Brown Dwarf Stars

A sub-stellar object, intermediate in mass between a star and a planet; brown dwarfs – actually dull red in colour - are "failed stars" because they are not massive enough to have initiated hydrogen fusion in their cores. The first brown dwarf to be confirmed, in 1995, on the basis of a combination of mass determination, spectroscopic studies, and direct imaging was Gliese 229B. Hundreds are now known.

A brown dwarf may be defined by its mass or its origin. The upper mass for a brown dwarf is that which is just insufficient for normal hydrogen fusion to be triggered in the core. This is believed to be between 0.075 and 0.080 solar masses, or 75 to 80 times the mass of Jupiter. The lower mass limit is somewhat arbitrary as there is no obvious point of transition between a high-mass planet and a low-mass brown dwarf, but it is generally taken to be about 0.013 solar masses, or 13 times the mass of Jupiter. Some astronomers argue that a more significant distinction between planets and brown dwarfs is their mode of formation. By this criterion, brown dwarfs are held to form in the same way as stars, as condensations in an interstellar gas cloud while planets accrete from material in a circumstellar disk.

Brown dwarfs are too cool to give off much visible light but they do emit substantial amounts of infrared radiation as a result of slow gravitational contraction and small-scale deuterium fusion. They can be detected by ground-based and spaceborne infrared telescopes and can also be found if they happen to be orbiting a star; the presence of the brown dwarf is indicated by wobbles that it causes in the motion of its companion star across the sky. The radial velocity method has been successful in identifying a large number of brown dwarfs in orbit around stars. Telescopes equipped with coronagraphs may be able to image the candidate brown dwarf at visible wavelengths, as was the case with Gliese 229B. Brown dwarfs do not glow, even dully for very long. As soon as they have used up their meager supply of deuterium, which takes about 10 million years, they fade from dim dark red to black. However, there are stars that start out as ordinary hydrogen-fusing red dwarfs and then get whittled away to brown dwarf size. The binary systems LL Andromedae and EF Eridani both contain white dwarf primaries that have looted material from their partners and reduced them to 40-Jupiter-mass objects, with surface temperatures of about 1,300 K and 1,650 K, respectively.

While some brown dwarfs, like Gliese 229B, are part of binary systems, others having been found floating around on their own, including a number in the Pleiades, the Sigma Orionis star cluster, and the Trapezium. PPl 15, in the Pleiades, is a binary system in which both components are brown dwarfs. S Ori 47, in the Sigma Orionis cluster, holds the record for the brown dwarf with the smallest known mass – a mere 0.015 solar mass. Some objects, such as the companion of HD 114762, lie close to the borderline between massive planets and low-mass brown dwarfs. A surprisingly high proportion of brown dwarfs have been found as companions to low-mass (red dwarf or other brown dwarf) stars, and, within these systems, the separation between the two components is typically very small, averaging about 4 AU. This goes against the prediction by some theorists that most very low-mass stars and brown dwarfs are solo objects, wandering though space alone after being ejected out of their stellar nurseries during the star formation process. Very few brown dwarf companions of larger, Sun-like stars have been found inside 5 AU, a deficiency that has been dubbed the "brown dwarf desert;" however, there is no such desert associated with low-mass stars. The observations to date strongly support the idea that low-mass binaries form in a process similar to that of more massive binaries, and that the percentage of binary systems is similar for bodies spanning the range from one solar mass to as little as 0.05 solar masses. There are also many lone brown dwarfs, such as KELU-1, discovered in 1997, only 33 light-years from the Sun. Such solitary dwarfs could be ejected stellar embryos – small infant stars that were still accreting material when they were ejected by the other bodies in multiple stellar systems. On the other hand, observations of some brown dwarfs in the Orion Nebula, which show an excess of near-infrared radiation, point to the presence of dusty disks around these objects. Not only does this suggest a normal stellar formation process but also the possibility that brown dwarfs might develop planetary systems.

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