Astronomers using the UK Infrared Telescope (UKIRT) in Hawaii have discovered two examples of a kind of star never previously observed. These small, cool stars look superficially like brown dwarfs but are actually the remnants of ordinary stars that have been whittled down to cool Jupiter-sized bodies over billions of years by spilling material over to a white dwarf companion star.

An artist conception of a normal cataclysmic variable. Here we see the mass losing star (left) and the white dwarf (right, in middle of disk) as well as the stream of gas between the two and the accretion disk formed around the white dwarf. In this binary, the two stars will typically orbit each other every 4-6 hours and they are about 1 solar diameter apart The white dwarf is about the size of the Earth yet contains 1/2 to 1 times the mass of the sun. The mass losing star is similar to our sun, but only about 8/10ths of its mass. Photo: Dana Berry

Dr Steve B. Howell, Head of the Astrophysics Group at the Planetary Science Institute in Tucson, Arizona, was one of several astronomers who predicted that stars of this kind would exist in such binary star systems. Now Dr Howell, working with Dr David Ciardi of the University of Florida and UKIRT staff scientists Chris Davis and Paul Hirst, has secured the first direct evidence of such stars by taking infrared spectra with the CGS4 instrument on UKIRT of two variable binary star systems: LL Andromedae and EF Eridani. The results are to be published in Astrophysical Journal Letters.

The observers took advantage of periods when the flow of material between the two stars in these binary systems temporarily stops. At these quiescent times, UKIRT can distinguish the radiation coming from the cool donor star. In the case of LL Andromedae, the signature of methane was detected at a wavelength of 2.2 microns. This shows that the donor star's temperature is around 1,300 K (1000 degrees C), similar to a 'T-type' methane brown dwarf. In EF Eridani, the donor star is a little warmer at around 1,650 K (1,350 degrees C), similar to an 'L-type' brown dwarf. According to theory, the estimated mass of these cool stars is near four hundredths the mass of the Sun, or 40 times the mass of the planet Jupiter.

Assuming that they give out about the same amount of radiation as more familiar young brown dwarfs, Howell estimates that both LL Andromedae and EF Eridani are between about 100 and 130 light years away - virtually neighbours of the solar system. To get a good feel for what these binary stars are like, Howell says "Imagine the Earth is a white dwarf star, which is about the same size as the Earth, and that Jupiter is where the Moon is, orbiting around Earth every 80 minutes."

An artist conception of a very old cataclysmic variable containing a brown dwarf-like secondary star (left) and a white dwarf (right). In this case, the two stars orbit each other every 80 minutes or so, have very little mass being transferred, and are only about as far apart as the Earth and Moon. The unique type of brown dwarf-like star contained in such a system is the size of the planet Jupiter (Jupiter is 100 times smaller than the sun) but has a mass of only 4/100ths that of the sun, about 40 Jupiter masses. Originally, the brown dwarf-like star probably contained about as much mass as our sun, but has lost the majority of it during its likely 7000-9000 million year lifetime. Photo: Dana Berry

These newly discovered stars are probably about 8 billion years old, as old as the Galaxy itself. Though as cool as brown dwarfs, and similar to them in size and mass, Howell emphasises that their structure and composition is likely to be different, and is not yet known.

Background Notes
Brown dwarfs - Ordinary brown dwarfs are intermediate between stars and planets. Often described as 'failed stars', they are more massive than Jupiter, the largest planet in the solar system, but they fall short of the minimum mass a true star needs - 8% of the Sun's mass. Stars can shine constantly for billions of years because they generate nuclear energy from the fusion of hydrogen into helium. But brown dwarfs cannot sustain nuclear power production. After a modest initial flush, they cool off and become progressively fainter.

Young brown dwarfs are now known to exist in the hundreds in the Sun's neighbourhood. Their surface temperatures are less than about 3,500 K (3,200 degrees C). As the surface of a brown dwarf cools below 1,500 K, a dramatic chemical change takes place: large amounts of methane form, considerably altering its appearance. The methane, or 'T-type', brown dwarfs are the coolest objects so far detected.

More about LL And and EF Eri - LL Andromedae is classified as a 'dwarf nova'. It is a binary star system in which material flows from one star to the other. The two members are a white dwarf primary, which receives material, and a cool dwarf secondary, which acts as 'donor'. Material flowing from the donor star forms a disc around the more massive white dwarf. Outbursts take place from time to time when hot regions develop on this accretion disc. The outbursts of LL Andromedae, which is a very old system, are spaced by several years, or even decades. In younger dwarf novae, outburst may occur much more frequently.

EF Eridani is classified as a 'polar'. Polars are similar to a dwarf novae except that their white dwarfs are strongly magnetic. The magnetic field prevents an accretion disc forming, Instead, material from the donor star flows directly onto the magnetic poles of the white dwarf. Though variable, polars do not have outbursts like dwarf novae. They are sources of X-rays.

The UK Infrared Telescope - The world's largest telescope dedicated solely to infrared astronomy, the 3.8-metre UK Infrared Telescope (UKIRT) is sited near the summit of Mauna Kea, Hawaii, at an altitude of 4194 m above sea level. It is owned by the United Kingdom Particle Physics and Astronomy Research Council and operated along with the James Clerk Maxwell Telescope (JCMT) by the staff of the Joint Astronomy Centre, located in Hilo. UKIRT produces images and spectra in a key 1-5 micron region of the electromagnetic spectrum for astronomy, covering wavelengths between 1 and 5 microns. Wavelength coverage will shortly be extended to the thermal infrared, at 10 and 20 microns. Further examples of UKIRT data and images can be found on the UKIRT image gallery: