Editing Brown dwarf (section)

=== Classification of brown dwarfs ===

==== Spectral class M ====
[[Image:late-M-dwarf-nasa-hurt.png|thumb|left|Artist's vision of a late-M dwarf]]
These are brown dwarfs with a spectral class of M5.5 or later; they are also called late-M dwarfs. Some scientists regard them as [[red dwarf]]s.{{citation needed|date=February 2020}} All brown dwarfs with spectral type M are young objects, such as [[Teide 1]], which is the first M-type brown dwarf discovered, and [[LP 944-20]], the closest M-type brown dwarf.

==== Spectral class L ====
{{main|L dwarf}}
[[Image:L-dwarf-nasa-hurt.png|thumb|Artist's concept of an L dwarf]]

The defining characteristic of [[spectral class]] M, the coolest type in the long-standing classical stellar sequence, is an optical spectrum dominated by absorption bands of [[titanium(II) oxide]] (TiO) and [[vanadium(II) oxide]] (VO) molecules. However, [[GD 165]]B, the cool companion to the white dwarf [[GD 165]], had none of the hallmark TiO features of M dwarfs. The subsequent identification of many objects like GD 165B ultimately led to the definition of a new [[spectral class]], the '''L dwarfs''', defined in the red optical region of the spectrum not by metal-oxide absorption bands (TiO, VO), but by metal [[hydride]] emission bands ([[FeH]], [[CrH]], [[magnesium hydride|MgH]], [[calcium hydride|CaH]]) and prominent atomic lines of [[alkali metal]]s (Na, K, Rb, Cs). {{As of|2013}}, over 900 L&nbsp;dwarfs had been identified,<ref name="DwarfArchives"/> most by wide-field surveys: the Two Micron All Sky Survey ([[2MASS]]), the [[Deep Near Infrared Survey of the Southern Sky]] (DENIS), and the [[Sloan Digital Sky Survey]] (SDSS). This spectral class also contains the coolest main-sequence stars (>&nbsp;80&nbsp;''M''<sub>J</sub>), which have spectral classes L2 to L6.<ref>{{cite journal |last1=Smart |first1=Richard L. |last2=Bucciarelli |first2=Beatrice |last3=Jones |first3=Hugh R. A. |last4=Marocco |first4=Federico |last5=Andrei |first5=Alexandre Humberto |last6=Goldman |first6=Bertrand |last7=Méndez |first7=René A. |last8=d'Avila |first8=Victor de A. |last9=Burningham |first9=Ben |last10=Camargo |first10=Julio Ignácio Bueno de |last11=Crosta |first11=Maria Teresa |first12=Mario |last12=Daprà |first13=James S. |last13=Jenkins |first14=Regis |last14=Lachaume |first15=Mario G. |last15=Lattanzi |first16=Jucira L. |last16=Penna |first17=David J. |last17=Pinfield |first18=Dario Nepomuceno |last18=da Silva Neto |first19=Alessandro |last19=Sozzetti |first20=Alberto |last20=Vecchiato |date=December 2018 |title=Parallaxes of Southern Extremely Cool objects III: 118 L and T dwarfs |journal=MNRAS |volume=481 |issue=3 |pages=3548–3562 |doi=10.1093/mnras/sty2520 |doi-access=free |arxiv=1811.00672 |bibcode=2018MNRAS.481.3548S |s2cid=119390019 |issn=0035-8711 }}</ref>

==== Spectral class T ====
{{main|T dwarf}}
[[Image:T-dwarf-nasa-hurt.png|thumb|left|Artist's concept of a T dwarf]]

As GD 165B is the prototype of the L dwarfs, [[Gliese 229]]B is the prototype of a second new spectral class, the '''T dwarfs'''. T&nbsp;dwarfs are pinkish-magenta. Whereas [[near-infrared]] (NIR) spectra of L dwarfs show strong absorption bands of H<sub>2</sub>O and [[carbon monoxide]] (CO), the NIR spectrum of Gliese 229B is dominated by absorption bands from [[methane]] (CH<sub>4</sub>), a feature which in the Solar System is found only in the giant planets and [[Titan (moon)|Titan]]. CH<sub>4</sub>, H<sub>2</sub>O, and molecular [[hydrogen]] (H<sub>2</sub>) collision-induced absorption (CIA) give Gliese 229B blue near-infrared colors. Its steeply sloped red optical spectrum also lacks the FeH and CrH bands that characterize L dwarfs and instead is influenced by exceptionally broad absorption features from the [[alkali]] metals [[sodium|Na]] and [[potassium|K]]. These differences led [[J. Davy Kirkpatrick]] to propose the T spectral class for objects exhibiting H- and K-band CH<sub>4</sub> absorption. {{As of|2013}}, 355 T&nbsp;dwarfs were known.<ref name="DwarfArchives"/> NIR classification schemes for T dwarfs have recently been developed by Adam Burgasser and Tom Geballe. Theory suggests that L dwarfs are a mixture of very-low-mass stars and sub-stellar objects (brown dwarfs), whereas the T dwarf class is composed entirely of brown dwarfs. Because of the absorption of [[sodium]] and [[potassium]] in the green part of the spectrum of T dwarfs, the actual appearance of T dwarfs to human [[visual perception]] is estimated to be not brown, but [[magenta]].<ref name=burrows>{{cite journal |last1=Burrows |first1=Adam |last2=Hubbard |first2=William B. |last3=Lunine |first3=Jonathan I. |last4=Liebert |first4=James |year=2001 |title=The theory of brown dwarfs and extrasolar giant planets |journal=[[Reviews of Modern Physics]] |volume=73 |issue=3 |pages=719–765 |doi=10.1103/RevModPhys.73.719 |bibcode=2001RvMP...73..719B |arxiv=astro-ph/0103383 |s2cid=204927572 }}</ref><ref>[http://spider.ipac.caltech.edu/staff/davy/2mass/science/comparison.html "An Artist's View of Brown Dwarf Types"] {{Webarchive |first=Robert |last=Hurt |newspaper=Infrared Processing and Analysis Center |url=https://web.archive.org/web/20111117180311/http://spider.ipac.caltech.edu/staff/davy/2mass/science/comparison.html |date=2011-11-17 }}</ref> Early observations limited how distant T-dwarfs could be observed. T-class brown dwarfs, such as [[WISE 0316+4307]], have been detected more than 100 light-years from the Sun. Observations with JWST have detected T-dwarfs such as [[UNCOVER-BD-1]] up to 4500 parsec distant from the sun.

==== Spectral class Y ====
{{Main|Y dwarf}}
[[File:WISE 1828+2650 Brown dwarf.jpg|thumb|Artist's vision of a Y dwarf]]

In 2009, the coolest-known brown dwarfs had estimated effective temperatures between {{cvt|500|and(-)|600|K|C F|lk=on}}, and have been assigned the spectral class T9. Three examples are the brown dwarfs [[CFBDS J005910.90–011401.3]], [[ULAS J133553.45+113005.2]] and [[ULAS J003402.77−005206.7]].<ref name=four600k>{{cite journal |doi=10.1088/0004-637X/695/2/1517 |bibcode=2009ApJ...695.1517L |title=The Physical Properties of Four ~600 K T Dwarfs |journal=The Astrophysical Journal |volume=695 |issue=2 |pages=1517–1526 |last1=Leggett |first1=Sandy K. |last2=Cushing |first2=Michael C. |last3=Saumon |first3=Didier |last4=Marley |first4=Mark S. |last5=Roellig |first5=Thomas L. |last6=Warren |first6=Stephen J. |last7=Burningham |first7=Ben |last8=Jones |first8=Hugh R. A. |last9=Kirkpatrick |first9=J. Davy |last10=Lodieu |first10=Nicolas |last11=Lucas |first11=Philip W. |last12=Mainzer |first12=Amy K. |last13=Martín |first13=Eduardo L. |last14=McCaughrean |first14=Mark J. |last15=Pinfield |first15=David J. |last16=Sloan |first16=Gregory C. |last17=Smart |first17=Richard L. |last18=Tamura |first18=Motohide |last19=Van Cleve |first19=Jeffrey |year=2009 |arxiv=0901.4093 |s2cid=44050900 }}.</ref> The spectra of these objects have absorption peaks around 1.55 micrometres.<ref name=four600k/> Delorme et al. have suggested that this feature is due to absorption from [[ammonia]] and that this should be taken as indicating the T–Y transition, making these objects of type Y0.<ref name=four600k/><ref name=tytrans>{{cite journal |doi=10.1051/0004-6361:20079317 |bibcode=2008A&A...482..961D |title=CFBDS J005910.90-011401.3: Reaching the T–Y brown dwarf transition? |journal=Astronomy and Astrophysics |volume=482 |issue=3 |pages=961–971 |year=2008 |last1=Delorme |first1=Philippe |last2=Delfosse |first2=Xavier |last3=Albert |first3=Loïc |last4=Artigau |first4=Étienne |last5=Forveille |first5=Thierry |last6=Reylé |first6=Céline |last7=Allard |first7=France |last8=Homeier |first8=Derek |last9=Robin |first9=Annie C. |last10=Willott |first10=Chris J. |last11=Liu |first11=Michael C. |last12=Dupuy |first12=Trent J. |arxiv=0802.4387 |s2cid=847552 }}</ref> However, the feature is difficult to distinguish from absorption by water and [[methane]],<ref name=four600k/> and other authors have stated that the assignment of class Y0 is premature.<ref name="Burninghametal2008"/>

[[File:WISE2010-040-rotate180.jpg|thumb|right|[[WISEPC J045853.90+643451.9|WISE 0458+6434]] is the first ultra-cool brown dwarf (green dot) discovered by [[Wide-field Infrared Survey Explorer|WISE]]. The green and blue comes from infrared wavelengths mapped to visible colors.]]

The first [[James Webb Space Telescope|JWST]] spectral energy distribution of a Y-dwarf was able to observe several bands of molecules in the atmosphere of the Y0-dwarf [[WISE 0359−5401]]. The observations covered spectroscopy from 1 to 12 μm and photometry at 15, 18 and 21 μm. The molecules water (H<sub>2</sub>O), methane (CH<sub>4</sub>), carbon monoxide (CO), carbon dioxide (CO<sub>2</sub>) and ammonia (NH<sub>3</sub>) were detected in WISE 0359−5401. Many of these features have been observed before in this Y-dwarf and warmer T-dwarfs by other observatories, but JWST was able to observe them in a single spectrum. Methane is the main reservoir of carbon in the atmosphere of WISE 0359−5401, but there is still enough carbon left to form detectable carbon monoxide (at 4.5–5.0 μm) and carbon dioxide (at 4.2–4.35 μm) in the Y-dwarf. Ammonia was difficult to detect before JWST, as it blends in with the absorption feature of water in the near-infrared, as well at 5.5–7.1 μm. At longer wavelengths of 8.5–12 μm the spectrum of WISE 0359−5401 is dominated by the absorption of ammonia. At 3 μm there is an additional newly detected ammonia feature.<ref>{{cite journal |last1=Beiler |first1=Samuel A. |last2=Cushing |first2=Michael C. |last3=Kirkpatrick |first3=J. Davy |last4=Schneider |first4=Adam C. |last5=Mukherjee |first5=Sagnick |last6=Marley |first6=Mark S. |date=2023-07-01 |title=The First JWST Spectral Energy Distribution of a Y Dwarf |journal=The Astrophysical Journal |volume=951 |issue=2 |pages=L48 |doi=10.3847/2041-8213/ace32c |issn=0004-637X|arxiv=2306.11807 |bibcode=2023ApJ...951L..48B |doi-access=free }}</ref>

==== Role of vertical mixing ====
[[File:Reaction methan to CO in brown dwarfs.png|thumb|225x225px|Major chemical pathways linking carbon monoxide and methane. The short-lived radicals are marked with a dot. Adopted from Zahnle & Marley<ref name=":9">{{cite journal |last1=Zahnle |first1=Kevin J. |last2=Marley |first2=Mark S. |date=2014-12-01 |title=Methane, Carbon Monoxide, and Ammonia in Brown Dwarfs and Self-Luminous Giant Planets |url=https://ui.adsabs.harvard.edu/abs/2014ApJ...797...41Z |journal=The Astrophysical Journal |volume=797 |issue=1 |pages=41 |doi=10.1088/0004-637X/797/1/41 |arxiv=1408.6283 |bibcode=2014ApJ...797...41Z |s2cid=118509317 |issn=0004-637X}}</ref>]]
In the hydrogen-dominated atmosphere of brown dwarfs a [[chemical equilibrium]] between [[carbon monoxide]] and [[methane]] exists. Carbon monoxide reacts with [[hydrogen]] molecules and forms methane and [[Hydroxyl radical|hydroxyl]] in this reaction. The hydroxyl radical might later react with hydrogen and form water molecules. In the other direction of the reaction, methane reacts with hydroxyl and forms carbon monoxide and hydrogen. The chemical reaction is tilted towards carbon monoxide at higher temperatures (L-dwarfs) and lower pressure. At lower temperatures (T-dwarfs) and higher pressure the reaction is tilted towards methane, and methane predominates at the T/Y-boundary. However, vertical mixing of the atmosphere can cause methane to sink into lower layers of the atmosphere and carbon monoxide to rise from these lower and hotter layers. The carbon monoxide is slow to react back into methane because of an energy barrier that prevents the breakdown of the [[Carbon–oxygen bond|C-O bonds]]. This forces the observable atmosphere of a brown dwarf to be in a chemical disequilibrium. The L/T transition is mainly defined with the transition from a carbon-monoxide-dominated atmosphere in L-dwarfs to a methane-dominated atmosphere in T-dwarfs. The amount of vertical mixing can therefore push the L/T-transition to lower or higher temperatures. This becomes important for objects with modest surface gravity and extended atmospheres, such as giant [[exoplanet]]s. This pushes the L/T transition to lower temperatures for giant exoplanets. For brown dwarfs this transition occurs at around 1200 K. The exoplanet [[HR 8799 c|HR 8799c]], on the other hand, does not show any methane, while having a temperature of 1100K.<ref name=":9" />

The transition between T- and Y-dwarfs is often defined as 500 K because of the lack of spectral observations of these cold and faint objects.<ref name=":10">{{cite journal |last1=Bardalez Gagliuffi |first1=Daniella C. |last2=Faherty |first2=Jacqueline K. |last3=Schneider |first3=Adam C. |last4=Meisner |first4=Aaron |last5=Caselden |first5=Dan |last6=Colin |first6=Guillaume |last7=Goodman |first7=Sam |last8=Kirkpatrick |first8=J. Davy |last9=Kuchner |first9=Marc |last10=Gagné |first10=Jonathan |last11=Logsdon |first11=Sarah E. |last12=Burgasser |first12=Adam J. |last13=Allers |first13=Katelyn |last14=Debes |first14=John |last15=Wisniewski |first15=John |date=2020-06-01 |title=WISEA J083011.95+283716.0: A Missing Link Planetary-mass Object |journal=The Astrophysical Journal |volume=895 |issue=2 |pages=145 |doi=10.3847/1538-4357/ab8d25 |arxiv=2004.12829 |bibcode=2020ApJ...895..145B |s2cid=216553879 |issn=0004-637X |doi-access=free }}</ref> Future observations with [[James Webb Space Telescope|JWST]] and the [[Extremely large telescope|ELTs]] might improve the sample of Y-dwarfs with observed spectra. Y-dwarfs are dominated by deep spectral features of methane, water vapor and possibly absorption features of [[ammonia]] and [[Ice|water ice]].<ref name=":10" /> Vertical mixing, clouds, metallicity, [[photochemistry]], [[lightning]], impact shocks and metallic [[Catalysis|catalysts]] might influence the temperature at which the L/T and T/Y transition occurs.<ref name=":9" />

==== Secondary features ====
{| class="wikitable floatright" style="width: 26em"
|+ Brown dwarf spectral types
|-
! colspan="2" |Secondary features
|-
|pec
|This suffix (e.g. L2pec) stands for "peculiar".<ref>{{cite web|url=http://simbad.u-strasbg.fr/simbad/sim-display?data=sptypes|title=Spectral type codes|website=simbad.u-strasbg.fr|access-date=2020-03-06}}</ref>
|-
|sd
|This prefix (e.g. sdL0) stands for [[subdwarf]] and indicates a low metallicity and blue color.<ref name=":5">{{cite journal |last1=Burningham |first1=Ben |last2=Smith |first2=Leigh |last3=Cardoso |first3=Cátia V. |last4=Lucas |first4=Philip W. |last5=Burgasser |first5=Adam J. |last6=Jones |first6=Hugh R. A. |last7=Smart |first7=Richard L. |date=May 2014 |title=The discovery of a T6.5 subdwarf |journal=[[Monthly Notices of the Royal Astronomical Society]] |volume=440 |issue=1 |pages=359–364 |doi=10.1093/mnras/stu184 |doi-access=free |arxiv=1401.5982 |bibcode=2014MNRAS.440..359B |s2cid=119283917 |issn=0035-8711 }}</ref>
|-
|β
|Objects with the beta (β) suffix (e.g. L4β) have an intermediate surface gravity.<ref name=":6">{{cite journal |last1=Cruz |first1=Kelle L. |last2=Kirkpatrick |first2=J. Davy |last3=Burgasser |first3=Adam J. |date=February 2009 |title=Young L Dwarfs Identified in the Field: A Preliminary Low-Gravity, Optical Spectral Sequence from L0 to L5 |journal=The Astronomical Journal |volume=137 |issue=2 |pages=3345–3357 |doi=10.1088/0004-6256/137/2/3345 |arxiv=0812.0364 |bibcode=2009AJ....137.3345C |s2cid=15376964 |issn=0004-6256 }}</ref>
|-
|γ
|Objects with the gamma (γ) suffix (e.g. L5γ) have a low surface gravity.<ref name=":6"/>
|-
|red
|The red suffix (e.g. L0red) indicates objects without signs of youth, but high dust content.<ref name=":7">{{cite journal |last1=Looper |first1=Dagny L. |last2=Kirkpatrick |first2=J. Davy |last3=Cutri |first3=Roc M. |last4=Barman |first4=Travis |last5=Burgasser |first5=Adam J. |last6=Cushing |first6=Michael C. |last7=Roellig |first7=Thomas |last8=McGovern |first8=Mark R. |last9=McLean |first9=Ian S. |last10=Rice |first10=Emily |last11=Swift |first11=Brandon J. |date=October 2008 |title=Discovery of Two Nearby Peculiar L Dwarfs from the 2MASS Proper-Motion Survey: Young or Metal-Rich? |journal=Astrophysical Journal |volume=686 |issue=1 |pages=528–541 |doi=10.1086/591025 |arxiv=0806.1059 |bibcode=2008ApJ...686..528L |s2cid=18381182 |issn=0004-637X }}</ref>
|-
|blue
|The blue suffix (e.g. L3blue) indicates unusual blue near-infrared colors for L dwarfs without obvious low metallicity.<ref name=":8">{{cite journal |last1=Kirkpatrick |first1=J. Davy|last2=Looper |first2=Dagny L. |last3=Burgasser |first3=Adam J. |last4=Schurr |first4=Steven D. |last5=Cutri |first5=Roc M. |last6=Cushing |first6=Michael C. |last7=Cruz |first7=Kelle L. |last8=Sweet |first8=Anne C. |last9=Knapp |first9=Gillian R. |last10=Barman |first10=Travis S. |last11=Bochanski |first11=John J. |date=September 2010 |title=Discoveries from a Near-infrared Proper Motion Survey Using Multi-epoch Two Micron All-Sky Survey Data |journal=Astrophysical Journal Supplement Series |volume=190 |issue=1 |pages=100–146 |doi=10.1088/0067-0049/190/1/100 |arxiv=1008.3591 |bibcode=2010ApJS..190..100K |s2cid=118435904 |issn=0067-0049 }}</ref>
|}
Young brown dwarfs have low [[Surface gravity|surface gravities]] because they have larger radii and lower masses than the field stars of similar spectral type. These sources are noted by a letter ''beta'' (β) for intermediate surface gravity or ''gamma'' (γ) for low surface gravity. Indicators of low surface gravity include weak CaH, K I and Na I lines, as well as a strong VO line.<ref name=":6"/> ''Alpha'' (α) denotes normal surface gravity and is usually dropped. Sometimes an extremely low surface gravity is denoted by a delta (δ).<ref name=":8"/> The suffix "pec" stands for "peculiar"; this suffix is still used for other features that are unusual, and summarizes different properties, indicating low surface gravity, subdwarfs and unresolved binaries.<ref name="Faherty 10">{{cite journal |last1=Faherty |first1=Jacqueline K. |last2=Riedel |first2=Adric R. |last3=Cruz |first3=Kelle L. |last4=Gagne |first4=Jonathan |last5=Filippazzo |first5=Joseph C. |last6=Lambrides |first6=Erini |last7=Fica |first7=Haley |last8=Weinberger |first8=Alycia |last9=Thorstensen |first9=John R. |last10=Tinney |first10=Chris G. |last11=Baldassare |first11=Vivienne |date=July 2016 |title=Population Properties of Brown Dwarf Analogs to Exoplanets |journal=Astrophysical Journal Supplement Series |volume=225 |issue=1 |pages=10 |doi=10.3847/0067-0049/225/1/10 |arxiv=1605.07927 |bibcode=2016ApJS..225...10F |s2cid=118446190 |issn=0067-0049 |doi-access=free }}</ref> The prefix sd stands for [[subdwarf]] and only includes cool subdwarfs. This prefix indicates a low [[metallicity]] and kinematic properties that are more similar to [[Galactic halo|halo]] stars than to [[Thin disk|disk]] stars.<ref name=":5"/> Subdwarfs appear bluer than disk objects.<ref>{{cite web |url=http://www.stsci.edu/~inr/cmd.html |title=Colour-magnitude data |first=Neill |last=Reid <!-- |date=15/07/02 (unless a typo, dates to ca. 2004, otherwise 2002 --> |website=www.stsci.edu |access-date=2020-03-06 }}</ref> The red suffix describes objects with red color, but an older age. This is not interpreted as low surface gravity, but as a high dust content.<ref name=":7"/><ref name=":8"/> The blue suffix describes objects with blue [[near-infrared]] colors that cannot be explained with low metallicity. Some are explained as L+T binaries, others are not binaries, such as [[2MASS J11263991−5003550]] and are explained with thin and/or large-grained clouds.<ref name=":8"/>