Editing Brown dwarf (section)

==== 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" />