An object with the spectral type L (also called L-dwarf) can be either a low-mass

star A star is a luminous spheroid of plasma (physics), plasma held together by Self-gravitation, self-gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked eye at night sk ...

, a

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, main-sequence stars. Their mass is approximately 13 to 80 Jupiter mass, times that of Jupiter ()not big en ...

or a young free-floating planetary-mass object. If a young

exoplanet An exoplanet or extrasolar planet is a planet outside the Solar System. The first confirmed detection of an exoplanet was in 1992 around a pulsar, and the first detection around a main-sequence star was in 1995. A different planet, first det ...

or planetary-mass companion is detected via direct imaging, it can also have an L spectral type, such as

Kappa Andromedae b Kappa Andromedae b is a directly imaged substellar object and likely superjovian-mass planet orbiting Kappa Andromedae, a young B9IV star in the Andromeda (constellation), Andromeda constellation, about 170 light-years away. The companion's mass ...

Spectral characteristics

Before

2MASS The Two Micron All-Sky Survey, or 2MASS, was an astronomical survey of the whole sky in infrared light. It took place between 1997 and 2001, in two different locations: at the U.S. Fred Lawrence Whipple Observatory on Mount Hopkins, Arizona, and ...

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 T-Type was the performance marker used on Buick automobiles during the 1980s. Marketed during a hiatus of the Gran Sport branding, the T-Type vehicles were the sportiest versions of each Buick model line, in line with Super Sport-branded Chevro ...

spectral types.

Kirkpatrick Kirkpatrick is an Irish (Ulster) and Scottish surname, and occasionally a given name, possibly a branch of the Cenél nEógain of the Northern Uí Néill. The name traditionally relates to a church ("kirk") dedicated to Saint Patrick.Way, George a ...

et al. defined the two spectral types in 1999. In these L-dwarfs the metallic

oxides An oxide () is a chemical compound containing at least one oxygen atom and one other element in its chemical formula. "Oxide" itself is the dianion (anion bearing a net charge of −2) of oxygen, an O2− ion with oxygen in the oxidation state o ...

( TiO, VO), which are present in late M-dwarfs, are replaced with metallic

hydrides In chemistry, a hydride is formally the anion of hydrogen (H−), a hydrogen ion with two electrons. In modern usage, this is typically only used for ionic bonds, but it is sometimes (and has been more frequently in the past) applied to all co ...

(e.g. CrH,

FeH Iron(I) hydride, systematically named iron hydride and poly(hydridoiron) is a solid inorganic compound with the chemical formula (also written or FeH). It is both thermodynamically and kinetically unstable toward decomposition at ambient temper ...

) and neutral

alkali metals The alkali metals consist of the chemical elements lithium (Li), sodium (Na), potassium (K),The symbols Na and K for sodium and potassium are derived from their Latin names, ''natrium'' and ''kalium''; these are still the origins of the names ...

(e.g. K, Rb, Cs). The transition between L- and T-dwarfs is defined with the appearance of

methane Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The abundance of methane on Earth makes ...

(CH₄) in the spectrum. M-dwarfs show absorption due to

water vapor Water vapor, water vapour, or aqueous vapor is the gaseous phase of Properties of water, water. It is one Phase (matter), state of water within the hydrosphere. Water vapor can be produced from the evaporation or boiling of liquid water or from th ...

(H₂O) in their near-infrared spectrum. This absorption feature gets stronger with later L spectral type. The absorption due to

carbon monoxide Carbon monoxide (chemical formula CO) is a poisonous, flammable gas that is colorless, odorless, tasteless, and slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom connected by a triple bond. It is the si ...

(CO) does show little variation over spectral type. CO is replaced by CH₄ in T-dwarfs. Initially it was estimated that the hottest L0-dwarfs have a temperature of around 2000 K and the coldest L8-dwarfs have a temperature of about 1500 K. Modern estimates range from 1100 K for L9, to a maximum of 2500 K for L0. L-dwarfs have a

red Red is the color at the long wavelength end of the visible spectrum of light, next to orange and opposite violet. It has a dominant wavelength of approximately 625–750 nanometres. It is a primary color in the RGB color model and a seconda ...

violet Violet may refer to: Common meanings * Violet (color), a spectral color with wavelengths shorter than blue * One of a list of plants known as violet, particularly: ** ''Viola'' (plant), a genus of flowering plants Places United States * Vi ...

purple Purple is a color similar in appearance to violet light. In the RYB color model historically used in the arts, purple is a secondary color created by combining red and blue pigments. In the CMYK color model used in modern printing, purple is ...

color due to absorption from the

sodium Sodium is a chemical element; it has Symbol (chemistry), symbol Na (from Neo-Latin ) and atomic number 11. It is a soft, silvery-white, highly reactive metal. Sodium is an alkali metal, being in group 1 element, group 1 of the peri ...

D-line, which is centered at 5890 Å, overlapping with the color

green Green is the color between cyan and yellow on the visible spectrum. It is evoked by light which has a dominant wavelength of roughly 495570 nm. In subtractive color systems, used in painting and color printing, it is created by a com ...

. Later work described L-dwarfs as having a violet color.

Subdwarfs

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 of hydrogen molecules leads to a suppression of the H- and K-band, which causes L-type subdwarfs to have blue near-infrared colors. 2MASS J0532+8246 was the first L-type subdwarf discovered. The

prefix A prefix is an affix which is placed before the stem of a word. Particularly in the study of languages, a prefix is also called a preformative, because it alters the form of the word to which it is affixed. Prefixes, like other affixes, can b ...

sd, esd and usd indicate subdwarfs, extreme subdwarfs and ultra subdwarfs. Objects with an usd-prefix have the lowest metallicity.

Main-sequence stars

The

hydrogen burning In astrophysics, stellar nucleosynthesis is the creation of chemical elements by nuclear fusion reactions within stars. Stellar nucleosynthesis has occurred since the original creation of hydrogen, helium and lithium during the Big Bang. As a ...

minimum mass lies at 0.075 (78.5 ) for objects with a solar metallicity. 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 239-25B (L0) for which they find a mass of 88.9 ±0.59 . 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. Old L-subdwarfs with an early L spectral type can be main-sequence stars. The brown dwarf SDSS J0104+1535 (usdL1.5, 0.086 ± 0.0015 ) for example is just below the hydrogen burning limit of around 0.088 , for its metallicity of e/H= -2.4 ± 0.2. The same team found that a third of known L-subdwarfs are substellar objects and two-thirds are low-mass stars. CWISE J1249+3621 (sdL1, ) is for example a star, because the hydrogen burning limit is at around 0.080 for /H-1. This star is also a

hypervelocity star In astronomy, stellar kinematics is the Observational astronomy, observational study or measurement of the kinematics or motions of stars through space. Stellar kinematics encompasses the measurement of stellar Velocity, velocities in the Milky ...

Brown dwarfs

Most L-dwarfs are brown dwarfs. Brown dwarfs are objects with a mass below 78.5 . Objects with a mass below 14 are often referred to as planetary-mass objects, but depending on their formation mechanism they are also called planetary-mass brown dwarfs. 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 . Additionally there are dozens of L-type brown dwarfs known that co-move with a star,

white dwarf A white dwarf is a Compact star, stellar core remnant composed mostly of electron-degenerate matter. A white dwarf is very density, dense: in an Earth sized volume, it packs a mass that is comparable to the Sun. No nuclear fusion takes place i ...

or brown dwarf. The first L-type brown dwarf discovered was GD 165B, which orbits a white dwarf. Its mass was later determined to be 62.58 ± 15.57 .

Planetary-mass objects and exoplanets

A planetary-mass object is commonly defined as an object with a mass below 14 . These objects can be free-floating or co-move with a star or brown dwarf (e.g. HD 106906 b). 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 A protoplanetary disk is a rotating circumstellar disc of dense gas and dust surrounding a young newly formed star, a T Tauri star, or Herbig Ae/Be star. The protoplanetary disk may not be considered an accretion disk; while the two are sim ...

. One exoplanet near this 100 AU boundary is

Delorme 1 Delorme 1 (2MASS J01033563-5515561) is a binary star with a planetary-mass object, planetary-mass companion (PMC) or protoplanet in a Circumbinary planet, circumbinary orbit. The PMC is notable for showing signs of accretion, despite bein ...

(AB)b, which could have formed via fragmentation of the circumstellar disk and is therefore considered an exoplanet. More close-in planets, such as the planets around

HR 8799 HR 8799 is a roughly 30 million-year-old main sequence, main-sequence star located away from Earth in the constellation of Pegasus (constellation), Pegasus. It has roughly 1.5 times the Sun's mass and 4.9 times its lumino ...

and

also resemble L-dwarfs or have an L spectral type. 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) or a young association (see List of nearby associations). Researchers can use the temperature-age or luminosity-age relation to determine if its mass is below 13 . For very young star clusters (<1

Myr Million years ago, abbreviated as Mya, Myr (megayear) or Ma (megaannum), is a unit of time equal to (i.e. years), or approximately 31.6 teraseconds. Usage Myr is in common use in fields such as Earth science and cosmology. Myr is also used w ...

) even an L0 spectral type corresponds to a planetary-mass and therefore all L-dwarfs in such a star cluster have a planetary-mass. Another method is to determine other indicators of a young age. A lower-mass object has for example a lower

surface gravity The surface gravity, ''g'', of an astronomical object is the gravitational acceleration experienced at its surface at the equator, including the effects of rotation. The surface gravity may be thought of as the acceleration due to gravity experi ...

, 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 In linguistics, a suffix is an affix which is placed after the stem of a word. Common examples are case endings, which indicate the grammatical case of nouns and adjectives, and verb endings, which form the conjugation of verbs. Suffixes can ca ...

β, γ and δ, indicating intermediate (β), low (γ) and very low (δ) gravity. Low-gravity L3-L5 dwarfs can also show

lithium Lithium (from , , ) is a chemical element; it has chemical symbol, symbol Li and atomic number 3. It is a soft, silvery-white alkali metal. Under standard temperature and pressure, standard conditions, it is the least dense metal and the ...

absorption. The so-called "lithium test" is less reliable to determine a low mass for young L-dwarfs. An example for a low gravity object is CWISE J0506+0738, which has a spectral type between L8γ and T0γ and probably a mass of 7±2 .

Variability and clouds

Iron Iron is a chemical element; it has symbol Fe () and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, forming much of Earth's o ...

clouds with

silicate A silicate is any member of a family of polyatomic anions consisting of silicon and oxygen, usually with the general formula , where . The family includes orthosilicate (), metasilicate (), and pyrosilicate (, ). The name is also used ...

clouds on top of it were theorized since the early 2000s for L-dwarfs. The presence of silicates in L-dwarfs is well established with Spitzer observations. Especially L4-L6 dwarfs often show silicate absorption. But silicate absorption can also be absent for any L-dwarf. 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 An aurora ( aurorae or auroras), also commonly known as the northern lights (aurora borealis) or southern lights (aurora australis), is a natural light display in Earth's sky, predominantly observed in high-latitude regions (around the Arc ...

. Especially young objects show variability. One of the most variable L-dwarf is the planetary-mass companion

VHS J1256–1257 VHS (Video Home System) is a discontinued standard for consumer-level analog video recording on tape cassettes, introduced in 1976 by JVC. It was the dominant home video format throughout the tape media period of the 1980s and 1990s. Magnet ...

b (L7), with an amplitude of 33-38%.

Magnetic field and aurorae

Radio emission can be detected in L-dwarfs and this radio emission sometimes shows rotational periodic radio pulses. Additionally the

H-alpha Hydrogen-alpha, typically shortened to H-alpha or Hα, is a deep-red visible spectral line of the hydrogen atom with a wavelength of 656.28 nm in air and 656.46 nm in vacuum. It is the first spectral line in the Balmer series and is em ...

emission seen commonly in L-dwarf is interpreted as chromospheric and coronal 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 (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 Jupiter is the fifth planet from the Sun and the List of Solar System objects by size, largest in the Solar System. It is a gas giant with a Jupiter mass, mass more than 2.5 times that of all the other planets in the Solar System combined a ...

. The other suggested energy source is the interaction with a

rocky planet A terrestrial planet, tellurian planet, telluric planet, or rocky planet, is a planet that is composed primarily of silicate, rocks or metals. Within the Solar System, the terrestrial planets accepted by the IAU are the inner planets closest to ...

around the brown dwarf, similar to the interaction between Io and Jupiter. Impacts of

electrons The electron (, or in nuclear reactions) is a subatomic particle with a negative one elementary charge, elementary electric charge. It is a fundamental particle that comprises the ordinary matter that makes up the universe, along with up qua ...

with hydrogen molecules can create a

trihydrogen cation The trihydrogen cation or protonated molecular hydrogen ( IUPAC name: hydrogen onium ion) is a cation (positive ion) with formula , consisting of three hydrogen nuclei (protons) sharing two electrons. The trihydrogen cation is one of the most a ...

(H). This could be detected in the infrared at 2 and 4 μm with

JWST The James Webb Space Telescope (JWST) is a space telescope designed to conduct infrared astronomy. As the largest telescope in space, it is equipped with high-resolution and high-sensitivity instruments, allowing it to view objects too old, di ...

. Destruction of H by gases, such as H₂O and CH₄, could mean that it is not built up significantly in brown dwarfs. No H was detected in any M, L or T dwarf with the

Keck Observatory The W. M. Keck Observatory is an astronomical observatory with two telescopes at an elevation of 4,145 meters (13,600 ft) near the summit of Mauna Kea in the U.S. state of Hawaii. Both telescopes have aperture primary mirrors, and, when c ...

, likely due to auroral electrons penetrating deeper into the brown dwarf atmospheres and being destroyed by gases. The first L-dwarf with radio emission was 2MASS J00361617+1821104 (L3.5).

Binaries

L-dwarfs are less often

binaries A binary file is a computer file that is not a text file. The term "binary file" is often used as a term meaning "non-text file". Many binary file formats contain parts that can be interpreted as text; for example, some computer document files ...

than M-dwarfs. Systems with an L-dwarf as a primary have a binary fraction of % with a typical separation of 5–8

astronomical units The astronomical unit (symbol: au or AU) is a unit of length defined to be exactly equal to . Historically, the astronomical unit was conceived as the average Earth-Sun distance (the average of Earth's aphelion and perihelion), before its mode ...

(AU). There are also L-dwarfs with a wider separation, such as WISE 2150−7520 (L1+T8), which has a separation of 341 AU. The closest L-dwarf to the

Solar System The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Sola ...

is the primary in the

Luhman 16 Luhman 16 (also designated WISE 1049−5319 or WISE J104915.57−531906.1) is a Binary system, binary brown dwarf, brown-dwarf system in the southern constellation Vela (constellation), Vela at a distance of from the Sun. These are th ...

AB binary. It has a spectral type of L8.

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