Editing Brown dwarf (section)

=== Brown dwarf–brown dwarf binaries ===
[[File:Hubble Space Telescope - Brown Dwarf Binaries.gif|thumb|Multi-epoch images of brown dwarf binaries taken with the [[Hubble Space Telescope]]. The binary Luhman 16 AB (left) is closer to the Solar System than the other examples shown here.]]
Brown dwarfs binaries of type M, L, and T are less common with a lower mass of the primary.<ref>{{cite web |title=Are the Coolest Brown Dwarfs Loners? |url=https://www.noirlab.edu/public/announcements/geminiann16003/ |access-date=2023-04-16 |website=www.noirlab.edu }}</ref> L-dwarfs have a binary fraction of about {{Val|24|6|2}}% and the binary fraction for late T, early Y-dwarfs (T5-Y0) is about {{val|8|6|u=%}}.<ref name=":11">{{cite journal |last1=Fontanive |first1=Clémence |last2=Biller |first2=Beth |last3=Bonavita |first3=Mariangela |last4=Allers |first4=Katelyn |date=2018-09-01 |title=Constraining the multiplicity statistics of the coolest brown dwarfs: binary fraction continues to decrease with spectral type |journal=Monthly Notices of the Royal Astronomical Society |volume=479 |issue=2 |pages=2702–2727 |doi=10.1093/mnras/sty1682 |doi-access=free |arxiv=1806.08737 |bibcode=2018MNRAS.479.2702F |issn=0035-8711}}</ref>

Brown dwarf binaries have a higher companion-to-host ratio <math>q=M_B/M_A</math> for lower mass binaries. Binaries with a [[Red dwarf|M-type star]] as a primary have for example a broad distribution of ''q'' with a preference of ''q''&nbsp;≥&nbsp;0.4. Brown dwarfs on the other hand show a strong preference for ''q''&nbsp;≥&nbsp;0.7. The separation is decreasing with mass: M-type stars have a separation peaking at 3–30 [[astronomical unit]]s (au), M-L-type brown dwarfs have a projected separation peaking at 5–8 au and T5–Y0 objects have a projected separation that follows a [[Log-normal distribution|lognormal distribution]] with a peak separation of about 2.9&nbsp;au.<ref name=":11" />

An example is the closest brown dwarf binary Luhman 16 AB with a primary L7.5 dwarf and a separation of 3.5&nbsp;au and ''q''&nbsp;=&nbsp;0.85. The separation is on the lower end of the expected separation for M-L-type brown dwarfs, but the mass ratio is typical.

It is not known if the same trend continues with Y-dwarfs, because their sample size is so small. The Y+Y dwarf binaries should have a high mass ratio q and a low separation, reaching scales of less than one au.<ref>{{cite journal |last1=Opitz |first1=Daniela |last2=Tinney |first2=C. G. |last3=Faherty |first3=Jacqueline |last4=Sweet |first4=Sarah |last5=Gelino |first5=Christopher R. |last6=Kirkpatrick |first6=J. Davy |date=2016-02-24 |title=Searching for Binary Y dwarfs with the Gemini Multi-Conjugate Adaptive Optics System (GeMS) |journal=The Astrophysical Journal |volume=819 |issue=1 |pages=17 |doi=10.3847/0004-637X/819/1/17 |arxiv=1601.05508 |bibcode=2016ApJ...819...17O |s2cid=3208550 |issn=1538-4357 |doi-access=free }}</ref> In 2023, the Y+Y dwarf [[WISE J0336−0143|WISE J0336-0143]] was confirmed as a binary with [[James Webb Space Telescope|JWST]], with a mass ratio of q=0.62±0.05 and a separation of 0.97 astronomical units. The researchers point out that the sample size of low-mass binary brown dwarfs is too small to determine if WISE J0336-0143 is a typical representative of low-mass binaries or a peculiar system.<ref name=":14">{{cite journal |last1=Calissendorff |first1=Per |last2=De Furio |first2=Matthew |last3=Meyer |first3=Michael |last4=Albert |first4=Loïc |last5=Aganze |first5=Christian |last6=Ali-Dib |first6=Mohamad |last7=Gagliuffi |first7=Daniella C. Bardalez |last8=Baron |first8=Frederique |last9=Beichman |first9=Charles A. |last10=Burgasser |first10=Adam J. |last11=Cushing |first11=Michael C. |last12=Faherty |first12=Jacqueline Kelly |last13=Fontanive |first13=Clémence |last14=Gelino |first14=Christopher R. |last15=Gizis |first15=John E. |date=2023-03-29 |title=JWST/NIRCam Discovery of the First Y+Y Brown Dwarf Binary: WISE J033605.05–014350.4 |journal=The Astrophysical Journal Letters |volume=947 |issue=2 |pages=L30 |doi=10.3847/2041-8213/acc86d |arxiv=2303.16923 |bibcode=2023ApJ...947L..30C |s2cid=257833714 |doi-access=free }}</ref>

Observations of the orbit of binary systems containing brown dwarfs can be used to measure the mass of the brown dwarf. In the case of [[2MASSW J0746425+2000321]], the secondary weighs 6% of the solar mass. This measurement is called a dynamical mass.<ref>{{cite press release |first=Hervé |last=Bouy |url=https://www.eso.org/public/news/eso0420/ |title=Weighing Ultra-Cool Stars – Large Ground-Based Telescopes and Hubble Team-Up to Perform First Direct Brown Dwarf Mass Measurement |publisher=European Southern Observatory |access-date=2019-12-11 }}</ref><ref>{{cite journal |last1=Bouy |first1=Hervé |last2=Duchêne |first2=Gaspard |last3=Köhler |first3=Rainer |last4=Brandner |first4=Wolfgang |last5=Bouvier |first5=Jérôme |last6=Martín |first6=Eduardo L. |last7=Ghez |first7=Andrea Mia |last8=Delfosse |first8=Xavier |last9=Forveille |first9=Thierry |last10=Allard |first10=France |last11=Baraffe |first11=Isabelle |first12=Gibor |last12=Basri |first13=Laird M. |last13=Close |first14=Caer E. |last14=McCabe |date=2004-08-01 |title=First determination of the dynamical mass of a binary L dwarf |journal=Astronomy & Astrophysics |volume=423 |issue=1 |pages=341–352 |arxiv=astro-ph/0405111 |doi=10.1051/0004-6361:20040551 |issn=0004-6361 |bibcode=2004A&A...423..341B |s2cid=3149721 }}</ref> The brown dwarf system closest to the Solar System is the binary Luhman 16. It was attempted to search for planets around this system with a similar method, but none were found.<ref>{{cite journal |last1=Bedin |first1=Luigi R. |last2=Pourbaix |first2=Dimitri |last3=Apai |first3=Dániel |last4=Burgasser |first4=Adam J. |last5=Buenzli |first5=Esther |last6=Boffin |first6=Henri M. J. |last7=Libralato |first7=Mattia |date=2017-09-01 |title=Hubble Space Telescope astrometry of the closest brown dwarf binary system – I. Overview and improved orbit |url=https://academic.oup.com/mnras/article/470/1/1140/3896221 |journal=Monthly Notices of the Royal Astronomical Society |volume=470 |issue=1 |pages=1140–1155 |arxiv=1706.00657 |doi=10.1093/mnras/stx1177 |doi-access=free |issn=0035-8711 |hdl=10150/625503 |s2cid=119385778 }}</ref>