially a new type of stellar object.

Szkody took part in observations at the Keck Observatory in September 2003, then helped to analyze data to determine the origin of mysterious features that other astronomers had seen previously from the Gemini North Observatory. Both observatories are on Hawaii’s extinct Mauna Kea volcano.

The binary system, known as EF Eridanus (abbreviated EF Eri), is 300 light-years from Earth in the constellation Eridanus. The system is a type called a cataclysmic variable, with two stars that have orbited each other for billions of years. As their orbits got closer, the white dwarf, about 60 percent the mass of our sun, began pulling matter from the other, which had a mass about half that of our sun.

“The result is a substellar object that now is only one-twentieth the mass of our sun,” Szkody said. “While this is expected to occur in a handful of close binary systems like this, it is difficult to observe in most systems because the mass being transferred from one star to the other actually obscures the stars. But, for some reason, EF Eri has had its mass transfer turned off for seven years now and is in the ideal state to look for its faint companion.”

The two bodies currently orbit each other in 81 minutes, but it is likely the orbital period was four or five hours when the white dwarf began pulling matter from the other star about 5 billion years ago.