Newly Discovered Brown Dwarf is
A brown dwarf discovered in the Pleiades star cluster by a team of astronomers at the University of Leicester is the coolest and faintest object ever found in the cluster, and possibly has the lowest mass of any known brown dwarf. Given the name PIZ 1 by its discoverers, the new brown dwarf's mass is estimated at 50 Jupiter masses and its surface temperature is put at only 2300 degrees K (2000 degrees C). Until now, Gliese 229B was regarded as the least massive brown dwarf known. But there is considerable uncertainty about that star's actual mass, which may be anywhere in the range 20 to 65 times the mass of Jupiter. So PIZ 1 is a definite challenger for the light-weight title.
The team of astronomers at Leicester are Drs Martin Cossburn, Simon Hodgkin, Richard Jameson and David Pinfield. David Pinfield will describe the new discovery at the UK's National Astronomy Meeting at the University of Southampton on Tuesday 8th April, and a paper on it will be published later in the Monthly Notices of the Royal Astronomical Society. The team is involved in an international project to search for brown dwarfs. Astronomers would like to know just how common these objects are. It is possible that they may make some significant contribution towards the unseen dark matter in galaxies, but this is as yet highly uncertain.
PIZ1 was discovered with the 2.5-metre Isaac Newton Newton Telescope on the island of La Palma during an observing run in December 1995 and January 1996. (Images were taken through filters known as I and Z, hence the origin of the name.) Further observations were made with the United Kingdom Infrared Telescope (UKIRT) in Hawaii in October 1996 and spectra were taken with the 4-metre William Herschel Telescope on La Palma in November 1996.
Brown dwarfs are sometimes referred to as 'failed stars' - in the sense that they are not massive enough for the nuclear fusion of hydrogen to take place in their cores. So once a brown dwarf has formed, it has no ongoing source of energy and spends the rest of its lifetime cooling down, getting fainter all the time. So the best time to find a brown dwarf is when it is young and at its brightest. The Pleiades cluster has proved a particularly good hunting ground, because the stars in it are relatively young on the astronomical scale - only 100 million years, and it is near enough for faint brown dwarfs to be detectable. Three other brown dwarfs have been discovered in the Pleiades already.
Even the brightest brown dwarfs are difficult to detect because they give out so little radiation. The search has gone on over the last 15 years, with limited success in terms of definite identifications. Recent detections have been as a direct result of improvements in the sensitivity of detectors and the use of larger telescopes.
Early indications from the international search suggest that many more new objects of similar mass to PIZ 1 are being found.
All at the Department of Physics and Astronomy, University of Leicester, Leicester LE1 7RH: