L dwarf

An object with the spectral type L (also called L-dwarf) can be either a low-mass star, a brown dwarf or a young free-floating planetary-mass object. 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.

Spectral characteristics

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, and this was done 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. 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. 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, violet, or purple color due to absorption from the sodium D-line, which is centered at 5890 Å, overlapping with the color green. 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 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 minimum mass lies at 0.075 M☉ (78.5 MJ) 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 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. 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 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. 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, 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.

Brown dwarfs

Most L-dwarfs are brown dwarfs. Brown dwarfs are objects with a mass below 78.5 MJ. Objects with a mass below 14 MJ 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 MJ. Additionally there are dozens of L-type brown dwarfs known that co-move with a star, white dwarf 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 MJ.

Planetary-mass objects and exoplanets

A planetary-mass object is commonly defined as an object with a mass below 14 MJ. 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. 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. More close-in planets, such as the planets around HR 8799 and Kappa Andromedae b 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 MJ. 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. 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. 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.

… excerpt ends here. Continue reading the full article.