Washington, June 3 : Astronomers have used ultra sharp images obtained with the Keck Telescope and Hubble Space Telescope to determine for the first time the masses of the coldest class of brown dwarfs, which are also known as "failed stars".
Brown dwarfs are a class of objects that represent the missing link between the lowest-mass stars and the gas-giant planets, such as Jupiter and Saturn. They are the faintest and coolest objects that can be directly observed outside the solar system.
Also, with masses as light as 3 percent the mass of the sun, these are the lowest mass free-floating objects ever weighed outside the solar system.
The observations are a major step in testing the theoretical predictions of objects that cannot generate their own internal energy, both brown dwarfs and gas-giant planets.
"Mass is the fundamental parameter that governs the life-history of a free-floating object, and thus after many years of patient measurements, we are delighted to report the first masses of the very faintest, coldest brown dwarfs," said Dr. Michael Liu of the Institute for Astronomy at the University of Hawaii (IfA/UH).
"Astronomers have measured the energy output and temperatures for a myriad of brown dwarfs. However, the most important property of all is the hardest one to measure - the mass," said Dr. Michael J. Ireland from the University of Sydney.
To determine the masses, the team has spent the last several years studying brown dwarfs that occur in binaries, that is two brown dwarfs that are mutually bound together by gravity and orbit each other, in a fashion similar to how Earth orbits the sun.
The astronomers obtained images using the 10-meter (400-inch) Keck II Telescope on Mauna Kea, Hawaii.
The team measured the masses of two brown dwarf binaries. They are located in the constellations of Libra (the Scales) and Bootes (the Herdsman), are about 45-60 light-years from Earth.
One binary is composed of two "methane" brown dwarfs, the coolest type of brown dwarf, which is characterized by the presence of methane gas in their atmospheres.
This is the first mass measurement for this type of brown dwarf.
The team found that the total mass of this binary is only 6 percent of the sun's mass, and each brown dwarf in it has a mass of about 3 percent of the sun's (about 30 times the mass of Jupiter).
The other binary system is a pair of slightly warmer "dusty" brown dwarfs with a total mass of only 11 percent of the sun's mass and individual masses of about 5.5 percent of the sun's.