Brown dwarfs: a second Pleiades candidate.

If only it had more mass, a brown dwarf could be a star.

A star forms when a cloud of gas and dust undergoes gravitational collapse, becoming hotter and denser at its core. Eventually, that core grows hot enough to ignite and sustain a powerful energy source--the fusion of light nuclei into heavier ones.

A brown dwarf arises from the same raw material, but its lower mass and weaker gravitational contraction prevent it from attaining a high enough core temperature to burn nuclear fuel steadily. Thus, when gravitational contraction comes to a halt, a dwarf is doomed to slowly cool and fade into oblivion.

That's exactly why the Pleiades star cluster has recently proved a fruitful hunting ground for such objects. The cluster's relative youth--about 100 million years--means that any brown dwarfs that formed there haven't lived long enough to have faded. Now, for the second time in 3 months, researchers report compelling evidence of a brown dwarf candidate that almost certainly resides in the cluster.

Raphael Rebolo and his colleagues at the Instituto de Astrofisica de Canarias in La Laguna, Spain, discovered the candidate while analyzing images of the Pleiades taken with a telescope at Teide Observatory in the Canary Islands. Using several telescopes to reexamine the object, now named Teide 1, the researchers conclude in the Sept. 14 Nature that it lies in the cluster and probably has a mass equal to 2 percent of the sun's.

That estimate places Teide 1, unlike previous candidates, well below 8 percent of the sun's mass--the maximum a brown dwarf can have, comments Lorne A. Nelson of Bishop's University in Lennoxville, Quebec. Because of uncertainties in calibrating the surface temperature of Teide 1, astronomers can't entirely rule out the possibility that the body may have a mass as large as 7 percent of the sun's, Nelson adds. But he notes that this still puts Teide 1 below the required mass limit.

Another group of astronomers reported recently that PPL 15, a Pleiades resident estimated to have a mass just under the dwarf limit, has retained most of its primordial allotment of lithium. The presence of lithium indicates that the body never had an opportunity to destroy the gas through nuclear burning and is either a brown dwarf or an object right at the transition between a star and a dwarf (SN: 6/24/95, p.389).

Nelson argues that if PPL 15 qualifies as a brown dwarf, then the seemingly less massive Teide 1 should certainly make the grade. He and other researchers agree, however, that the ultimate test--finding the fingerprints of lithium in Teide 1--still lies ahead.

Astronomers may not have long to wait, notes Gibor S. Basri of the University of California, Berkeley, a member of the team that detected the lithium content of PPL 15. He says that he and his collaborators, including Rebolo, plan to examine the abundance of lithium in Teide 1 with the same telescope, the W.M. Keck atop Hawaii's Mauna Kea, this November.