Brown dwarf


Brown dwarfs are substellar objects that have more mass than the biggest gas giant planets, but less than the least massive main-sequence stars. Their mass is approximately 13 to 80 times that of Jupiter not big enough to sustain nuclear fusion of hydrogen into helium in their cores, but massive enough to emit some light and heat from the fusion of deuterium, 2H, an isotope of hydrogen with a neutron as well as a proton, that can undergo fusion at lower temperatures. The most massive ones can fuse lithium.
Astronomers classify self-luminous objects by spectral type, a distinction intimately tied to the surface temperature, and brown dwarfs occupy types M , L , T , and Y . As brown dwarfs do not undergo stable hydrogen fusion, they cool down over time, progressively passing through later spectral types as they age.
The "brown" in brown dwarf was meant to name a color between red and black. To the naked eye, most brown dwarfs would appear to be magenta or purple with others in different colors depending on their temperature. Brown dwarfs may be fully convective, with no layers or chemical differentiation by depth.
Though their existence was initially theorized in the 1960s, it was not until 1994 that the first unambiguous brown dwarfs were discovered. As brown dwarfs have relatively low surface temperatures, they are not very bright at visible wavelengths, emitting most of their light in the infrared. However, with the advent of more capable infrared detecting devices, thousands of brown dwarfs have been identified. The nearest known brown dwarfs are located in the Luhman 16 system, a binary of L- and T-type brown dwarfs about from the Sun. Luhman 16 is the third closest system to the Sun after Alpha Centauri and Barnard's Star.

History

Early theorizing

In the 1960s Shiv Kumar theorized the existence of objects now called brown dwarfs; they were originally called black dwarfs,
a classification for dark substellar objects floating freely in space that were not massive enough to sustain hydrogen fusion. However:
Because of this, alternative names for these objects were proposed, including and substar. In 1975 Jill Tarter, as part of her PhD thesis at University of California at Berkeley was the first to suggest that the term to describe these objects should be brown dwarf, using brown as a color "somewhere between red and black", suggesting that the dwarfs appeared dim, dark, and dull, even though not exactly brown.
The term black dwarf continues to be used to refer to a white dwarf that has cooled to the point that it no longer emits significant amounts of light. However, the time required for even the lowest-mass white dwarf to cool to this temperature is calculated to be longer than the current age of the universe; hence such objects are expected to not yet exist.
Early theories concerning the nature of the lowest-mass stars and the hydrogen-burning limit suggested that a object with a mass less than 0.07 solar masses or a object less than would never go through normal stellar evolution and would become a completely degenerate star.
The resulting brown dwarf star is sometimes called a failed star.
The first self-consistent calculation of the hydrogen-burning minimum mass confirmed a value between 0.07–0.08 solar masses for population I objects.

Deuterium fusion

The discovery of deuterium burning down to and the impact of dust formation in the cool outer atmospheres of brown dwarfs in the late 1980s brought these theories into question. However, such objects were hard to find because they emit almost no visible light. Their strongest emissions are in the infrared 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 these objects. These methods included multi-color imaging surveys around field stars, imaging surveys for faint companions of main-sequence dwarfs and white dwarfs, surveys of young star clusters, and radial velocity monitoring for close companions.

Teide 1 and class M

For many years, efforts to discover brown dwarfs were fruitless.
The first confirmed class "M" brown dwarf was discovered by Spanish astrophysicists Rafael Rebolo, María Rosa Zapatero-Osorio, and Eduardo L. Martín in 1994. This object, found in the Pleiades open cluster, received the name Teide 1. Nature highlighted "Brown dwarfs discovered, official" on the front page of that issue.
Teide 1 was discovered in images collected by the IAC team on 6 January 1994 using the 80 cm telescope 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. 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 substellar evolution models at that moment, the team estimated for Teide 1 a mass of, which is below the stellar-mass limit. The object became a reference in subsequent young brown dwarf related works.
In theory, a brown dwarf below is unable to burn lithium by thermonuclear fusion at any time during its evolution. This fact is one of the lithium test principles used to judge the substellar nature of 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 still had the initial lithium abundance of the original molecular cloud from which Pleiades stars formed, proving the lack of thermonuclear fusion in its core. These observations fully confirmed that Teide 1 is a brown dwarf, as well as the efficiency of the spectroscopic lithium test.
For some time, Teide 1 was the smallest known object outside the Solar System that had been identified by direct observation. Since then, over 1,800 brown dwarfs have been identified, even some very close to Earth, like Epsilon Indi Ba and Bb, a pair of brown dwarfs gravitationally bound to a Sun-like star 12 light-years from the Sun, and Luhman 16, a binary system of brown dwarfs at 6.5 light-years from the Sun.

Gliese 229B and class T

The first class "T" brown dwarf was discovered in 1994 by Caltech astronomers Shrinivas Kulkarni, Tadashi Nakajima, Keith Matthews and Rebecca Oppenheimer, and Johns Hopkins scientists Samuel T. Durrance and David Golimowski. It was confirmed in 1995 as a substellar companion to Gliese 229. Gliese 229b is one of the first two instances of clear evidence for a brown dwarf, along with Teide 1. Confirmed in 1995, both were identified by the presence of the 670.8 nm lithium line. The latter was found to have a temperature and luminosity well below the stellar range.
Its near-infrared spectrum clearly exhibited a methane absorption band at 2 micrometres, a feature that had previously only been observed in the atmospheres of giant planets and that of Saturn's moon Titan. Methane absorption is not expected at any temperature of a main-sequence star. This discovery helped to establish yet another spectral class even cooler than L dwarfs, known as "T dwarfs", for which Gliese 229b is the prototype.

GD 165B and class L

In 1988 a faint companion to the white dwarf star GD 165 was found in an infrared search of white dwarfs. The spectrum of the companion GD 165B was very red and enigmatic, showing none of the features expected of a low-mass red dwarf. 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 in 1997, which discovered many objects with similar colors and spectral features.
Although 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, because observationally it is very difficult to distinguish between the two. Today, GD 165B is recognized as the prototype of a class of objects now called "L dwarfs".

Theory

Formation

Five different theories for the formation of brown dwarf stars have been proposed:
  • A massive collapsing core produces fragments of different mass; tidal shear prevent the low mass fragments from growing;
  • Low mass fragments in collapsing core are ejected before they can grow by accretion;
  • Low mass fragments in a collapsing core are stripped of surrounding gas by ionizing radiation from hot massive OB stars;
  • Fragmentation and subsequent ejection of material in the circumstellar disc of a large star;
  • Gravitational collapse of turbulent molecular gas clouds results in range a masses for the collapsing core; the smallest masses lead to low mass stars and brown dwarf stars.
The last mechanism, turbulent fragmentation, predicts the observations of brown dwarf stars in many more circumstances than the other mechanisms, which matches observations. The other mechanisms could also occur but clear evidence has not been found.

Evolution

If the initial mass of the protostar is less than about, normal hydrogen thermonuclear fusion reactions will not ignite in the core. Gravitational contraction does not heat the small protostar very effectively, and before the temperature in the core can increase enough to trigger fusion, the density reaches the point where electrons become closely packed enough to create quantum electron degeneracy pressure. According to the brown dwarf interior models, typical conditions in the core for density, temperature and pressure are expected to be the following:
This means that the protostar is not massive or dense enough ever to reach the conditions needed to sustain hydrogen fusion. The infalling matter is prevented, by electron degeneracy pressure, from reaching the densities and pressures needed.
Further gravitational contraction is prevented and the result is a brown dwarf that simply cools off by radiating away its internal thermal energy. Note that, in principle, it is possible for a brown dwarf to slowly accrete mass above the hydrogen burning limit without initiating hydrogen fusion. This could happen via mass transfer in a binary brown dwarf system.