New type of object spotted in space
By Kevin Dayton
Advertiser Big Island Bureau
HILO, Hawai'i — Astronomers using two telescopes on Mauna Kea have discovered a new type of celestial object that defies normal classification: It's too big to be a superplanet and too feeble to be a star.
The object is part of a binary star system known as EF Eridanus, or EF Eri. Most stars are part of binary systems, in which two stars orbit each other, but astronomers say this system is something new. Gravity from the stronger partner has stripped away so much of the mass of the weaker star that it is essentially "dead" and cannot sustain nuclear fusion at its core. Scientists describe it as a strange, inert body that does not resemble any known star type.
Astronomers expect the object to orbit its more powerful partner for millions of years, leaving them some mysteries to ponder.
"Like the classic line about the aggrieved partner in a romantic relationship, the smaller donor star gave and gave and gave some more, until it had nothing left to give," said Steve B. Howell, research astronomer with the National Optical Astronomy Observatory in Tucson, Ariz.
"Now the donor star has reached a dead end: It is far too massive to be considered a superplanet, its composition does not match known brown dwarfs (stars), and it is far too low in mass to be a star. There's no true category for an object in such limbo."
EF Eridanus was discovered in the 1960s as a very bright system that changed its brightness regularly, and it had one of the strongest sources of electromagnetic radiation in the sky — telltale signs of one star in a binary system pulling mass away from its partner.
Astronomers believe the stripping of mass went on for 500 million to 5 billion years, then stopped about eight years ago. No one knows why, Howell said.
He and Thomas E. Harrison of New Mexico State University used the Gemini North telescope and W.M. Keck Observatory on Mauna Kea in 2002 and 2003 to study what is left of the system, and co-authored a paper on the subject to be published in the Oct. 20 Astrophysical Journal.
What remains now is a faint white dwarf star — the dominant partner — with a mass about 60 percent of the sun's. It is being orbited by an object with about 5 percent of the sun's mass. Scientists believe the smaller object originally may have had a mass about the same as the sun's.
The star system is about 300 light years away, in the constellation Eridanus. The two objects orbit each other from a distance about the same as that between the Earth and moon.
Scientists have not been able to create a computer model that replicates the EF Eri phenomenon. "I think it means the universe has won again — clearly we don't know what we're doing," Howell said. "I think that this indicates that this object, whatever it is, is something that we have to create some entirely new view of, and try to figure out what it is about our old models that don't work."
Co-authors of the paper on EF Eri are Paula Szkody of the University of Washington in Seattle and Joni Johnson and Heather Osborne of New Mexico State.
Reach Kevin Dayton at kdayton@honoluluadvertiser.com or (808) 935-3916.
