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L dwarf

From Wikipedia, the free encyclopedia

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

Spectral characteristics

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Spectrum of Kelu-1 (L-type binary, bottom line) in comparison to an M6-dwarf, which shows much stronger TiO and sodium absorption.

Before 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 spectral types. Kirkpatrick et al. defined the two spectral types in 1999. In these L-dwarfs the metallic oxides (TiO, VO), which are present in late M-dwarfs, are replaced with metallic hydrides (e.g. CrH, FeH) and neutral alkali metals (e.g. K, Rb, Cs). The transition between L- and T-dwarfs is defined with the appearance of methane (CH4) in the spectrum.[5] M-dwarfs show absorption due to water vapor (H2O) in their near-infrared spectrum. This absorption feature gets stronger with later L spectral type. The absorption due to carbon monoxide (CO) does show little variation over spectral type.[6] CO is replaced by CH4 in T-dwarfs.[7] 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.[5] Modern estimates range from 1100 K for L9, to a maximum of 2500 K for L0.[8][9]

L-dwarfs have a red, violet or purple color due to absorption from sodium D-line, which is centered at 5890 Å, overlapping with the color green.[7] Later work described L-dwarf as having a violet color.[10]

Subdwarfs

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Main article: subdwarf

Subdwarfs are objects with a low metallicity. These objects are usually old and their metallicity influences different absorption features. Especially the collision induced absorption of hydrogen molecules leads to a suppression of the H- and K-band, which causes L-type subdwarf to have blue near-infrared colors. 2MASS J0532+8246 was the first L-type subdwarf discovered. The prefix sd, esd and usd indicate subdwarfs, extreme subdwarfs and ultra subdwarfs. Objects with an usd-prefix have the lowest metallicity.[11]

Main-sequence stars

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The hydrogen burning minimum mass lies at 0.075 M☉ (78.5 MJ) for objects with a solar metallicity.[12] The table of ultracool fundamental parameters lists several objects with an infrared spectral type of L0 to L4 and a mass above 78.5 MJ. 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 MJ.[8][9] 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.[13] Old L-subdwarfs with an early L spectral type can be main-sequence stars.[14] The brown dwarf SDSS J0104+1535 (usdL1.5, 0.086 ± 0.0015 M☉) for example is just below the hydrogen burning limit of around 0.088 M☉, for its metallicity of [Fe/H] = -2.4 ± 0.2.[13] The same team found that ⅓ of known L-subdwarfs are substellar objects and ⅔ are low-mass stars.[1] CWISE J1249+3621 (sdL1, 0.082+0.002
−0.003 M☉) 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.[14]

Brown dwarfs

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Main article: brown dwarfs

Most L-dwarfs are brown dwarfs. Brown dwarfs are objects with a mass below 78.5 MJ.[12] Objects with a mass below 14 MJ are often referred as planetary-mass objects,[15] but depending on their formation mechanism they are also called planetary-mass brown dwarfs.[16]

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 MJ.[8][9] Additionally there are dozens of L-type brown dwarfs known that co-move with a star, white dwarf or brown dwarf.[2] The first L-type brown dwarf discovered was GD 165B, which orbits a white dwarf.[17] Its mass was later determined to be 62.58 ± 15.57 MJ.[18]

Planetary-mass objects and exoplanets

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Free-floating planetary-mass object PSO J318.5−22, which is an L-dwarf
See also: List of directly imaged exoplanets, sub-brown dwarf, and Rogue planet

A planetary-mass object is commonly defined as an object with a mass below 14 MJ. These objects can be free-floating[15] or co-move with a star or brown dwarf (e.g. HD 106906 b).[19][20] 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.[21] One exoplanet near this 100 AU boundary is Delorme 1 (AB)b, which could have formed via fragmentation of the circumstellar disk and is therefore considered an exoplanet.[22] More close-in planets, such as the planets around HR 8799[23] and 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)[24] 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 MJ.[15] For very young star clusters (<1 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.[24]

Another method is to determine other indicators of a young age. A lower-mass object has for example a lower 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 β, γ and δ, indicating intermediate (β), low (γ) and very low (δ) gravity. Low-gravity L3-L5 dwarfs can also show lithium absorption. The so-called "lithium test" is less reliable to determine a low mass for young L-dwarfs.[25] 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 MJ.[15]

Variability and clouds

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Iron clouds with silicate clouds on top of it were theorized since the early 2000s for L-dwarfs.[26] 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.[27] 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. Especially young objects show variability.[28]

Binaries

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The L-type binary CWISE J0146-0508AB (L4+L8 blue)[29]

L-dwarfs are less often binaries than M-dwarfs. Systems with an L-dwarf as a primary have a binary fraction of 24+6
−2% with a typical separation of 5–8 astronomical units (AU).[30] There are also L-dwarfs with a wider separation, such as WISE 2150−7520 (L1+T8), which has a separation of 341 AU.[31] The closest L-dwarf to the Solar System is the primary in the Luhman 16 AB binary. It has a spectral type of L8.[32]

See also

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References

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