Brown dwarfs, essentially stunted stars, were most likely ejected from newborn, multiple-star systems before they had a chance to accumulate enough mass to ignite the hydrogen in their interiors and flower, according to a new study.

University of Colorado at Boulder astronomer Bo Reipurth said that most newborn stars are spawned in binary or multiple systems involving two, three, four, five or even more stars. Just as newborn mammals on Earth compete with each other for milk, newborn stars in multiple systems compete for gaseous matter that generates speedy growth, he said.

“What we are proposing is that brown dwarfs don’t require special conditions to form,” said Reipurth, a research professor at CU-Boulder’s Center for Astrophysics and Space Astronomy. “Most stars in our Milky Way Galaxy began either as binary or multiple-star systems, and soon after birth a tug-of-war starts between stellar embryos over which ones can accumulate the most star-forming material.”

Whether singly or in small groups, stars form out of dense cores that collapse, resulting in a powerful rain of gaseous material falling toward the stars’ centers, he said. The more massive star embryos generally move around the center of such cores, allowing them to accrete more matter and bulk up.

“The smaller and weaker star embryos are constantly flung out of the central feeding ground by gravitational slingshots, and thus grow slower,” Reipurth said.

Reipurth and Cathie Clark of the Institute of Astronomy in Cambridge, England, co-authored a paper on the subject in the July 2001 issue of The Astronomical Journal, published by the American Astronomical Society.

“Amazingly, calculations show that the gravitational horseplay between stellar
embryos almost always end up with the lightest member being violently flung out of the little group,” said Reipurth. Sometimes the kick merely sends it into an extended orbit around the other embryos, but more often it is completely booted out of the system.

“While we know the eventual outcome, it is impossible to precisely predict when such an ejection will occur,” he said. “There is an element of randomness in this process, much like a lottery.” After 50,000 to 100,000 years, many of these multiple-star systems have decayed, leaving behind smaller binary or multiple-star systems.

Stars require roughly 8 percent of our sun’s mass to begin the process of nuclear ignition, said Reipurth. “If they donít make it, they don’t ignite. If such small star embryos are ejected by their siblings so early that they have not built up the necessary mass and fuel, they become brown dwarfs,” he said.

“With better luck in the disintegration lottery, a brown dwarf could have become a normal star, shining for billions of years. But without nuclear ignition, brown dwarfs are left to merely glimmer darkly forever,” he said.

Binary stars are very common, but scientists have puzzled for years over why brown dwarfs are rarely found as close companions to normal stars, Reipurth said. “Our model can now explain that, because a stellar embryo very close to one that succeeds in becoming a star would normally also get sufficiently fed to become a star.”

During the last year, astronomers have pinpointed several brown dwarfs that are distant companions to normal stars. “These are brown dwarfs that also were kicked out, but not quite strongly enough to avoid the pull of their siblings,” he said.

Close examination with large telescopes of free-floating brown dwarfs has shown that some are binaries, Reipurth said. “These are two small stellar embryos which formed a pair before they got ejected together. Widely spaced pairs of stellar embryos are more unlikely to survive an ejection intact, consistent with the observation that all known binary brown dwarfs are relatively close to each other.”

The last few years have seen a flurry of discoveries of giant planets orbiting other stars, and it has been hotly debated whether such giant planets are simply very small brown dwarfs. “This is not the case,” said Reipurth.

“Even though brown dwarfs failed in the end to become stars, they still were formed the same way as stars. In contrast, giant planets are frequently found as very close companions to stars, implying they grew as planets from circumstellar disks like planets in our own solar system.”

Contact: Bo Reipurth

Jim Scott

University of Colorado at Boulder


  • The Formation of Brown Dwarfs as Ejected Stellar Embryos

    Authors: Bo Reipurth (Univ. of Colorado), Cathie Clarke (Inst. of Astronomy, Cambridge)
    Comments: 8 pages, 1 figure, Accepted by the Astronomical Journal

    “We conjecture that brown dwarfs are substellar objects because they have been ejected from small newborn multiple systems which have decayed in dynamical interactions. In
    this view, brown dwarfs are stellar embryos for which the star formation process was aborted before the hydrostatic cores could build up enough mass to eventually start
    hydrogen burning. The disintegration of a small multiple system is a stochastic process, which can be described only in terms of the half-life of the decay. A stellar embryo
    competes with its siblings in order to accrete infalling matter, and the one that grows slowest is most likely to be ejected. With better luck, a brown dwarf would therefore have
    become a normal star. This interpretation of brown dwarfs readily explains the rarity of brown dwarfs as companions to normal stars (aka the “brown dwarf desert”), the
    absence of wide brown dwarf binaries, and the flattening of the low mass end of the initial mass function. Possible observational tests of this scenario include statistics of brown
    dwarfs near Class 0 sources, and the kinematics of brown dwarfs in star forming regions while they still retain a kinematic signature of their expulsion. Because the ejection
    process limits the amount of gas brought along in a disk, it is predicted that substellar equivalents to the classical T Tauri stars should be very rare. “