Brown dwarf

Brown dwarfs are celestial bodies that occupy a special position between stars and planets .

At less than 75 Jupiter's masses, their masses are  not sufficient to start a hydrogen fusion in their interior , as in the lightest stars, the red dwarfs . On the other hand, with at least 13 masses of Jupiter (i.e. more massive than planetary gas giants ), they are heavy enough to start the deuterium fusion .

Demarcation

Gliese 229 B : left: discovery at the Palomar observatory
right: Hubble Space Telescope ( NASA ) (center of each picture)

All objects below the mass limit for hydrogen fusion and above the mass limit for deuterium fusion are classified as brown dwarfs:

• Hydrogen fusion is the characterizing process for a star. It counteracts gravity for at least part of the life of the star and thus stabilizes it. The minimum temperature for hydrogen fusion is reached - with a composition similar to our sun - at a mass of about 0.07 times that of the sun - or 75 times the mass of Jupiter (approx. 1.39 · 10 29  kg). From this minimum mass upwards, a star is created. However, the upper mass limit for a brown dwarf depends on its metallicity : for a metallicity of 0, i.e. H. for objects from the initial phase of the universe , it is around 90 times the mass of Jupiter.
• In brown dwarfs, however, some fusion processes take place that take place at lower temperatures than hydrogen fusion. These are essentially
• the lithium fusion , from about the 65 times the mass of Jupiter, or at core temperatures in excess of 2 million in Kelvin a lithium -7- core with a proton reacts, and
• the deuterium fusion , in which from about 13 times the mass of Jupiter a deuterium nucleus and a proton fuse to form a helium 3 nucleus.

Objects with less than 13 Jupiter masses are called

Many known exoplanets have - in addition to large masses, some of which could even be in the range of the brown dwarfs - with high eccentricities and small distances from the central star, orbital parameters that one would expect from a stellar companion rather than from planets; in fact, at least one exoplanet is also classified as a candidate for a brown dwarf. In the case of objects under 13 times the mass of Jupiter, however, no uniform naming is foreseeable.

In the first studies of brown dwarfs, the creation criterion was used: all objects were called brown dwarfs, which, like stars, are formed by the contraction of a gas cloud ( H-II region , molecular cloud ), but in which no hydrogen fusion begins - in contrast to rocks - and gas planets that are formed in the accretion disks of the stars. However, this definition is very problematic, since the history of the origins of the lighter objects can only be clarified with great effort, if at all. The merger criterion is not yet generally used, but it is used much more frequently at the beginning of the 21st century than the emergence criterion, which is only used by a few older pioneers in this field of research.

Emergence

The development process of the brown dwarfs has not yet been clearly clarified, but there are essentially six possibilities:

• They are formed from a gas cloud (see molecular cloud ) using the same mechanisms as the stars, with the only difference that the mass of the resulting body is not sufficient for hydrogen fusion.
• They begin their development as part of a multiple system in a globule . However, they are thrown out of the system before they reach the mass necessary to ignite the hydrogen fusion.
• Similar to planets, they arise in a protoplanetary disk and are ejected from the planetary system at a later stage of development.
• In young, massive star clusters , the ionizing radiation from massive O and B stars can destroy the protostellar accretion disks before these objects can accumulate sufficient mass for hydrogen fusion.
• Close encounters with other stars in a young star cluster can destroy the accretion disk before the hydrogen fusion limit is reached.
• In tight binary systems one can white dwarf from a red dwarf mass accrete and from the red dwarf so much mass remove that this mutated into a brown dwarf. This process takes place in many cataclysmic variables , which develop into a binary star system consisting of a white and a brown dwarf over a period of several hundred million years.

In the star- forming region Chamaeleon  I , which is only a few million years old, 34 brown dwarfs were found; in three of them an accretion disk could also be detected, which is typical for young stars.

The evidence of a T-Tauri phase in several brown dwarfs, which was previously only known in young stars on their way to the main sequence , is evidence that at least some of the brown dwarfs have the same history of formation as stars.

properties

Young brown dwarfs can hardly be distinguished from the stars near them when observed: The approximately 12 million year old brown dwarf TWA 5B (above) on an X-ray ( Chandra , NASA)

Brown dwarfs have a comparable element composition as stars. Brown dwarfs formed in accretion disks could have a rock core, although no evidence exists for this path of origin.

For very light dwarf stars, an equilibrium temperature of around 3 million Kelvin is established in the core, regardless of the mass , at which the hydrogen fusion processes begin by leaps and bounds. The constancy of the temperature means approximately proportionality between mass and radius, i. that is, the lower the mass, the higher the density in the core. With increasing nuclear density, the electrons exert an additional pressure against the gravitational contraction, which is caused by a partial degeneration of the electrons due to the Pauli principle and leads to less heating of the nucleus. With a metallicity similar to the sun at less than 75 times the mass of Jupiter, this means that the temperatures required for hydrogen fusion are no longer reached and a brown dwarf is created. (Since neither the course of electron degeneracy nor the properties of the lightest stars are fully understood, older literature values ​​vary between 70 and 78 times the mass of Jupiter, and newer values ​​between 72 and 75 times.)

Although the fusion processes make a contribution to the energy balance of young brown dwarfs, they cannot be compared with the contribution of gravitational energy in any development phase. As a result, brown dwarfs already begin to cool down towards the end of the accretion phase; the fusion processes only slow this process down for about 10 to 50 million years.

Heat transfer

With brown dwarfs and stars with less than 0.3 times the solar mass, no shell structure forms as with heavier stars. They are completely convective , which means that matter is transported from the core to the surface, which leads to complete mixing and determines the temperature distribution throughout the interior.

Investigations of the methane dwarfs such. B. Gliese 229 B , however, suggest that in older, cooler brown dwarfs, this convection zone no longer extends to the surface and that an atmosphere similar to the gas giants may develop instead .

size

Size comparison of the sun, brown dwarf, Jupiter and earth (from left, NASA)
Size and temperature comparison of planets, brown dwarfs and stars. Estimated relative sizes of Jupiter and the brown dwarfs WISE1828, Gliese 229B and Teide 1 compared to the Sun and a red dwarf. (Source: MPIA / V. Joergens)

In brown dwarfs, the degeneracy of the electrons leads to a mass dependence of the radius of . This weak reciprocal mass dependence causes an approximately constant radius over the entire mass range, which roughly corresponds to the radius of Jupiter ; the lighter brown dwarfs are larger than the heavier ones . ${\ displaystyle R \ propto {\ frac {1} {\ sqrt [{3}] {M}}}}$

Only below the mass limit of the brown dwarfs does the degeneration lose its importance, and at constant density a mass dependence of occurs . ${\ displaystyle R \ propto {\ sqrt [{3}] {M}}}$

Spectral classes

The spectral classes defined for stars are not strictly applicable to brown dwarfs, since they are not stars. At surface temperatures above 1800 to 2000 K, however, they fall into the range of the L and M stars when observed, since the optical properties only depend on the temperature and the composition. The spectral classes are therefore also applied to brown dwarfs, although these do not provide any direct information about the mass, but only about the combination of mass and age.

A heavy young brown dwarf starts in the middle M range at around 2900 K and goes through all later M and L types, lighter brown dwarfs start with a later type. The lower end of the main sequence is not exactly known, but it is probably between L2 and L4, i.e. H. at temperatures below 1800 to 2000 K. Later, cooler types are definitely brown dwarfs.

For the cooler brown dwarfs such as B. Gliese 229B with a temperature of about 950 K, another spectral class was introduced with the T-type , which is no longer applicable to stars with temperatures below about 1450 K. Since the spectrum in this temperature range is mainly characterized by strong methane lines, T-type brown dwarfs are mostly called methane dwarfs .

Until 2011, 2MASS J04151954-0935066 was considered the coolest known brown dwarf. As a T9 dwarf, it already shows deviations from the other T dwarfs at a temperature of 600 to 750 K. Before 2MASS J0415-0935, Gliese 570D was considered the coolest known brown dwarf at around 800 K.

In 2011, the spectral class Y was introduced for extremely cold brown dwarfs. Since they only have surface temperatures of 25 to 170 ° C, they do not emit visible light , only infrared radiation and are very difficult to observe. They were therefore only predicted theoretically for a long time before the first observation by the Wise Observatory was possible in 2011 . One of these Y-dwarfs, WISE 1828 + 2650, has a surface temperature of 27 ° C according to the measurements of the satellite. The WISE 0855-0714 found in 2014 even has a surface temperature of no more than −13 ° C, although due to its low mass (3 to 10 Jupiter's masses) it is unclear whether it should be classified as a brown dwarf or as an object of planetary mass .

Rotation periods

All brown dwarfs with an age of more than 10 million to several billion years have periods of rotation of less than one day and in this capacity correspond more closely to gas planets than to stars.

While the period of rotation of red dwarfs increases with age, probably due to magnetic activity , this association is not observed in brown dwarfs .

variability

The low temperatures in the atmospheres of brown dwarfs with a spectral type from late L to T suggest that cloud formations will occur. In combination with the rotation of the brown dwarfs, a variable luminosity in the near infrared, as in Jupiter, should be detectable, whereby the rotation time should be in the order of hours. In the case of 2MASS J21392676 + 0220226 with a spectral type T1.5, a period of 7.72 hours over several nights could be detected. The variability of the amplitude from cycle to cycle supports the interpretation that it is a result of high-contrast cloud formation in the atmosphere of brown dwarfs.

In addition, brown dwarfs also show fluctuations in the intensity of their radio radiation . Flares have been observed from 2MASS J10475385 + 2124234 with a spectral type of T6.5 in combination with a very low basic intensity. Magnetic activity is assumed to be the cause of these phenomena, but this cannot be stimulated by an alpha-omega dynamo , since the completely convective brown dwarfs lack the necessary tachocline region .

frequency

There is a simple mass function to describe the relative number of star-like objects with regard to their mass, the original mass function . This mass function should continue unchanged into the area of ​​the heavier brown dwarfs, since at least the initial phase of the star formation process with the collapse of a gas cloud is independent of the type of object being formed; d. In other words, the cloud cannot “know” whether a star or a brown dwarf will emerge in the end.

However, this mass function will show deviations in the area of ​​the lighter brown dwarfs, because on the one hand the other formation processes could also make a contribution ( see section Formation ), and on the other hand not much is known about the minimum masses of the objects that can be formed in star formation processes.

A precise determination of the frequency or the mass function of the brown dwarfs is therefore not only important for the formation processes of the brown dwarfs, but also contributes to an understanding of the star formation processes in general.

Several hundred brown dwarfs have been found since the discovery of Gliese 229B, mainly during the star surveys 2MASS ( 2 Micron All Sky Survey ), DENIS ( DEep Near Infrared Sky survey ) and SDSS ( Sloan Digital Sky Survey ) as well as during intensive surveys of open areas Star clusters and star formation regions.

The Citizen Science project Backyard Worlds: Planet 9 of NASA, which started in February 2017 to evaluate images from the Space Telescope Wide-Field Infrared Survey Explorer (WISE), revealed the discovery of 95 brown dwarfs within a radius of 65 light years as of August 2020. This points to the existence of up to 100 billion brown dwarfs in the Milky Way.

Detection methods

Brown dwarfs have a very low luminosity and are therefore difficult to observe, and in the early stages of development they are easy to confuse with red dwarfs . There are several possibilities for the clear detection of brown dwarfs:

Luminosity
In brown dwarfs, fusion processes only play a subordinate role in the release of energy, the luminosity of these objects therefore decreases in the course of their development. If the measured luminosity is below that of the lightest stars, which corresponds to 10 −4 times the luminosity of the sun, then it can only be a brown dwarf.
However, the luminosity can only be used as a criterion if the distance is known, e.g. B. in star clusters. This method was used in the first attempts to detect brown dwarfs in the 1980s and has proven to be very unreliable, with most of the candidates found an incorrect determination of the distance could later be detected.
temperature
The luminosity L can be assigned an effective surface temperature  T eff using the Stefan-Boltzmann law , but this changes significantly less than the luminosity; however, the temperature can very easily be determined from the spectrum of the object. If the measured temperature is significantly lower than the minimum temperature of around 1800 K for stars, then it can only be brown dwarfs.${\ displaystyle T_ {eff} \ propto {\ sqrt [{4}] {L}}}$
Dimensions
In the case of double systems with a brown dwarf, the mass can be determined by measuring the movement around the common center of gravity , even if the brown dwarf itself cannot be observed, a situation similar to that of exoplanets . The direct determination of the mass is the only way to detect young brown dwarfs at the upper mass limit.
methane
More complex molecules , especially methane, can form in the atmosphere of brown dwarfs . Since this is not possible in stellar atmospheres, the detection of methane in the spectra clearly indicates a brown dwarf. It is then an old and cool brown dwarf of the T-type.
lithium
The detection of neutral lithium in the spectrum offers a very good possibility to identify brown dwarfs and can be used in a very wide range. The lithium test was proposed by Rafael Rebolo in 1992 and first used by Gibor Basri in 1996. At masses more than 65 times the mass of Jupiter, lithium -7 is converted into helium -4. As a result of this process, the lithium reserves of very light stars are used up after around 50 million years; in brown dwarfs, this period is extended to up to 250 million years. Since light stars, like brown dwarfs, are completely convective, the lithium abundance increases in contrast to heavier stars such as B. B. the sun not only in the fusion area of ​​the core, but can be observed directly on the surface.
The detection of lithium alone does not provide a clear result.On the one hand, lithium can also be detected in very young stars, and on the other hand, lithium is no longer detectable in older brown dwarfs with masses more than 65 times the mass of Jupiter.
However, if one can detect pronounced lithium-7 lines in a star-like object with a temperature of less than 2800 K, then it is clearly a brown dwarf. The lines of the neutral lithium are also in the red spectral range and can therefore be examined very well with terrestrial telescopes . Due to its good traceability, this method has established itself as the standard for the detection of brown dwarfs.

distribution

Star clusters

Many brown dwarfs were already found in young star clusters such as B. detected the Pleiades , but so far no pile has been completely searched. In addition, many other candidates are known in these areas, whose membership of the brown dwarfs or the star cluster itself has not yet been clarified. Initial analyzes can be reconciled with the stellar mass function within the scope of the error estimation, but there are sometimes strong deviations. It is still too early to clearly conclude that there is a change in mass function in the area of ​​the brown dwarfs.

Star Formation Areas

The detection of brown dwarfs is very difficult in star formation regions, as they differ very little from light stars due to their small age and the associated high temperature. Another problem in these regions is the high proportion of dust , which makes observation difficult due to high extinction rates. The methods used here are strongly model-dependent, which is why only very few candidates are unequivocally confirmed as brown dwarfs. The mass functions derived so far deviate largely from the stellar mass function, but are still subject to high errors.

Double systems

For systems with brown dwarfs, the first results of the star surveys show the following picture:

• With complete surveys of the F to M0 stars in the solar environment, only a few brown dwarfs were found in close double systems with a distance of less than three astronomical units  (AU) from one another, while these distances occur in 13 percent of all binary star systems; this observation is mostly described in the literature as the Brown Dwarf Desert . At very large distances of more than 1000 AU there does not seem to be any difference between stellar companions and brown dwarfs, but this conclusion is based on an extrapolation of very few observations and is therefore still very uncertain.
• About 20 percent of the L-dwarfs, most of which are probably brown dwarfs, have another brown dwarf as a companion, but no double systems with a distance of more than 20 AU were found.

Although the numerical values ​​of the results are still very uncertain, a fundamental difference between the two systems F-M0 star / brown dwarf and L-dwarf / brown dwarf is considered certain. The causes are presumably to be found in the formation process of the brown dwarfs, especially the followers of the "cast out star embryos", i. H. the emergence in a multiple system and the catapulting out in an early phase of development, consider these distributions as a natural consequence of this theory.

Isolated brown dwarfs

The 2MASS and DENIS surveys have already found hundreds of brown dwarfs, although the surveys are not yet complete. Initial analyzes indicate that the stellar mass function extends very far into the range of the brown dwarfs. The formation process of the brown dwarfs, with the exception of the very light ones, seems to be very closely related to the star formation processes, which therefore presumably also explain the population of the brown dwarfs.

Age determination of young star clusters

For star clusters, the lithium test provides a "side effect" of a mass limit up to which lithium can be detected and which is called the lithium depletion boundary . With this mass one can determine the age of the heap. However, this method only works if the cluster is younger than about 250 million years, otherwise the mass limit is constantly 65 times the mass of Jupiter.

In this way, the age of the Pleiades was corrected upwards by more than 50 percent in 1999 to around 125 million years. Similar corrections were then made for other star clusters, among others. a. for α Persei and IC 2391 . Although brown dwarfs will be difficult to detect at greater distances and the lithium test can only be used to determine the age of very young clusters, this method still enables other dating methods to be calibrated very well .

history

In 1963, Shiv Kumar first considered that the process of star formation could also create objects that, due to their low mass, would not reach the temperature required for hydrogen fusion, but the name brown dwarf was onlyproposedby Jill Tarter in 1975. The name is actually incorrect, since brown dwarfs also appear red, but the term red dwarf was already given for the lightest stars.

Various attempts were made in the 1980s to find these hypothetical bodies, but it was not until 1995 that  the first brown dwarf, Gliese 229 B, was proven beyond doubt. Decisive for this were, on the one hand, significant advances in the sensitivity of the telescopes , and on the other hand, the theoretical models were also improved, which made it possible to better distinguish between weakly shining stars. Several hundred brown dwarfs were detected within a few years, and the number of other possible candidates is also in this range.

The two brown dwarfs closest to the Sun form the double system Luhman 16 at a distance of 6.6  light years (as of 2017).

The investigation of the brown dwarfs is still in its infancy, but has already contributed a great deal to our knowledge and understanding of the universe, comparable to the opening of new observation windows or the discovery of other new effects.

literature

• Ben R. Oppenheimer, SR Kulkarni, John R. Stauffer: Brown Dwarfs. In: Protostars and Planets. Volume 4. University of Arizona Press, Tucson 1999, Academic Press, San Diego Cal 2000 (good and very extensive overview of the state of knowledge from 1998, arxiv : astro-ph / 9812091 ).
• Shiv S. Kumar: The Bottom of the Main Sequence and Beyond. Speculations, Calculations, Observations, and Discoveries (1958-2002). In: ASP Conference Series. Volume 30. Astronomical Society of the Pacific, San Francisco 2002, (Full account of scientific acceptance in the 1960s, arxiv : astro-ph / 0208096 ).
• Gilles Chabrier: The Physics of Brown Dwarfs. In: Journal of physics. Condensed matter. Volume 10, 1998, , p 11263 (PDF, physical theory of brown dwarfs, very formulaic heavy, arxiv : astro-ph / 9902015 ).
• Bo Reipurth, Cathie Clarke: The Formation of Brown Dwarfs as Ejected Stellar Embryos. In: The Astronomical Journal. 2001, , pp. 432-439 (basics and discussion of this development model, arxiv : astro-ph / 0103019 ).
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Commons : Brown dwarf  album with pictures, videos and audio files

Individual evidence

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 This version was added to the list of articles worth reading on September 13, 2009 .