Brown Dwarf - History

History

What became known as Brown dwarfs were theorized to exist in the 1960s.

Brown dwarfs, a term coined by Jill Tarter in 1975, were originally called black dwarfs, a classification for dark substellar objects floating freely in space which were too low in mass to sustain stable hydrogen fusion (the term black dwarf currently refers to a white dwarf that has cooled down so that it no longer emits significant heat or visible light). Alternative names have been proposed, including planetar and substar.

Early theories concerning the nature of the lowest-mass stars and the hydrogen-burning limit suggested that a Population I object with a mass less than 0.07 solar masses or a Population II object less than 0.09 solar masses would never go through normal stellar evolution and would become a completely degenerate star (Kumar 1963). The role of deuterium-burning down to 0.012 solar masses and the impact of dust formation in the cool outer atmospheres of brown dwarfs was understood by the late 1980s. However, such objects would be hard to find in the sky, as they would emit almost no visible light. Their strongest emissions would be in the infrared (IR) spectrum, and ground-based IR detectors were too imprecise at that time to readily identify any brown dwarfs.

Since then, numerous searches by various methods have sought to find these objects. These methods included multi-color imaging surveys around field stars, imaging surveys for faint companions to main sequence dwarfs and white dwarfs, surveys of young star clusters, and radial velocity monitoring for close companions.

For many years, efforts to discover brown dwarfs were frustrating and searches to find them seemed fruitless. In 1988, however, University of California, Los Angeles professors Eric Becklin and Ben Zuckerman identified a faint companion to GD 165 in an infrared search of white dwarfs. The spectrum of GD 165B was very red and enigmatic, showing none of the features expected of a low-mass red dwarf star. It became clear that GD 165B would need to be classified as a much cooler object than the latest M dwarfs then known. GD 165B remained unique for almost a decade until the advent of the Two Micron All Sky Survey (2MASS) when Davy Kirkpatrick, of the California Institute of Technology, and others discovered many objects with similar colors and spectral features.

Today, GD 165B is recognized as the prototype of a class of objects now called "L dwarfs". While the discovery of the coolest dwarf was highly significant at the time, it was debated whether GD 165B would be classified as a brown dwarf or simply a very low-mass star, since observationally, it is very difficult to distinguish between the two.

Soon after the discovery of GD 165B, other brown dwarf candidates were reported. Most failed to live up to their candidacy, however, and with further checks for substellar nature, such as the lithium test, many turned out to be stellar objects and not true brown dwarfs. When still young, up to a billion years (a gigayear) old, brown dwarfs can have temperatures and luminosities similar to some stars, so other distinguishing characteristics are necessary, such as the presence of lithium. Stars will burn lithium in a little over 100 Myr, at most, while most brown dwarfs will never acquire high enough core temperatures to do so. Thus, the detection of lithium in the atmosphere of a candidate object ensures its status as a brown dwarf.

In 1995 the study of brown dwarfs changed substantially with the discovery of two incontrovertible substellar objects (Teide 1 and Gliese 229B), some of which were identified by the presence of the 670.8 nm lithium line. The most notable of these objects was Gliese 229B, which was found to have a temperature and luminosity well below the stellar range. Remarkably, its near-infrared spectrum clearly exhibited a methane absorption band at 2 micrometres, a feature that had previously only been observed in gas giant atmospheres and the atmosphere of Saturn's moon, Titan. Methane absorption is not expected at the temperatures of main-sequence stars. This discovery helped to establish yet another spectral class even cooler than L dwarfs known as "T dwarfs" for which Gl 229B is the prototype.

The first confirmed brown dwarf was discovered by Spanish astrophysicists Rafael Rebolo (head of team), Maria Rosa Zapatero Osorio, and Eduardo Martín in 1994. They called this object Teide 1 and it was found in the Pleiades open cluster. The discovery article was submitted to Nature in spring 1995, and published on September 14, 1995. Nature highlighted "Brown dwarfs discovered, official" in the front page of that issue.

Teide 1 was discovered in images collected by the IAC team on January 6, 1994 using the 80 cm telescope (IAC 80) at Teide Observatory and its spectrum was first recorded in December 1994 using the 4.2 m William Herschel Telescope at Roque de los Muchachos Observatory (La Palma).
The distance, chemical composition, and age of Teide 1 could be established because of its membership in the young Pleiades star cluster. Using the most advanced stellar and sub-stellar evolution models at that moment, the team estimated for Teide 1 a mass 55 times the mass of Jupiter, which is clearly below the sub-stellar / stellar mass limit. The object became a reference in subsequent young brown dwarf related works.

In theory, a brown dwarf below 65 Jupiter masses is unable to burn lithium by thermonuclear fusion at any time during its evolution. This fact is one of the lithium test principles to examine sub-stellar nature in low luminosity and low surface temperature astronomical bodies.

High quality spectral data acquired by the Keck 1 telescope in November 1995 showed that Teide 1 had kept the initial lithium amount of the original molecular cloud from which Pleiades stars formed, proving the lack of thermonuclear fusion in its core. These observations confirmed the brown dwarf nature of Teide 1 as well as the efficiency of the spectroscopic lithium test.

Teide 1 was considered for some time the smallest object out of our Solar System that had been identified by direct observation. Since then over a thousand brown dwarfs have been identified, even very close to Earth like Epsilon Indi Ba and Bb, a pair of brown dwarfs gravitationally bound to a sunlike star, around 12 light-years from the Sun.