Brown dwarf

A brown dwarf is a celestial body that occupies a special position between planets and stars with a mass of 13-75 Jupiter masses. The same applies to the processes taking place in the interior of processes which do not have hydrogen fusion. Brown dwarfs are more massive than the gas giant planetary and less massive than stellar red dwarfs.

• 5.1 Detection Methods 5.1.1 Lithium Test

• 5.2.1 Clusters
• 5.2.2 Star formation regions
• 5.2.3 binaries
• 5.2.4 Isolated brown dwarfs

Demarcation

As brown dwarfs, all objects are classified, which are below the mass limit for hydrogen fusion and above the mass limit for the deuterium fusion (about 13 Jupiter masses). The hydrogen fusion is the process of characterizing a star. It prevents, at least for a part of the lifetime rating of the gravitational force, thereby stabilizing the star. The minimum temperature for hydrogen fusion is achieved at one of our sun-like composition with a mass of about 0.07 solar and 75 Jupiter masses (about 0.139 × 1030 kg ). From this minimum mass upwards creates a star. The upper mass limit for a brown dwarf is, however, of the metallicity dependent and is for a metallicity of 0, ie when objects from the early phase of the universe, at about 90 Jupiter masses.

In brown dwarfs nevertheless fusion processes take place, as there are some fusion reactions that occur at lower temperatures than hydrogen fusion. These are substantially the lithium fusion, in which a lithium -7- core with a proton, reacts from about 65 Jupiter mass or core temperatures in excess of 2 million degrees Kelvin, and the deuterium fusion, in which from about 13 Jupiter masses deuterium nucleus and a proton to form a helium - 3- core merge.

The mass lower limit in this definition at about 13 Jupiter masses, the mass limit for the deuterium fusion. Objects with a lower mass planet is called when they are companions of stars. Objects that are not part of a planetary system, but move freely around the center of the Milky Way, are objects of planetary mass called because nothing is known about the origin of these objects. Many exoplanets have not only large masses, which could in some cases be in the range of brown dwarfs, at high eccentricities and small distances from the central star orbital parameters, one would rather expect from a stellar companion as planets. In fact, at least one exoplanet is also classified as a candidate for a brown dwarf. The objects under 13 Jupiter masses, however, no consistent naming is foreseeable.

In the first studies of brown dwarfs, the emergence criteria were used: one called all objects brown dwarfs, which arise as the stars by contraction of a gas cloud ( H II region molecular cloud ), in which, however, does not use hydrogen fusion - in contrast to the rock - and gas planets that arise in the accretion disks of stars. This definition is very problematic, mainly because the legislative history of the lighter objects, if any, can be resolved only at very high cost. The fusion criterion is not yet widely used, but it is the beginning of the 21st century far more frequently used than the emergence criterion, which is applied only on some older pioneers of this field.

Formation

The formation process of brown dwarfs is not yet clearly understood, but essentially there are six options:

• They are using the same mechanisms of a gas cloud (see molecular cloud ) is formed as the stars, with the only difference that the mass of the resulting body is not enough 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 necessary mass to ignite the hydrogen fusion.
• They are created like the planets in a protoplanetary disk, and are thrown out in a later stage of development of the planetary system.
• In young massive star clusters, the ionizing radiation of massive O and B stars destroy the protostellar accretion disks before these objects could accumulate sufficient mass for hydrogen fusion.
• Close encounters with other stars in a young star clusters can destroy the accretion disk before the hydrogen fusion limit is reached.
• In close binary systems, a white dwarf from a red dwarf accrete mass and thus remove from the red dwarf so much mass that it mutates into a dwarf eyebrows. This process takes place in many cataclysmic variables that evolve over a period of several hundred million years to a binary system consisting of a white dwarf and a brow.

In the Chamaeleon I star-forming region, which is only a few million years old, 34 brown dwarfs have been found; in a three accretion disk could also be detected, which is typical for young stars. Evidence of a T Tauri phase at several brown dwarfs, which was previously known on its way to the main sequence only in young stars, is further evidence of the same genesis of at least some of the brown dwarfs.

Properties

Brown dwarfs have a similar elemental composition like stars. In accretion disks resulting brown dwarfs could have a rocky core, which for this formation mechanism but so far there is no evidence.

For very light dwarf star is at the core independent of the mass an equilibrium temperature of about 3 million Kelvin, in the use of hydrogen fusion processes by leaps and bounds. The constancy of the temperature is approximately proportionality between mass and radius, that is, the lower the weight, the higher the density in the core. With increasing density, the core electrons exert an additional counter-pressure against the gravitational contraction, which is caused by a partial degeneration of the electrons due to the Pauli exclusion principle, and resulting in lower heating of the core. This leads to a metallicity similar to the Sun in less than 75 Jupiter masses means that the temperatures required for hydrogen fusion no longer be achieved and a brown dwarf is formed. Since neither the course of the degeneracy of the electrons nor the properties of the lightest stars are understood in all aspects, vary older literature values ​​70-78, newer 72-75 Jupiter masses.

Although the fusion processes in young brown dwarfs provide a contribution to the energy balance, but they are comparable in any development phase with the contribution of the gravitational energy. This leads to the fact that brown dwarfs begin to cool towards the end of the accretion phase, the fusion processes slow down this process for about 10 to 50 million years.

Temperature transport

For brown dwarfs and stars with less than 0.3 solar masses no shell structure is formed, as in heavier stars. They are fully convection, that is, it is a matter transport of the core rather than to the surface, which leads to a complete mixing and determining the temperature distribution throughout the interior. Studies of methane dwarfs such as Gliese 229 B, however, suggest that in older, cooler brown dwarfs these convection no longer extends to the surface and instead possibly forming a gas giant like atmosphere.

Size

The degeneracy of the electrons leads to a mass dependence of the radius of brown dwarfs by R ~ M -1/ 3 Only below the mass limit of the brown dwarfs lose the degeneracy in importance and it turns at a constant density of a mass dependence of R ~ M 1 / 3. The weak dependence of the reciprocal mass brown dwarfs leads to a over the entire mass range of approximately constant radius, which roughly corresponds to the Jupiter radius, the lighter brown dwarfs are larger than the Heavier.

Spectral

The spectral classes that are defined for stars, in the narrower sense does not apply to brown dwarfs, since it is not to star. At surface temperatures exceeding 1800 to 2000 K, however, they fall in the observation area in the L and M stars, since the optical properties depend only on the temperature and the composition. One applies the spectral therefore also on brown dwarfs, whereby these, however, does not provide direct information about the mass, but only through the combination of mass and age.

A serious young brown dwarf starts in the mid- M area at about 2900 K and passes through all the later M- and L- types, lighter brown dwarfs start already at a later type. The lower end of the main sequence is not exactly known, but it is probably between L2 and L4, ie at temperatures below 1800-2000 K. For later, cooler types are sure to brown dwarfs.

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

Before 2011 was the coolest known brown dwarf, 2MASS J04151954 - 0935066th He points at a temperature of 600-750 K as T9 dwarf already deviations from the other T- Dwarfs. Before 2MASS J0415 - 0935 Gliese 570D was around 800 K as coolest known brown dwarf.

2011 then the spectral class Y has been introduced for extremely cold brown dwarfs. Since they only have surface temperatures of 25-170 ° C, they emit no visible light but infrared radiation and are very difficult to observe. They have therefore been a long time predicted theoretically before 2011, the first observation was achieved by the Wise Observatory. One of the Y dwarfs, WISE 1828 2650, has according to the measurements of the satellite has a surface temperature of 27 ° C.

Variability

The low temperatures in the atmospheres of brown dwarfs, it is expected that there will be cloud formations with a spectral type of late L to T. In combination with the rotation of the brown dwarfs a variable luminosity in the near infrared should be detectable as with Jupiter, and the rotation period is likely to be in the order of hours. 2MASS J21392676 0220226, in the case of a spectral type T1, 5 a period of 7.72 hours was detected over several nights. The variability of the amplitude from cycle to cycle supports the interpretation that it is a consequence of a high-contrast cloud formation in the atmosphere of brown dwarfs. In addition, brown dwarfs also show variations in the intensity of their radio emission. From 2MASS J10475385 2124234 with a spectral type of T6.5 flares have been observed in combination with a very low background intensity. The cause of these phenomena, a magnetic activity is assumed, but can not be excited by an alpha - omega- dynamo, since the fully convective brown dwarfs lack the necessary tachocline region.

Rotation periods

During the period of rotation of red dwarfs probably due to magnetic activity with age is longer, this correlation is not observed in brown dwarfs. All brown dwarfs with an age of more than 10 million to a few billion years rotation periods of less than one day and meet in that capacity rather than the gas planets the stars.

Frequency

There is a simple mass function to describe the relative number of star-like objects with respect to their mass, the original mass function. This mass function is expected to continue unchanged in the range of the heavier brown dwarfs, since at least the early stages of the star formation process is independent of the type of the resulting object with the collapse of a gas cloud, ie, the cloud can not "know" whether at the end of a star or a brown dwarf is formed. This mass function is, however, show in the lighter brown dwarfs deviations, on the one hand and the other development processes (see section origin ) could make a contribution, and on the other hand not much is known about the minimum masses of the objects that can occur in a star formation processes. An accurate determination of the frequency and the mass function of brown dwarfs is therefore not only for the formation processes of the brown dwarfs important, but also contributes to our understanding of star formation processes in general at.

Since the discovery of Gliese 229B several hundred brown dwarfs have been found, especially in the star surveys 2MASS (2 Micron All Sky Survey ), DENIS ( DEep Near Infrared Sky survey ) and SDSS ( Sloan Digital Sky Survey ) and during intensive surveys of open star clusters and star-forming regions.

Detection methods

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

Lithium test

For masses greater than 65 Jupiter masses lithium - 7 is converted into helium -4. Through this process, the lithium supply is exhausted after about 50 million years ago with some very light stars, brown dwarfs at this time extended to up to 250 million years. Since light stars are just like brown dwarfs fully convective, the lithium abundance decreases in contrast to the heavier stars such as the sun not only in the fusion region of the core, but can be directly observed on the surface. However, the lithium detection alone does not provide a clear result, on the one lithium is detectable even in very young stars, on the other hand no more lithium is detectable in older brown dwarfs with masses greater than 65 Jupiter masses.

Can we, however, in a star-like object with a temperature less than 2800 K distinct lithium -7 lines demonstrate it is clearly a brown dwarf. The lines of neutral lithium are also in the red spectral region and are therefore to investigate very well with terrestrial telescopes. Through the good detectability of this method has become the standard for the detection of brown dwarfs.

Distribution

Star clusters

There are already many brown dwarfs were detected in young star clusters such as the Pleiades, but it is not yet a bunch been completely searched. In addition, many other candidates are known in these areas, whose affiliations are not yet clear to brown dwarfs or the cluster itself. Initial analyzes are within the error estimate of the stellar mass function compatible, but there are some significant deviations. It is still too early to conclude unambiguously on a modified mass function used in the brown dwarfs.

Star-forming regions

The detection of brown dwarfs is difficult, as in star-forming regions, since they differ only slightly from mild stars due to their low age and high temperature involved. Another problem in these regions is the high dust content, the more difficult by high extinction rates that observation. The applied methods are strongly model-dependent, so very few candidates are only unequivocally confirmed as brown dwarfs. The previously derived mass functions differ for the most part very much on the stellar mass function from, but are still subject to considerable errors.

Double systems

For systems with brown dwarfs offers after the first results of the star surveys as follows:

• For complete surveys of the F- to - M0 stars in the solar neighborhood just a few brown dwarfs were at a distance of less than three astronomical units (AU ) found in close binary systems, while these distances occur in 13 percent of double-star systems; this observation is usually described in the literature as the Brown Dwarf Desert. For very wide intervals over 1000 AE, however, no difference between stellar companions and brown dwarfs seems to be, this conclusion is however based on an extrapolation of very few observations and is therefore still very uncertain.
• About 20 percent of the L dwarfs, where they are probably in large part to brown dwarfs, have another brown dwarf as a companion, but no binaries were at a distance of more than 20 AU found.

Although the numerical values ​​of the results are still very uncertain, a fundamental difference between the two systems F-M0-Stern/Brauner dwarf and dwarf L-Zwerg/Brauner is considered safe. The reasons are probably in the development process of the brown dwarfs, especially the followers of the " outcast star embryos ," that is, the formation in a multi- system and the catapult in an early development phase, 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 suggest that the stellar mass function very well continue in the field of brown dwarfs. The development process of the brown dwarfs, so to be related with the exception of very light, seems very closely with the star formation processes probably explain why the population of brown dwarfs.

Age determination of young star clusters

The lithium test applied to star clusters as a " side effect" of a mass limit can be detected up to the lithium and the lithium depletion boundary is called. With this mass, one can determine the age of the cluster. This method will only work if the pile is younger than about 250 million years since this mass limit is otherwise constant at 65 Jupiter masses.

In this way one has corrected 1999, the age of the Pleiades by more than 50 percent to about 125 million years after the top. Similar corrections were made in the wake of more clusters, among other things, for α Persei and IC 2391st Although brown dwarfs will be difficult to be detected at greater distances and the lithium test is applicable only in very young clusters to determine the age, this method still allows a very good calibration of other dating methods.

History

Shiv Kumar introduced first time in 1963 thinking that the formation process of stars and objects could arise that do not reach the required temperature for hydrogen fusion due to their low mass, brown dwarf name, however, was proposed in 1975 by Jill Tarter. The name is in the literal sense does not work properly because brown dwarfs appear red, but the term Red Dwarf was already assigned to the lightest stars.

In the 1980s, several attempts have been made ​​to find these hypothetical body, but only in 1995 was detected with Gliese 229 B of the first brown dwarf doubt. Crucial for this development were significant advances in the sensitivity of the telescopes, on the other, the theoretical models have been improved, which allowed a better distinction to faint stars. Within a few years several hundred brown dwarfs were detected, the number of other possible candidates is also of this magnitude. The sun next brown dwarfs ( early 2004 ) form the Epsilon Indi -B twin system in 11.8 light years away.

The study of brown dwarfs is still in its infancy, but has comparable to the opening of new observation window or the discovery of other new effects, already contributed much to our knowledge and understanding of the universe.

Swell