# L dwarf

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An object with the **spectral type L** (also called **L-dwarf**) can be either a low-mass [star](/source/Star),[1] a [brown dwarf](/source/Brown_dwarf)[2] or a young [free-floating planetary-mass object](/source/Rogue_planet).[3] If a young [exoplanet](/source/Exoplanet) or planetary-mass companion is detected via direct imaging, it can also have an L spectral type, such as [Kappa Andromedae b](/source/Kappa_Andromedae_b).[4]

## Spectral characteristics

L dwarf properties by spectral type[5] Subclass Temp Luminosity M8 2300 K ~0.0007 L☉ L0 2100 K ~0.00035 L☉ L2 1950 K ~0.0001 L☉ L4 1800 K ~0.000075 L☉ L6 1700 K ~0.000065 L☉ L8 1500 K ~0.000045 L☉

Before [2MASS](/source/2MASS) there were only six known objects with a spectral type later than M9.5V. With the discovery of 20 new late-type objects it was necessary to define the L-type and [T-type](/source/T-dwarf) spectral types, and this was done in 1999. In these L-dwarfs the metallic [oxides](/source/Oxide) ([TiO](/source/Titanium(II)_oxide), [VO](/source/Vanadium(II)_oxide)), which are present in [late M-dwarfs](/source/Red_dwarfs), are replaced with metallic [hydrides](/source/Hydride) (e.g. [CrH](/source/Chromium(I)_hydride), [FeH](/source/Iron(I)_hydride)) and neutral [alkali metals](/source/Alkali_metal) (e.g. [K](/source/Potassium), [Rb](/source/Rubidium), [Cs](/source/Cesium)). The transition between L- and T-dwarfs is defined with the appearance of [methane](/source/Methane) (CH4) in the spectrum.[6] M-dwarfs show absorption due to [water vapor](/source/Water_vapor) (H2O) in their near-infrared spectrum. This absorption feature gets stronger with later L spectral type. The absorption due to [carbon monoxide](/source/Carbon_monoxide) (CO) does show little variation over spectral type.[7] CO is replaced by CH4 in T-dwarfs.[8] Initially it was estimated that the hottest L0-dwarfs have a temperature of around 2000 [K](/source/Kelvin) and the coldest L8-dwarfs have a temperature of about 1500 K.[6] Modern estimates range from 1100 K for L9, to a maximum of 2500 K for L0.[9][10]

L-dwarfs have a [red](/source/Red), [violet](/source/Violet_(color)), or [purple](/source/Purple) color due to absorption from the [sodium](/source/Sodium) D-line, which is centered at 5890 [Å](/source/Angstrom), overlapping with the color [green](/source/Green).[8] Later work described L-dwarfs as having a violet color.[11]

### Subdwarfs

Main article: [subdwarf](/source/Subdwarf)

Subdwarfs are objects with a low metallicity. These objects are usually old and their metallicity influences different absorption features. In particular, the [collision induced absorption](/source/Collision-induced_absorption_and_emission) of [hydrogen molecules](/source/Hydrogen) leads to a suppression of the [H-](/source/H_band_(infrared)) and [K-band](/source/K_band_(infrared)), which causes L-type subdwarfs to have blue near-infrared colors. [2MASS J0532+8246](/source/2MASS_J05325346%2B8246465) was the first L-type subdwarf discovered. The [prefix](/source/Prefix) sd, esd and usd indicate subdwarfs, extreme subdwarfs and ultra subdwarfs. Objects with an usd-prefix have the lowest metallicity.[12]

## Main-sequence stars

The [hydrogen burning](/source/Stellar_nucleosynthesis) minimum mass lies at 0.075 (78.5 ) for objects with a solar metallicity.[13] The table of ultracool fundamental parameters lists several objects with an infrared spectral type of L0 to L4 and a mass above 78.5 . One of the highest mass L-dwarfs in this list is [G 239-25](/source/G_239-25)B (L0) for which they find a mass of 88.9 ±0.59 .[9][10] The hydrogen burning-limit is dependent on metallicity and objects with a low metallicity can have a higher hydrogen burning limit. Another factor is that a lower metallicity causes the atmosphere to be more transparent. Therefore older objects have temperatures that are higher.[14] Old L-subdwarfs with an early L spectral type can be main-sequence stars.[15] The brown dwarf [SDSS J0104+1535](/source/SDSS_J0104%2B1535) (usdL1.5, 0.086 ± 0.0015 ) for example is just below the hydrogen burning limit of around 0.088 , for its metallicity of [Fe/H] = -2.4 ± 0.2.[14] The same team found that a third of known L-subdwarfs are substellar objects and two-thirds are low-mass stars.[1] [CWISE J1249+3621](/source/CWISE_J1249%2B3621) (sdL1, 0.082 ± 0.002 ) is for example a star, because the hydrogen burning limit is at around 0.080 for [M/H]=-1. This star is also a [hypervelocity star](/source/Hypervelocity_star).[15]

## Brown dwarfs

Main article: [brown dwarfs](/source/Brown_dwarfs)

Most L-dwarfs are brown dwarfs. Brown dwarfs are objects with a mass below 78.5 .[13] Objects with a mass below 14 are often referred to as planetary-mass objects,[16] but depending on their formation mechanism they are also called [planetary-mass brown dwarfs](/source/Sub-brown_dwarf).[17]

In the table of ultracool fundamental parameters there are currently 422 objects with an infrared spectral type of L and a mass range of 14-78.5 .[9][10] Additionally there are dozens of L-type brown dwarfs known that co-move with a star, [white dwarf](/source/White_dwarf) or brown dwarf.[2] The first L-type brown dwarf discovered was [GD 165B](/source/GD_165B), which orbits a white dwarf.[18] Its mass was later determined to be 62.58 ± 15.57 .[19]

## Planetary-mass objects and exoplanets

See also: [List of directly imaged exoplanets](/source/List_of_directly_imaged_exoplanets), [sub-brown dwarf](/source/Sub-brown_dwarf) and [Rogue planet](/source/Rogue_planet)

A planetary-mass object is commonly defined as an object with a mass below 14 . These objects can be free-floating[16] or co-move with a star or brown dwarf (e.g. [HD 106906 b](/source/HD_106906_b)).[20][21] If such an object orbits a star within about 100 AU, it is referred to as an exoplanet. Beyond 100 AU, it is referred to as a planetary-mass companion since theories predict that these objects form on their own and not from material of a [protoplanetary disk](/source/Protoplanetary_disk).[22] One exoplanet near this 100 AU boundary is [Delorme 1](/source/Delorme_1) (AB)b, which could have formed via fragmentation of the circumstellar disk and is therefore considered an exoplanet.[23] More close-in planets, such as the planets around [HR 8799](/source/HR_8799)[24] and [Kappa Andromedae b](/source/Kappa_Andromedae_b) also resemble L-dwarfs or have an L spectral type.[4]

These objects are usually identified by their young age. An object can for example be present in a young star cluster (e.g. [NGC 1333](/source/NGC_1333))[25] or a young association (see [List of nearby associations](/source/List_of_nearby_stellar_associations_and_moving_groups)). Researchers can use the temperature-age or luminosity-age relation to determine if its mass is below 13 .[16] For very young star clusters (<1 [Myr](/source/Myr)) even an L0 spectral type corresponds to a planetary-mass and therefore all L-dwarfs in such a star cluster have a planetary-mass.[25]

Another method is to determine other indicators of a young age. A lower-mass object has for example a lower [surface gravity](/source/Surface_gravity), which leads to a more extended atmosphere and more vertical mixing. This will affect the depth of certain spectral features and can lead to red near-infrared colors. A low-gravity L-dwarf is often denoted with the [suffix](/source/Suffix) β, γ and δ, indicating intermediate (β), low (γ) and very low (δ) gravity. Low-gravity L3-L5 dwarfs can also show [lithium](/source/Lithium) absorption. The so-called "lithium test" is less reliable to determine a low mass for young L-dwarfs.[26] An example for a low gravity object is [CWISE J0506+0738](/source/CWISE_J0506%2B0738), which has a spectral type between L8γ and T0γ and probably a mass of 7±2 .[16]

## Variability and clouds

[Iron](/source/Iron) clouds with [silicate](/source/Silicate) clouds on top of it were theorized since the early 2000s for L-dwarfs.[27] The presence of silicates in L-dwarfs is well established with [Spitzer](/source/Spitzer_Space_Telescope) observations. Especially L4-L6 dwarfs often show silicate absorption. But silicate absorption can also be absent for any L-dwarf.[28] Variability is often connected to the presence of clouds in L- and T-dwarfs. There are however other possible explanations, such as hot spots, temperature variations and [aurorae](/source/Aurora). Especially young objects show variability.[29] One of the most variable L-dwarf is the planetary-mass companion [VHS J1256–1257](/source/VHS_J1256%E2%80%931257)b (L7), with an amplitude of 33-38%.[30]

## Magnetic field and aurorae

[Radio emission](/source/Radio_astronomy) can be detected in L-dwarfs and this radio emission sometimes shows rotational periodic radio pulses. Additionally the [H-alpha](/source/Hydrogen-alpha) emission seen commonly in L-dwarf is interpreted as [chromospheric](/source/Chromosphere) and [coronal](/source/Stellar_corona) in early L-dwarfs, but with later spectral type it will become more and more an auroral feature. Therefore L4–T8 objects with H-alpha emission are often also radio sources. Radio pulses from brown dwarfs are highly circular polarized and likely come from [electron cyclotron maser instability](/source/Magnetosphere_of_Jupiter#Jupiter_at_radio_wavelengths) (ECMI), which is connected to aurorae. It is currently not known what powers auroral radio emission in brown dwarfs. One suggestion is the breakdown of the co-rotation with a plasma disk, which also powers the main aurora on [Jupiter](/source/Jupiter). The other suggested energy source is the interaction with a [rocky planet](/source/Terrestrial_planet) around the brown dwarf, similar to the interaction between [Io](/source/Io_(Moon)) and Jupiter. Impacts of [electrons](/source/Electron) with hydrogen molecules can create a [trihydrogen cation](/source/Trihydrogen_cation) (H+3). This could be detected in the infrared at 2 and 4 μm with [JWST](/source/James_Webb_Space_Telescope). Destruction of H+3 by gases, such as H2O and CH4, could mean that it is not built up significantly in brown dwarfs.[31] No H+3 was detected in any M, L or T dwarf with the [Keck Observatory](/source/W._M._Keck_Observatory), likely due to auroral electrons penetrating deeper into the brown dwarf atmospheres and being destroyed by gases.[32][33] The first L-dwarf with radio emission was [2MASS J00361617+1821104](/source/2MASS_J00361617%2B1821104) (L3.5).[34]

## Binaries

L-dwarfs are less often [binaries](/source/Binary_star) than M-dwarfs. Systems with an L-dwarf as a primary have a binary fraction of 24 ± 6% with a typical separation of 5–8 [astronomical units](/source/Astronomical_unit) (AU).[35] There are also L-dwarfs with a wider separation, such as [WISE 2150−7520](/source/WISE_2150%E2%88%927520) (L1+T8), which has a separation of 341 AU.[36] The closest L-dwarf to the [Solar System](/source/Solar_System) is the primary in the [Luhman 16](/source/Luhman_16) AB binary. It has a spectral type of L8.[37]

## See also

- [List of brown dwarfs](/source/List_of_brown_dwarfs)

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Adapted from the Wikipedia article [L dwarf](https://en.wikipedia.org/wiki/L_dwarf) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/L_dwarf?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.