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Astronomers identify new type of star

Astronomers from New Mexico State University and the University of California at Riverside have discovered a new type of star locked in a tight dance with a more powerful partner about 300 light-years from Earth.


sn't look like any star we've ever seen before," said New Mexico State astronomer Tom Harrison. "It has the temperature of a brown dwarf and the mass of a brown dwarf, but it's about five times more luminous than a brown dwarf."

Further study of the unusual object may help scientists understand some of the recently discovered extra-solar planets that orbit closely around their suns, Harrison said.

This previously unknown type of star -- or stellar end-product, since it seems to be a dying star -- is a partner in a binary star system known as EF Eridanus, or EF Eri for short. In a binary system, two stars bound by gravity orbit each other around a common point. In the case of EF Eri, a hot, very dense white dwarf star and its cooler, brown dwarf-like partner are orbiting each other once every 81 minutes -- a very short period for a binary system.

EF Eri is a member of a small family of binary star systems known as "magnetic cataclysmic variables." In the system's normal state, the more massive white dwarf, which has a magnetic field 15 million times more intense than Earth's, is furiously stripping matter away from the brown dwarf-like star. As the companions circle each other, the transferred matter is funneled down the magnetic field lines and eventually crashes into the surface at the magnetic poles of the white dwarf star, causing sudden explosive brightenings of the system.

Normally all this activity emits so much radiation that it is impossible for astronomers to see the faint companion star. But for reasons that astronomers do not yet understand, once in a while the transfer of matter in these types of binary systems shuts down.

"They usually only turn off for a year or two and then resume, but this one has been quiet for seven years," Harrison said.

During this quiet phase, the overall brightness of EF Eri has dimmed by a factor of 30, making it possible to investigate the nature of the white dwarf's faint companion. Harrison and Steve Howell of the Institute of Geophysics and Planetary Physics at UC Riverside obtained simultaneous observations of EF Eri using an infrared camera on the 2.1-meter telescope at Kitt Peak National Observatory in southern Arizona and an optical camera on the 1-meter New Mexico State University telescope at Apache Point Observatory in southern New Mexico.

They were surprised to find that the binary system was varying in brightness by a factor of two with each orbit. This is a much bigger variation than usually seen in these types of systems in their quiet states, Harrison said.

Harrison and Howell were able to construct a model to explain their observations, but to do so required that the secondary star had to be very cool, with a temperature between 800 and 1,200 Kelvin (980 to 1,700 degrees Fahrenheit). Normal stars have temperatures above 2,500 K.

These low temperatures imply that the companion star is like a brown dwarf. The large variations in brightness would be caused by the hot white dwarf heating one side of the brown dwarf-like star to a temperature of about 1,600 K while the other side remains cool, at 700 to 1,000 K, Harrison said.

Infrared spectroscopy obtained at the United Kingdom Infrared Telescope in Hawaii confirmed that the hot side of the star does indeed have a temperature near 1,600 K.

EF Eri's two stars "stay locked, like our Earth-Moon system, with one side of the companion star always facing the white dwarf," Harrison said. As the brown dwarf orbits the white dwarf, observers on Earth alternately see the hot and cool sides, and the total brightness rises and falls.

The companion star probably was once a normal low-mass star but has lost so much mass that it now resembles a brown dwarf, he said. Brown dwarfs, sometimes referred to as failed stars, do not have enough mass to sustain the hydrogen fusion that fuels a normal star. They are similar in some ways to giant, gaseous planets.

Recently discovered extra-solar planets tend to be giant planets and some of them have orbits very close to their suns. Astronomers currently are not able to directly observe these distant extra-solar planets, because of the overpowering light emitted by the stars that the planets orbit.

Because the white dwarf star in EF Eri is about 100 times fainter than most of the primary stars in extra-solar planetary systems, "we are better able to study its brown dwarf companion," and this can give astronomers a better understanding of how extra-solar planets respond to the extreme heating caused by their close proximity to their suns, Harrison said.

The astronomers' research was presented last Tuesday at a meeting of the American Astronomical Society in Seattle. Harrison, Howell and NMSU graduate student Heather Osborne are continuing to study EF Eridanus, using new data from the 8-meter Gemini telescope in Hawaii.

So far, the companion star is proving to be a bit strange.

"We are really in uncharted territory with this one," Harrison said. "It has the temperature of a brown dwarf and the mass of a brown dwarf, so we expected to see the spectra of a brown dwarf, but we don't. We might be looking at the remnant of the core of a low-mass star."