The first T-dwarf discovered was Gliese 229B, which was discovered in 1995.[6] This object had a temperature below 1000K and showedmethane (CH4),water vapor (H2O) andcarbon monoxide (CO) in its spectrum. In the upper atmosphere CO is converted into CH4 and H2O, while the opposite is true for the hotter lower atmosphere.[7][8][9] It also showed absorption due tocaesium (Cs), but absorption features commonly found inM-dwarfs (CaH,FeH,TiO, andVO) were missing.[10]Ammonia (NH3) was included in the analysis of the spectrum.[11]Sodium (Na) andpotassium (K) are also detected in this T-dwarf.[12] Later work found a dynamical mass of 70 ± 5MJ for Gliese 229B, which is much higher than the cooling models would suggest.[2] The spectral type is somewhat ambiguous. This is because it shows strong CH4 absorption at 1.3 and 1.6 μm, indicative of a T7 type, but weaker CH4 and H2O features at 1.1, 1.4, 1.9, and 2.2 μm, indicative of a T5-T6 type.[13] It is also suspected that Gliese 229B is a binary, which could explain its high mass and its unusual spectrum.[14] The binarity was confirmed in 2024 with instruments on theVery Large Telescope, which resolved the pair and constrained their orbit to be a tightsemi-major axis of about 16 Earth-Moon distances and an orbital period of about 12.1 days. Gliese 229Ba has a mass of about 38MJ and Gliese 229Bb has a mass of about 34MJ.[15]
Spectra of mid to late T-dwarfs, showing absorption due to methane, water vapor, CIA hydrogen and potassium
The spectral type "T" was first proposed in 1999 with Gliese 229B as its only representative at the time.[1] Next came the discovery ofGliese 570D,[16]SDSS 1624+00 (first field T-dwarf)[17] andSDSS 1346-00 (second field T-dwarf).[18] These were however mid- to late T-dwarfs and the first early T-dwarfs (SDSS 0837,SDSS 1254, andSDSS 1021) were discovered in data of theSloan Digital Sky Survey in 2000. These objects show weaker CH4 absorption than previously discovered T-dwarfs.[19] CH4 appears first in theK-band in L8 dwarfs and L- and T-dwarfs are distinguished by the appearance of CH4 in theH-band for T-dwarfs. T-dwarfs show an increasing absorption of H2O and CH4 from T0 to T8. Neutral Na and K features broaden in L- and T-dwarfs and the Na feature increases in depth for L/T-dwarfs with increasing spectral type.[20] One of the coldest T-dwarfs was discovered withUKIDSS, calledUGPS 0722-05.[21][22] Researchers usedWISE to discover additional late T-dwarfs and the objects of the newly discovered Y-dwarfs. The transition between T- and Y-dwarfs is defined with the help of UGPS 0722-05 as the T9 standard andWISE 1738+2732 as the Y0 standard. Late T and early Y-dwarfs show deep H2O and CH4 absorption features and the transition between T- and Y-dwarfs occurs near 500 K.[5][23] Another important T-dwarf isLuhman 16B, which is the closest T-dwarf. It has a spectral type of T0.5, near the L/T transition. It shows a hint of FeH in the spectrum, which weakens in late L dwarfs, but strengthens in early to mid T-dwarfs due to cloud disruption.[24][25] Observations of T-dwarfs in the near- and mid-infrared withJWST clearly show additional absorption features due to NH3, CH4, H2O, CO andcarbon dioxide (CO2).[26] Observations withGemini showed the first detection ofhydrogen sulfide (H2S) and molecularhydrogen (H2) in the T6 dwarfDENIS J081730.0−615520.[27]
Subdwarfs with a T spectral type are known, with2MASSI J0937347+293142 being the first T-type subdwarf. It shows blue near-infrared colors due to suppression of the 2.1 μm peak, likely caused by enhancedcollision induced absorption (CIA) ofhydrogen (H2).[28][29] Subdwarfs have a low metallicity and at first only a small sample with moderate low metallicity was known. In 2020 thebackyard worlds citizen science project discovered the first extreme subdwarfs of spectral type T, calledWISEA 0414−5854 andWISEA 1810−1010. These objects have unusual blue colors, indicative of a lower absorption from CH4.[30] Follow-up observations of WISEA 1810−1010 show that it only shows absorption due to H2O and H2 in the optical and infrared spectra. CH4 is missing completely, which stays in contrast to the definition of T-dwarfs as "methane dwarfs" and WISEA 1810−1010 was instead called a "water vapor dwarf".[31] In 2024 Burgasser et al. introduced a classification system for T subdwarfs, which allows the classification into mild subdwarfs (d/sdT), subdwarfs (sdT) and extreme subdwarfs (esdT). The signature of a low metallicity are a strongcollision induce absorption (CIA) of hydrogen molecules, obscured methane and water features, and weak potassium K I absorption. This work also identified three brown dwarfs that are candidate members ofstellar streams. Future works withJWST,Euclid,Rubin andRoman will increase the sample of T subdwarfs to thousands.[32] JWST has already discovered the first distant T subdwarfs such asUNCOVER-BD-1.[33]
Most T-dwarfs are brown dwarfs. Brown dwarfs have a mass lower than the hydrogen burning minimum mass (0.075M☉ or 78.5MJ).[34] There are currently 920 objects in the UltracoolSheet with an infrared spectral type of T.[35] The table of ultracool fundamental parameters lists objects with an infrared spectral type of T that have masses between 2 and 58MJ.[36][37] Additional T-type brown dwarfs that orbitstars orwhite dwarfs are known and the age of the primary can help to determine the mass of the T-dwarfs.[38][39][40] One of the oldest known T-dwarfs isWolf 1130C, which is around 10billion years old.[41]
One of the first objects that was conclusively determined to be a young isolated planetary-mass object with spectral type T wasSDSS J1110+0116 (T5.5), which is a member of the 120 Myr oldAB Doradus moving group.[42] Another significant discovery is one of the closest planetary-mass objects, calledSIMP J013656.5+093347 (T2.5, 12.7 ±1.0MJ), which is part of the 200 Myr oldCarina-Near moving group.[3] This object is also variable with a period of 2.4 hours, likely due to clouds.[43] It also showsradio emission due toaurorae.[44] Additional young T-dwarf candidates are known from otheryoung stellar associations and these objects show red colors compared to field T-dwarfs.[45] Young directly imaged exoplanets and planetary-mass companions sometimes show a T spectral type, such as51 Eridani b (T4.5-T6).[4]
Two of the most variable brown dwarfs are the T-dwarfs Luhman 16B, showing a variation up to 20%[46] and2MASS J2139+02, which varies with an amplitude as high as 26%.[47] T-dwarfs, especially young early-type T-dwarfs are often variable. The variability has been connected to the presence of clouds, but other explanations were proposed, such as hot spots and aurorae.[48] These early T-dwarfs are thought to have aniron cloud deck and a patchysilicate cloud layer on top of it. The silicate clouds are thought to dissipate near the L/T transition, resulting into the patchy silicate cloud layer and high amplitude variability for late L and early T dwarfs.[49] The disruption of clouds make deeper layers accessible for observations. These deeper layers are warmer and contain FeH. This explains the reappearance and strengthening of FeH and the blue near-infrared color for early to mid T-dwarfs.[25] Late T-dwarfs should also have cloud layers made ofchromium,potassium chloride and differentsulfides. These cloud layers are thin and exist above the silicate clouds.[49] One late T-dwarf that is variable isWISE 0458+6434 (T8.5), which varied with 13% in one epoch.[50]
The first T-dwarf detected inradio emission was2MASS J1047+21 (T6.5), which was discovered with theArecibo radio telescope.[51] Since then several other T-dwarfs with radio emission were discovered, including the planetary-mass object SIMP J013656.5+093347 (T2.5)[44] and the discovery of the T-dwarfBDR J1750+3809 with the help of radio emission.[52] The coldest T-dwarf with a radio emission isWISEPA J062309.94-045624.6 (T8).[53] Radio emission in T-dwarfs is thought to be generated by anaurora, similar to late L-dwarfs. AdditionallyH-alpha emission is often connected to radio emission in L4-T8 dwarfs and is thought to come from aurorae.[54]2MASS 1237+6526 (T6.5) is an unusual strong H-alpha emitting T-dwarf that was discovered in 2000.[55] It was theorized that the H-alpha emission,UV emission and radio emission come either from a cold companion (1-2.8R🜨; <500 K) or from an aurora.[56]
Late T dwarf binaries are less common than L-type binaries. Only 8±6% systems with a T5–Y0 primary are binaries and these systems usually have a separation of a fewastronomical units (AU).[57] One well-known T dwarf binary isEpsilon Indi B.[58] This binary consists of a T1 and a T6 dwarf that orbit each other with a separation of 2.65 AU.[59] T dwarf triple systems also exist, with2M0838+15 being the first fully resolved triple T dwarf that was discovered.[60]
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