Editing Brown dwarf (section)

=== High-mass brown dwarfs versus low-mass stars ===
[[Lithium]] is generally present in brown dwarfs and not in low-mass stars. Stars, which reach the high temperature necessary for fusing hydrogen, rapidly deplete their lithium. Fusion of [[lithium-7]] and a [[proton]] occurs, producing two [[helium-4]] nuclei. The temperature necessary for this reaction is just below that necessary for hydrogen fusion. Convection in low-mass stars ensures that lithium in the whole volume of the star is eventually depleted. Therefore, the presence of the lithium spectral line in a candidate brown dwarf is a strong indicator that it is indeed a substellar object.

==== Lithium test ====
The use of lithium to distinguish candidate brown dwarfs from low-mass stars is commonly referred to as the '''lithium test''', and was pioneered by [[Rafael Rebolo López|Rafael Rebolo]], [[Eduardo Martín]] and [[Antonio Magazzu]]. However, lithium is also seen in very young stars, which have not yet had enough time to burn it all.

Heavier stars, like the Sun, can also retain lithium in their outer layers, which never get hot enough to fuse lithium, and whose convective layer does not mix with the core where the lithium would be rapidly depleted. Those larger stars are easily distinguishable from brown dwarfs by their size and luminosity.

Conversely, brown dwarfs at the high end of their mass range can be hot enough to deplete their lithium when they are young. Dwarfs of mass greater than {{Jupiter mass|65}} can burn their lithium by the time they are half a billion years old;<ref>{{cite journal |last1=Kulkarni |first1=Shrinivas R. |author-link=Shrinivas Kulkarni |title=Brown Dwarfs: A Possible Missing Link Between Stars and Planets |journal=Science |date=30 May 1997 |volume=276 |issue=5317 |pages=1350–1354 |doi=10.1126/science.276.5317.1350 |bibcode=1997Sci...276.1350K }}</ref> thus the lithium test is not perfect.

==== Atmospheric methane ====
Unlike stars, older brown dwarfs are sometimes cool enough that, over very long periods of time, their atmospheres can gather observable quantities of [[methane]], which cannot form in hotter objects. Dwarfs confirmed in this fashion include [[Gliese 229]]B.

==== Iron, silicate and sulfide clouds ====
Main-sequence stars cool, but eventually reach a minimum [[bolometric luminosity]] that they can sustain through steady fusion. This luminosity varies from star to star, but is generally at least 0.01% that of the Sun.{{citation needed|reason=as in talk page|date=April 2013}} Brown dwarfs cool and darken steadily over their lifetimes; sufficiently old brown dwarfs will be too faint to be detectable.
[[File:Brown dwarf clouds.png|thumb|300x300px|Cloud models for the early T-type brown dwarfs [[SIMP J013656.5+093347|SIMP J0136+09]] and [[2MASS J21392676+0220226|2MASS J2139+02]] (left two panels) and the late T-type brown dwarf 2M0050–3322.]]
Clouds are used to explain the weakening of the [[Iron(I) hydride|iron hydride]] (FeH) spectral line in late L-dwarfs. [[Iron]] clouds deplete FeH in the upper atmosphere, and the cloud layer blocks the view to lower layers still containing FeH. The later strengthening of this chemical compound at cooler temperatures of mid- to late T-dwarfs is explained by disturbed clouds that allows a telescope to look into the deeper layers of the atmosphere that still contains FeH.<ref>{{cite journal |last1=Burgasser |first1=Adam J. |last2=Marley |first2=Mark S. |last3=Ackerman |first3=Andrew S. |last4=Saumon |first4=Didier |last5=Lodders |first5=Katharina |last6=Dahn |first6=Conard C. |last7=Harris |first7=Hugh C. |last8=Kirkpatrick |first8=J. Davy |date=2002-06-01 |title=Evidence of Cloud Disruption in the L/T Dwarf Transition |journal=The Astrophysical Journal |volume=571 |issue=2 |pages=L151–L154 |doi=10.1086/341343 |arxiv=astro-ph/0205051 |bibcode=2002ApJ...571L.151B |issn=0004-637X|doi-access=free }}</ref> Young L/T-dwarfs (L2-T4) show high [[Variable star|variability]], which could be explained with clouds, hot spots, magnetically driven [[aurora]]e or [[Thermochemistry|thermochemical]] instabilities.<ref>{{cite journal |last1=Vos |first1=Johanna M. |last2=Faherty |first2=Jacqueline K. |last3=Gagné |first3=Jonathan |last4=Marley |first4=Mark |last5=Metchev |first5=Stanimir |last6=Gizis |first6=John |last7=Rice |first7=Emily L. |last8=Cruz |first8=Kelle |date=2022-01-01 |title=Let the Great World Spin: Revealing the Stormy, Turbulent Nature of Young Giant Exoplanet Analogs with the Spitzer Space Telescope |journal=The Astrophysical Journal |volume=924 |issue=2 |pages=68 |doi=10.3847/1538-4357/ac4502 |arxiv=2201.04711 |bibcode=2022ApJ...924...68V |issn=0004-637X|doi-access=free }}</ref> The clouds of these brown dwarfs are explained as either iron clouds with varying thickness or a lower thick iron cloud layer and an upper [[silicate]] cloud layer. This upper silicate cloud layer can consist out of [[quartz]], [[enstatite]], [[corundum]] and/or [[Forsterite|fosterite]].<ref>{{cite journal |last1=Vos |first1=Johanna M. |last2=Burningham |first2=Ben |last3=Faherty |first3=Jacqueline K. |last4=Alejandro |first4=Sherelyn |last5=Gonzales |first5=Eileen |last6=Calamari |first6=Emily |last7=Bardalez Gagliuffi |first7=Daniella |last8=Visscher |first8=Channon |last9=Tan |first9=Xianyu |last10=Morley |first10=Caroline V. |last11=Marley |first11=Mark |last12=Gemma |first12=Marina E. |last13=Whiteford |first13=Niall |last14=Gaarn |first14=Josefine |last15=Park |first15=Grace |date=2023-02-01 |title=Patchy Forsterite Clouds in the Atmospheres of Two Highly Variable Exoplanet Analogs |journal=The Astrophysical Journal |volume=944 |issue=2 |pages=138 |doi=10.3847/1538-4357/acab58 |arxiv=2212.07399 |bibcode=2023ApJ...944..138V |issn=0004-637X|doi-access=free }}</ref><ref>{{cite journal |last1=Manjavacas |first1=Elena |last2=Karalidi |first2=Theodora |last3=Vos |first3=Johanna M. |last4=Biller |first4=Beth A. |last5=Lew |first5=Ben W. P. |date=2021-11-01 |title=Revealing the Vertical Cloud Structure of a Young Low-mass Brown Dwarf, an Analog to the β-Pictoris b Directly Imaged Exoplanet, through Keck I/MOSFIRE Spectrophotometric Variability |journal=The Astronomical Journal |volume=162 |issue=5 |pages=179 |doi=10.3847/1538-3881/ac174c |arxiv=2107.12368 |bibcode=2021AJ....162..179M |issn=0004-6256|doi-access=free }}</ref> It is however not clear if silicate clouds are always necessary for young objects.<ref>{{cite journal |last1=Tremblin |first1=P. |last2=Chabrier |first2=G. |last3=Baraffe |first3=I. |last4=Liu |first4=Michael. C. |last5=Magnier |first5=E. A. |last6=Lagage |first6=P. -O. |last7=Alves de Oliveira |first7=C. |last8=Burgasser |first8=A. J. |last9=Amundsen |first9=D. S. |last10=Drummond |first10=B. |date=2017-11-01 |title=Cloudless Atmospheres for Young Low-gravity Substellar Objects |journal=The Astrophysical Journal |volume=850 |issue=1 |pages=46 |doi=10.3847/1538-4357/aa9214 |arxiv=1710.02640 |bibcode=2017ApJ...850...46T |issn=0004-637X|doi-access=free }}</ref> Silicate absorption can be directly observed in the [[Infrared astronomy|mid-infrared]] at 8 to 12 μm. Observations with [[Spitzer Space Telescope#Instruments|Spitzer IRS]] have shown that silicate absorption is common, but not ubiquitous, for L2-L8 dwarfs.<ref>{{cite journal |last1=Suárez |first1=Genaro |last2=Metchev |first2=Stanimir |date=2022-07-01 |title=Ultracool dwarfs observed with the Spitzer infrared spectrograph - II. Emergence and sedimentation of silicate clouds in L dwarfs, and analysis of the full M5-T9 field dwarf spectroscopic sample |journal=Monthly Notices of the Royal Astronomical Society |volume=513 |issue=4 |pages=5701–5726 |doi=10.1093/mnras/stac1205 |doi-access=free |issn=0035-8711|arxiv=2205.00168 |bibcode=2022MNRAS.513.5701S }}</ref> Additionally, [[Mid-Infrared Instrument|MIRI]] has observed silicate absorption in the planetary-mass companion [[VHS J1256–1257|VHS 1256b]].<ref>{{cite journal |last1=Miles |first1=Brittany E. |last2=Biller |first2=Beth A. |last3=Patapis |first3=Polychronis |last4=Worthen |first4=Kadin |last5=Rickman |first5=Emily |last6=Hoch |first6=Kielan K. W. |last7=Skemer |first7=Andrew |last8=Perrin |first8=Marshall D. |last9=Whiteford |first9=Niall |last10=Chen |first10=Christine H. |last11=Sargent |first11=B. |last12=Mukherjee |first12=Sagnick |last13=Morley |first13=Caroline V. |last14=Moran |first14=Sarah E. |last15=Bonnefoy |first15=Mickael |date=2023-03-01 |title=The JWST Early-release Science Program for Direct Observations of Exoplanetary Systems II: A 1 to 20 μm Spectrum of the Planetary-mass Companion VHS 1256-1257 b |journal=The Astrophysical Journal |volume=946 |issue=1 |pages=L6 |doi=10.3847/2041-8213/acb04a |arxiv=2209.00620 |bibcode=2023ApJ...946L...6M |issn=0004-637X|doi-access=free }}</ref>

'''Iron rain''' as part of atmospheric convection processes is possible only in brown dwarfs, and not in small stars. The spectroscopy research into iron rain is still ongoing, but not all brown dwarfs will always have this atmospheric anomaly. In 2013, a heterogeneous iron-containing atmosphere was imaged around the B component in the nearby Luhman 16 system.<ref>{{cite journal |last1=Biller |first1=Beth A. |last2=Crossfield |first2=Ian J. M. |last3=Mancini |first3=Luigi |last4=Ciceri |first4=Simona |last5=Southworth |first5=John |last6=Kopytova |first6=Taisiya G. |last7=Bonnefoy |first7=Mickaël |last8=Deacon |first8=Niall R. |last9=Schlieder |first9=Joshua E. |last10=Buenzli |first10=Esther |last11=Brandner |first11=Wolfgang |last12=Allard |first12=France |last13=Homeier |first13=Derek |last14=Freytag |first14=Bernd |last15=Bailer-Jones |first15=Coryn A. L. |last16=Greiner |first16=Jochen |last17=Henning |first17=Thomas |last18=Goldman |first18=Bertrand |title=Weather on the Nearest Brown Dwarfs: Resolved Simultaneous Multi-Wavelength Variability Monitoring of WISE J104915.57–531906.1AB |journal=[[The Astrophysical Journal Letters]] |date=6 November 2013 |volume=778| issue=1 |pages=L10 |doi=10.1088/2041-8205/778/1/l10 |arxiv=1310.5144 |bibcode=2013ApJ...778L..10B |s2cid=56107487 }}</ref>

For late T-type brown dwarfs only a few variable searches were carried out. Thin cloud layers are predicted to form in late T-dwarfs from [[chromium]] and [[potassium chloride]], as well as several [[sulfide]]s. These sulfides are [[Manganese(II) sulfide|manganese sulfide]], [[sodium sulfide]] and [[zinc sulfide]].<ref>{{cite journal |last1=Morley |first1=Caroline V. |last2=Fortney |first2=Jonathan J. |last3=Marley |first3=Mark S. |last4=Visscher |first4=Channon |last5=Saumon |first5=Didier |last6=Leggett |first6=S. K. |date=2012-09-01 |title=Neglected Clouds in T and Y Dwarf Atmospheres |url=https://ui.adsabs.harvard.edu/abs/2012ApJ...756..172M |journal=The Astrophysical Journal |volume=756 |issue=2 |pages=172 |doi=10.1088/0004-637X/756/2/172 |issn=0004-637X|arxiv=1206.4313 |bibcode=2012ApJ...756..172M |s2cid=118398946 }}</ref> The variable T7 dwarf [[2M0050–3322]] is explained to have a top layer of potassium chloride clouds, a mid layer of sodium sulfide clouds and a lower layer of manganese sulfide clouds. Patchy clouds of the top two cloud layers could explain why the methane and water vapor bands are variable.<ref>{{cite journal |last1=Manjavacas |first1=Elena |last2=Karalidi |first2=Theodora |last3=Tan |first3=Xianyu |last4=Vos |first4=Johanna M. |last5=Lew |first5=Ben W. P. |last6=Biller |first6=Beth A. |last7=Oliveros-Gómez |first7=Natalia |date=2022-08-01 |title=Top-of-the-atmosphere and Vertical Cloud Structure of a Fast-rotating Late T Dwarf |journal=The Astronomical Journal |volume=164 |issue=2 |pages=65 |doi=10.3847/1538-3881/ac7953 |arxiv=2206.07566 |bibcode=2022AJ....164...65M |issn=0004-6256|doi-access=free }}</ref>

At the lowest temperatures of the Y-dwarf [[WISE 0855−0714|WISE 0855-0714]] patchy cloud layers of sulfide and [[Ice|water ice]] clouds could cover 50% of the surface.<ref>{{cite journal |last1=Faherty |first1=Jacqueline K. |last2=Tinney |first2=C. G. |last3=Skemer |first3=Andrew |last4=Monson |first4=Andrew J. |date=2014-09-01 |title=Indications of Water Clouds in the Coldest Known Brown Dwarf |url=https://ui.adsabs.harvard.edu/abs/2014ApJ...793L..16F |journal=The Astrophysical Journal |volume=793 |issue=1 |pages=L16 |doi=10.1088/2041-8205/793/1/L16 |issn=0004-637X|arxiv=1408.4671 |bibcode=2014ApJ...793L..16F |s2cid=119246100 }}</ref>