Astronomers Identify New Type Of Star

Astronomers Steve B. Howell of the University of California, Riverside and Tom Harrison of New Mexico State University, Las Cruces, announced today at the American Astronomical Society Meeting in Seattle, Wash., that they have confirmed the existence of a new variety of stellar end-product. This previously unknown type of star has some properties similar to brown dwarf stars and may help astronomers understand some of the recently discovered extra-solar planets in close proximity to their suns.

The newly discovered type of star resides in a binary star system known as EF Eridanus with an estimated distance of 300 light years from Earth. EF Eri, with an orbital period of 81 minutes, belongs to a class of binary star called magnetic cataclysmic variables, so named for their sudden explosive brightenings caused by mass accretion events and their very strong magnetic fields.

Using telescopes at the National Science Foundation's Kitt Peak National Observatory near Tucson, Ariz., and at Apache Point Observatory in Sunspot, N.M., Howell and Harrison simultaneously obtained observations of EF Eridanus using an infrared camera at the Kitt Peak 2.1 meter (84-inch) telescope and an optical camera on the New Mexico State University 1-m (36-inch) telescope.

In typical magnetic cataclysmic variables, the more massive component is a white dwarf star, the remnant of a once massive star with probably 3-5 times the mass of the sun. Most stars, including the sun, will end their lives as white dwarfs - small degenerate objects with about as much mass as our sun smashed into a size equal to that of the Earth. A teaspoon full of white dwarf material would weigh about as much as ~100 elephants. In these magnetic binaries, the white dwarf has a strong magnetic field of order 10-200 million times that of the Earth.

The other component in a magnetic cataclysmic variable is typically a normal star similar to our sun, but smaller and with only about 1/2 the mass. This companion spends its life transferring mass to the white dwarf at a rate of approximately 6 billion tons per second. The white dwarf in EF Eri has an extremely intense magnetic field, 15-20 million times that of the Earth, and when mass transfer occurs, the matter is funneled down the magnetic field lines where it eventually crashes onto the surface of the white dwarf near its magnetic poles within an area about the size of California.

Gravitational energy released from the accreted material produces copious amounts of radiation (equal to about 20 billion megaton bombs per second) coming from the binary, generally swamping the light emitted by either star from the X-ray to the optical and infrared regions. The two stars are, in effect, invisible when mass is being transferred, and observations reveal only strong emission from 1 million degree regions on the white dwarf surface.

For reasons that astronomers do not yet fully understand, the flow of matter from the mass donor to the white dwarf occasionally shuts off in these magnetic systems. The mass flow starts again within a few weeks to a few months in most systems. EF Eridanus stopped transferring matter in 1995, became 30 times fainter, and has remained relatively inactive for the last seven years. With the stars in the binary now exposed, Howell and Harrison had the first good opportunity to directly detect the low mass companion in EF Eri.

The results of the observations performed by Howell and Harrison have led to a view of the mass donor star as being very cool with a temperature between 800 and 1,200 Kelvin (980F to 1700F). "Large light variations that we observed in our data are caused by the heating to 1600K of one side of the mass donor by the near-by white dwarf," says Harrison, observatory specialist at New Mexico State University, Las Cruces, N.M., "while the other side remains very cool near 900K." Follow-up infrared spectroscopy, obtained at the United Kingdom Infrared Telescope on Mauna Kea in Hawaii, confirmed that the heated side of the companion has a temperature near 1,600 K.

At present, EF Eri appears nothing like a magnetic cataclysmic variable; optical observations show only a white dwarf while infrared observations reveal only an odd, cool brown dwarf-like object. Without detailed study, one would not know the stars are in a binary or that EF Eri used to be one of the brightest X-ray sources in the sky.

The mass-losing companion star in EF Eridanus started out life as a normal low-mass star similar to our sun, but has lost over 90% of its mass during the last 5-8 billion years. The remaining object probably has no internal energy generation making it a semi-degenerate star, sort of halfway between a normal star and a white dwarf.

"Theoretical models calculated a few years ago by myself and collaborators predicted the existence of such objects," says Howell, who is research professor at the Institute of Geophysics and Planetary Physics at the University of California, Riverside. "It is extremely gratifying to actually find one of them. Many more should be out there, so our search has just started."

This new type of star is about the size and temperature of a brown dwarf given its very different formation process, and has interior and atmospheric structures which are yet unknown. This object, and others like it, will provide an important test for understanding the response of the atmospheres of low mass, cool objects (such as brown dwarfs and large extra-solar planets) to heating by their parent stars. Many of the recent extra-solar planets have large masses (several times that of Jupiter) and are in relatively tight orbits around their suns. The atmospheres of such planets should suffer intense heating like that seen in the mass donor of EF Eridanus.

Howell, Harrison, and NMSU graduate student Ms. Heather Osborne are continuing to study EF Eridanus, and will soon have new infrared data obtained using the National Science Foundation's 8-m (320-inch) Gemini telescope in Hawaii. The new data will allow them to confirm and accurately derive how the temperature varies from one side of EF Eridanus to the other and provide a detailed study of the atmospheric structure of this new type of star. Observations of additional systems believed to be similar to EF Eri are also underway.

This work was supported by the National Science Foundation's Stellar Astronomy and Astrophysics program.