Big Telescope Is First to Find Brown Dwarf, Team Reports
By JOHN NOBLE WILFORD
Published: June 14, 1995
PITTSBURGH, June 13— In the first major discovery by the new Keck Observatory in Hawaii, the world's most powerful telescope, astronomers reported today what they said was the first confirmed sighting of a brown dwarf, one of those elusive theorized objects bigger than a planet but not quite a star.
So small and dim are the supposed brown dwarfs against the dark background of space that the search for them has taken astronomers to the limits of their telescopes' observing capabilities. Several brown-dwarf candidates have been reported in recent years, only to be eliminated by subsequent research.
But with the 400-inch Keck telescope, capable of seeing objects one-fourth as bright as anything detectable by the 200-inch telescope on Mount Palomar in California, and with a new search strategy, astronomers think they are now able to prove the existence of brown dwarfs, understand their characteristics and perhaps determine eventually how much of the so-called missing mass in the universe they could account for.
A team of astronomers from the University of California at Berkeley and San Francisco State University identified a faint low-mass object in the Pleiades star cluster, the familiar "Seven Sisters," as a brown dwarf based on an analysis of the chemical element lithium in its atmosphere. The fact that lithium is still detectable indicates that little if any hydrogen burning is taking place inside the object.
A star by definition is a massive sphere of dense gas glowing from the thermonuclear fusion of hydrogen, a process that would quickly destroy all the original lithium in small stars and virtually all in larger stars like the Sun. The smaller a star or a low-mass substellar object like the theorized brown dwarf, the cooler is its center, and the longer it takes to destroy lithium.
The team leader, Dr. Gibor Basri, a Berkeley astronomer, justified the claim that this was a "confirmed sighting of a brown dwarf" on the ground that the object is not only extremely dim, in the nature of brown dwarfs, but also "it has not destroyed lithium in its center."
The observations were reported here at a meeting of the American Astronomical Society. The other team members are Dr. Geoffrey W. Marcy of San Francisco State and Dr. James R. Graham, also of Berkeley.
"This is a tremendously exciting result," said Dr. James Liebert, an astronomer at the University of Arizona at Tucson who is an authority on brown dwarfs and other dim, cool stellar objects. But he said the many implications of the discovery were not yet clear, and he stopped short of agreeing with the interpretation that this was indeed a confirmed brown dwarf.
Because the new findings appeared to show a greater age for stars and the possible brown dwarf in the Pleiades than had been estimated, Dr. Liebert suggested that the object in question could be a small star with low hydrogen fusion that had not yet consumed all its original lithium. In that case, he said, the object was not a brown dwarf now, but might become one when its fusion furnace finally cooled off, in perhaps another billion years.
The Pleiades, which had been thought to be 76 million years old, now appeared to be 115 million years old, the California team reported on the basis of the new observations. Stars get fainter with time, and so to have its luminosity at the greater age, the reported brown dwarf must have a higher mass that approaches the boundary between low-mass objects of the brown-dwarf class and the lowest-mass true stars.
Responding to these comments, Dr. Basri affirmed, "Either age, it's a brown dwarf."
But he and his colleagues acknowledged that if the Pleiades cluster is younger than the new estimate, then the identification of the brown dwarf becomes more compelling because it would presumably have an even lower mass.
Dr. Liebert said it was "absolutely crucial to find other fainter ones" and determine that they also display a clear lithium signature. Dr. Basri said that was the next step in the team's research. "A couple of new candidates" have already been detected and are being studied, he said.
If they turn out to be fainter and smaller and have lithium, Dr. Liebert said this would be a more clear-cut result showing that brown dwarfs had at last been discovered.
The object thought to be a brown dwarf, designated PPL 15, was first detected at Palomar Observatory in 1989 by Dr. John Stauffer, an astronomer at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. Nothing definitive could be said about it until it was examined in visible and infrared light by the Keck telescope on Mauna Kea in Hawaii, which began full operation last year. By breaking up the object's faint light with a high-resolution spectrograph, astronomers found the critical signature of lithium, which they consider the most direct method available for identifying brown dwarfs.
"This really opens the door to those looking for brown dwarfs," Dr. Graham said of the new lithium search technique.
The mass of PPL 15 is estimated to be slightly less than 8 percent that of the Sun and about 80 times the mass of the planet Jupiter. Because Jupiter has one-hundredth the mass of the lowest-mass star known, Dr. Marcy said, "this discovery begins to fill the gap" between the least massive stars and the most massive planets."
Diagram: "A CLOSER LOOK: Brown Dwarfs -- Cool Little Heavenly Bodies" Brown dwarfs are theorized starlike objects, too cool and too small to ignite nuclear fusion in their cores. Therefore, they do not destroy lithium. It is this presence of lithium that allows astronomers to identify them. (Source: University of California at Berkeley)