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{{Short description|Star of luminosity class VI under the Yerkes spectral classification system}}
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A '''subdwarf''', sometimes denoted by "sd", is a star with [[luminosity class]]&nbsp;VI under the [[stellar classification#Yerkes spectral classification|Yerkes spectral classification]] system. They are defined as [[star]]s with [[luminosity]]&nbsp;1.5 to 2 [[Absolute magnitude|magnitudes]] lower than that of main-sequence stars of the same [[spectral type]]. On a [[Hertzsprung–Russell diagram]] subdwarfs appear to lie below the [[main sequence]].{{efn|name=no_mark_HR_note}}

The term "subdwarf" was coined by [[Gerard Kuiper]] in 1939, to refer to a series of stars with anomalous spectra that were previously labeled as "intermediate [[white dwarfs]]".<ref name=Croswell-1995/>{{rp|style=ama|p= 87}}

Since [[Gerard Kuiper|Kuiper]] coined the term, the subdwarf type has been extended to lower-mass stars than were known at the time. Astronomers have also discovered an entirely different group of blue-white subdwarfs, making two distinct categories:
* [[#cool_subdwarf_anchor|Cool subdwarfs]]
* [[#hot_subdwarf_anchor|Hot subdwarfs]]{{efn|
name=no_mark_HR_note|
[[#hot_subdwarf_anchor|Hot subdwarfs]] are as yet fairly rarely seen stars, and their place on the [[Hertzsprung–Russell diagram|HR diagram]] at present is usually unmarked. Their place would be a streak lower than the main sequence, under the label "sequence" on the HR diagram at the right.
}}

== <span class="anchor" id="cool_subdwarf_anchor">Cool (red) subdwarfs</span> ==
{{anchor|cool subdwarf|cool subdwarfs|cool subdwarf star|cool subdwarf stars}}
Like ordinary [[main sequence|main-sequence]] stars, cool subdwarfs (of spectral types&nbsp;G to M) produce their energy from [[hydrogen]] [[nuclear fusion|fusion]]. The explanation of their underluminosity lies in their low [[metallicity]]: These stars are not enriched in elements heavier than [[helium]]. The lower metallicity decreases the [[Opacity (optics)|opacity]] of their outer layers and decreases the [[radiation pressure]], resulting in a smaller, hotter star for a given mass.<ref>{{cite book |author-link=James B. Kaler |first=James |last=Kaler |year=1989 |title=Stars and their Spectra |place=Cambridge, UK |publisher=Cambridge UP |page=122}}</ref> This lower opacity also allows them to emit a higher percentage of [[ultraviolet]] light for the same [[spectral type]] relative to a [[Population I]] star, a feature known as the [[ultraviolet excess]].<ref name=Croswell-1995>{{cite book |author=Croswell, K. |author-link=Ken Croswell |year=1995 |title=The Alchemy of the Heavens |title-link=The Alchemy of the Heavens |place=New York, NY |publisher=Oxford UP |pages=87–92}}</ref>{{rp|style=ama|p= 87–92}}
Usually members of the Milky Way's [[Galactic halo|halo]], they frequently have high space velocities relative to the [[Sun]].<ref name=Burningham-Smith-etal-2014>{{cite journal |last1=Burningham |first1=Ben |last2=Smith |first2=L. |last3=Cardoso |first3=C.V. |last4=Lucas |first4=P.W. |last5=Burgasser |first5=Adam J. |last6=Jones |first6=H.R.A. |last7=Smart |first7=R.L. |date=May 2014 |title=The discovery of a T6.5 subdwarf |journal=[[Monthly Notices of the Royal Astronomical Society]] |lang=en |volume=440 |issue=1 |pages=359–364 |doi=10.1093/mnras/stu184 |doi-access=free |arxiv=1401.5982 |bibcode=2014MNRAS.440..359B |issn=0035-8711}}</ref>

Cool subdwarfs of spectral type&nbsp;L and T exist, for example [[ULAS J131610.28+075553.0]] with spectral type&nbsp;sdT6.5.<ref name=Burningham-Smith-etal-2014/>

Subclasses of cool subdwarfs are as following:<ref>{{cite journal |last1=Burgasser |first1=Adam J. |last2=Kirkpatrick |first2=J. Davy |year=2006 |title=Discovery of the coolest extreme subdwarf |journal=[[The Astrophysical Journal]] |volume=645 |issue=2 |pages=1485–1497 |bibcode=2006ApJ...645.1485B |doi=10.1086/504375 |arxiv=astro-ph/0603382 |s2cid=10911965}}</ref><ref name=Lépine-Rich-Shara-2007>{{cite journal |last1=Lépine |first1=Sébastien |last2=Rich |first2=R. Michael |last3=Shara |first3=Michael M. |date=November 2007 |title=Revised metallicity classes for low-mass stars: Dwarfs (dM), subdwarfs (sdM), extreme Subdwarfs (esdM), and ultrasubdwarfs (usdM) |journal=[[Astrophysical Journal]] |lang=en |volume=669 |issue=2 |pages=1235–1247 |doi=10.1086/521614 |doi-access=free |arxiv=0707.2993 |bibcode=2007ApJ...669.1235L |issn=0004-637X}}</ref>
; cool subdwarf:  Examples: [[Kapteyn's Star]] (sdM1), [[GJ 1062]] (sdM2.5)
; extreme subdwarf:  Example: [[APMPM J0559-2903]] (esdM7)<ref>{{cite journal |last1=Schweitzer |first1=A. |last2=Scholz |first2=R.-D. |last3=Stauffer |first3=J. |last4=Irwin |first4=M. |last5=McCaughrean |first5=M.J. |year=1999 |title=APMPM&nbsp;J0559-2903: The coolest extreme subdwarf known |journal=[[Astronomy and Astrophysics]] |volume=350 |page=L62 |bibcode=1999A&A...350L..62S}}</ref>
; ultrasubdwarf: Example: [[LSPM J0822+1700]] (usdM7.5)<ref name=Lépine-Rich-Shara-2007/>

=== Subdwarfs of type L, T and Y ===
The low [[metallicity]] of subdwarfs is coupled with their old age. The early universe had a low content of elements heavier than helium and formed stars and [[Brown dwarf|brown dwarfs]] with lower metallicity. Only later [[supernova]]e, [[planetary nebula]]e and [[neutron star merger]]s enriched the universe with heavier elements. The old subdwarfs belong therefore often to the older structures in our Milky Way, mainly the [[thick disk]] and the [[galactic halo]]. Objects in the thick disk or the halo have a high space velocity compared to the [[Sun]], which belongs to the younger [[thin disk]]. A high [[proper motion]] can be used to discover subdwarfs. Additionally the subdwarfs have spectral features that make them different from subdwarfs with solar metallicity. All subdwarfs share the suppression of the near-infrared spectrum, mainly the [[Photometric system|H-band]] and K-band. The low metallicity increase the [[Collision-induced absorption and emission|collision induced absorption]] of [[hydrogen]], causing this suppressed near-infrared spectrum. This is seen as blue infrared colors compared to brown dwarfs with solar metallicity. The low metallicity also change other absorption features, such as deeper [[Calcium monohydride|CaH]] and [[Titanium(II) oxide|TiO]] bands at 0.7&nbsp;μm in L-subdwarfs, a weaker [[Vanadium(II) oxide|VO]] band at 0.8&nbsp;μm in early L-subdwarfs and stronger [[Iron(I) hydride|FeH]] band at 0.99&nbsp;μm for mid- to late L-subdwarfs.<ref name=Zhang-Pinfield-etal-2017>{{cite journal |last1=Zhang |first1=Z.H. |last2=Pinfield |first2=D.J. |last3=Gálvez-Ortiz |first3=M.C. |last4=Burningham |first4=B. |last5=Lodieu |first5=N. |last6=Marocco |first6=F. |last7=Burgasser |first7=A.J. |last8=Day-Jones |first8=A. C. |last9=Allard |first9=F. |last10=Jones |first10=H.R.A. |last11=Homeier |first11=D. |display-authors=6 |date=January 2017 |title=Primeval very low-mass stars and brown dwarfs - I. Six new L&nbsp;subdwarfs, classification and atmospheric properties |bibcode=2017MNRAS.464.3040Z |journal=[[Monthly Notices of the Royal Astronomical Society]] |volume=464 |issue=3 |pages=3040–3059 |doi=10.1093/mnras/stw2438 |doi-access=free |issn=0035-8711 |arxiv=1609.07181}}</ref> [[2MASS J05325346+8246465|2MASS J0532+8246]] was discovered in 2003 as the first L-type subdwarf,<ref name=Burgasser-Kirkpatrick-etal-2003>{{cite journal |last1=Burgasser |first1=Adam J. |last2=Kirkpatrick |first2=J. Davy |last3=Burrows |first3=Adam |last4=Liebert |first4=James |last5=Reid |first5=I. Neill |last6=Gizis |first6=John E. |last7=McGovern |first7=Mark R. |last8=Prato |first8=L. |last9=McLean |first9=Ian S. |display-authors=6 |date=August 2003 |title=The first substellar subdwarf? Discovery of a metal-poor L&nbsp;dwarf with halo kinematics |journal=[[The Astrophysical Journal]] |volume=592 |issue=2 |pages=1186–1192 |doi=10.1086/375813 |bibcode=2003ApJ...592.1186B |arxiv=astro-ph/0304174 |s2cid=11895472 |issn=0004-637X}} </ref> which was later re-classified as an extreme subdwarf.<ref name=Zhang-Pinfield-etal-2017/> The L-type subdwarfs have subtypes similar to M-type subdwarfs: The subtypes subdwarf (sd), extreme subdwarfs (esd) and ultra subdwarfs (usd), which are defined by their decreasing [[metallicity]], compared to solar metallicity, which is defined on a [[logarithmic scale]]:<ref name=Zhang-Pinfield-etal-2017/>
* subdwarfs have <math>\ -1.0 < \bigl[ \tfrac{ \mathsf{Fe} }{ \mathsf{H} } \bigr]_\star \leq -0.3\ ,</math>
* extreme subdwarfs have <math>\ -1.7 < \bigl[ \tfrac{ \mathsf{Fe} }{\mathsf{H} } \bigr]_\star \leq -1.0\ ,</math> and
* ultra subdwarfs have <math>\ \bigl[ \tfrac{ \mathsf{Fe} }{\mathsf{H} } \bigr]_\star \leq -1.7 ~.</math>
* The [[Sun]] sets the scale at <math>\ \bigl[ \tfrac{\mathsf{Fe} }{\mathsf{H}} \bigr]_\odot \equiv 0\ ,</math> by definition.

For T-type subdwarfs only a small sample of subdwarfs and extreme subdwarfs is known.<ref name=Schneider-Burgasser-etal-2020>{{cite journal |last1=Schneider |first1=Adam C. |last2=Burgasser |first2=Adam J. |last3=Gerasimov |first3=Roman |last4=Marocco |first4=Federico |last5=Gagne |first5=Jonathan |last6=Goodman |first6=Sam |last7=Beaulieu |first7=Paul |last8=Pendrill |first8=William |last9=Rothermich |first9=Austin |last10=Sainio |first10=Arttu |last11=Kuchner |first11=Marc J. |display-authors=6 |date=2020-07-24 |df=dmy-all |title=WISEA J041451.67-585456.7 and WISEA J181006.18-101000.5: The first extreme T-type subdwarfs? |journal=[[The Astrophysical Journal]] |volume=898 |issue=1 |page=77 |doi=10.3847/1538-4357/ab9a40 |bibcode=2020ApJ...898...77S |s2cid=220403370 |arxiv=2007.03836 |issn=1538-4357 |doi-access=free }}</ref>

[[2MASS J09373487+2931409|2MASSI J0937347+293142]] is the first object that was discovered in 2002 as a T-type subdwarf candidate<ref name=Burgasser-Kirkpatrick-etal-2003/> and in 2006 it was confirmed to have low metallicity.<ref>{{cite journal |last1=Burgasser |first1=Adam J. |last2=Burrows |first2=Adam |last3=Kirkpatrick |first3=J. Davy |year=2006 |title=Method for determining the physical properties of the coldest known brown dwarfs |journal=[[The Astrophysical Journal]] |volume=639 |issue=2 |pages=1095–1113 |doi=10.1086/499344 |bibcode=2006ApJ...639.1095B |arxiv=astro-ph/0510707 |s2cid=9291848 |issn=0004-637X}}</ref> The first two extreme subdwarfs of type&nbsp;T were discovered in 2020 by scientists and volunteers of the [[Backyard Worlds]] project. The first extreme subdwarfs of type&nbsp;T are [[WISEA 0414−5854]] and [[WISEA 1810−1010]].<ref name=Schneider-Burgasser-etal-2020/> Subdwarfs of type&nbsp;T and Y have less [[methane]] in their atmosphere, due to the lower concentration of [[carbon]] in these subdwarfs. This leads to a bluer W1-W2 ([[Wide-field Infrared Survey Explorer|WISE]]) or ch1-ch2 ([[Spitzer Space Telescope|Spitzer]]) color, compared to objects with similar temperature, but with solar metallicity.<ref>{{cite journal |last1=Meisner |first1=Aaron M. |last2=Schneider |first2=Adam C. |last3=Burgasser |first3=Adam J. |last4=Marocco|first4=Federico |last5=Line |first5=Michael R. |last6=Faherty |first6=Jacqueline K. |author6-link=Jackie Faherty |last7=Kirkpatrick |first7=J. Davy |last8=Caselden |first8=Dan |last9=Kuchner |first9=Marc J. |last10=Gelino |first10=Christopher R. |last11=Gagne |first11=Jonathan |display-authors=6 |date=2021-06-02 |df=dmy-all |title=New Candidate Extreme T Subdwarfs from the Backyard Worlds: Planet 9 Citizen Science Project |journal=The Astrophysical Journal |volume=915 |issue=2 |page=120 |doi=10.3847/1538-4357/ac013c |arxiv=2106.01387 |doi-access=free |bibcode=2021ApJ...915..120M }}</ref> The color of T-types as a single classification criterion can be misleading. The closest [[List of directly imaged exoplanets|directly imaged]] exoplanet, [[COCONUTS-2b]], was first classified as a subdwarf of type&nbsp;T due to its color, while not showing a high tangential velocity. Only in 2021 it was identified as an exoplanet.<ref>{{Cite journal |last1=Zhang |first1=Zhoujian |last2=Liu |first2=Michael C. |last3=Claytor |first3=Zachary R. |last4=Best |first4=William M.J. |last5=Dupuy |first5=Trent J. |last6=Siverd |first6=Robert J. |date=2021-07-01 |title=The second discovery from the COCONUTS Program: A cold wide-orbit exoplanet around a young field M&nbsp;dwarf at 10.9&nbsp;pc |journal=[[The Astrophysical Journal Letters]] |volume=916 |issue=2 |page=L11 |doi=10.3847/2041-8213/ac1123 |arxiv=2107.02805 |bibcode=2021ApJ...916L..11Z |s2cid=236464073 |doi-access=free }}</ref>

The first Y-type subdwarf candidate was discovered in 2021, the brown dwarf [[WISE 1534–1043]], which shows a moderate red [[Spitzer Space Telescope]] color (ch1-ch2 = 0.925±0.039&nbsp;mag). The very red color between J and ch2 (J-ch2 > 8.03&nbsp;mag) and the absolute brightness would suggest a much redder ch1-ch2 color of about 2.4 to 3 mag. Due to the agreement with new subdwarf models, together with the high tangential velocity of 200&nbsp;km/s, Kirkpatrick, Marocco ''et al''. (2021) argue that the most likely explanation is a cold very low-metal brown dwarf, maybe the first subdwarf of type&nbsp;Y.<ref>{{cite journal |last1=Kirkpatrick |first1=J. Davy |last2=Marocco |first2=Federico |last3=Caselden |first3=Dan |last4=Meisner |first4=Aaron M. |last5=Faherty|first5=Jacqueline K. |last6=Schneider |first6=Adam C. |last7=Kuchner |first7=Marc J. |last8=Casewell |first8=S.L. |last9=Gelino |first9=Christopher R. |last10=Cushing |first10=Michael C. |last11=Eisenhardt |first11=Peter R. |display-authors=6 |date=June 2021 |title=The enigmatic brown dwarf WISEA&nbsp;J153429.75-104303.3 (a.k.a. "the Accident") |journal=[[The Astrophysical Journal Letters]] |lang=en |volume=915 |issue=1 |page=L6 |doi=10.3847/2041-8213/ac0437 |arxiv=2106.13408 |bibcode=2021ApJ...915L...6K |s2cid=235651911 |issn=2041-8205 |doi-access=free }}</ref>

Binaries can help to determine the age and mass of these subdwarfs. The subdwarf [[VVV 1256−62|VVV 1256−62B]] (sdL3) was discovered as a companion to a [[Galactic halo|halo]] [[white dwarf]], allowing the age to be measured at 8.4 to 13.8 billion years. It has a mass of 84 to 87 {{Jupiter mass|link=true}}, making VVV 1256−62B likely a [[red dwarf]] star.<ref name="Zhang2024">{{Cite journal |journal=MNRAS |last=Zhang |first=Z. H. |title=Primeval very low-mass stars and brown dwarfs -- VIII. The first age benchmark L subdwarf, a wide companion to a halo white dwarf |date=2024-07-27 |last2=Raddi |first2=R. |last3=Burgasser |first3=A. J. |last4=Casewell |first4=S. L. |last5=Smart |first5=R. L. |last6=Galvez-Ortiz |first6=M. C. |last7=Jones |first7=H. R. A. |last8=Baig |first8=S. |last9=Lodieu |first9=N.|first10=B. |last10=Gauza |first11=Ya. V. |last11=Pavlenko |first12=Y. F. |last12=Jiao |first13=Z. K. |last13=Zhao |first14=S. Y. |last14=Zhou |first15=D. J. |last15=Pinfield|arxiv=2407.19219}}</ref> The subdwarf [[Wolf 1130|Wolf 1130C]] (sdT8) is the companion of an old subdwarf-white dwarf binary, which is estimated to be older than 10 billion years. It has a mass of 44.9 {{Jupiter mass}}, making it a brown dwarf.

=== Examples of cool subdwarfs ===
* [[Kapteyn's Star]]
* [[Groombridge 1830]]
* [[Mu Cassiopeiae]]
* [[2MASS J05325346+8246465]], a possible [[galactic halo|halo]] [[brown dwarf]] and the first substellar subdwarf.<ref name=Burgasser-Kirkpatrick-etal-2003/>
* [[SSSPM J1549-3544]]

== Hot (blue) subdwarfs<span class="anchor" id="hot subdwarf anchor"></span><span class="anchor" id="hot subdwarf"></span><span class="anchor" id="hot subdwarfs"></span><span class="anchor" id="hot subdwarf star"></span><span class="anchor" id="hot subdwarf stars"></span> ==
{{Main|B-type subdwarf|O-type subdwarf}}
Hot subdwarfs, of bluish spectral types&nbsp;O and B are an entirely different class of object than cool subdwarfs; they are also called ''"extreme [[horizontal branch|horizontal-branch]] stars"''.
Hot subdwarf stars represent a late stage in the evolution of some stars, caused when a [[red giant]] star loses its outer [[hydrogen]] layers before the core begins to fuse [[helium]].

The reasons for their premature loss of their hydrogen envelope are unclear, but the interaction of stars in a [[binary star]] system is thought to be one of the main mechanisms. Single subdwarfs may be the result of a merger of two [[white dwarf]]s or gravitational influence from substellar companions. B-type subdwarfs, being more luminous than white dwarfs, are a significant component in the hot star population of old stellar systems, such as [[globular cluster]]s and [[elliptical galaxy|elliptical galaxies]].<ref>{{cite journal |last1=Jeffery |first1=C. Simon |year=2005 |title=Pulsations in Subdwarf B Stars |journal=[[Journal of Astrophysics and Astronomy]] |volume=26 |issue=2–3 |pages=261–271 |bibcode=2005JApA...26..261J |doi=10.1007/BF02702334 |s2cid=13814916}}</ref><ref>{{cite journal |last1=Geier |first1=S. |last2=Edelmann |first2=H. |last3=Heber |first3=U. |last4=Morales-Rueda |first4=L. |year=2009 |title=Discovery of a close substellar companion to the hot subdwarf star HD&nbsp;149382 — the decisive influence of substellar objects on late stellar evolution |journal=[[The Astrophysical Journal Letters]] |volume=702 |issue=1 |pages=L96–L99  |arxiv=0908.1025 |bibcode=2009ApJ...702L..96G |doi=10.1088/0004-637X/702/1/L96 |s2cid=119282460}}</ref>

===Heavy metal subdwarfs===
{{anchor|heavy metal subdwarfs|heavy metal subdwarf star|heavy metal subdwarf stars}}
{{expand section|date=March 2024}}
The heavy metal subdwarfs are a type of hot subdwarf star with high concentrations of [[heavy metals]]. The metals detected include [[germanium]], [[strontium]], [[yttrium]], [[zirconium]] and [[lead]]. Known heavy metal subdwarfs include [[HE 2359-2844]], [[LS IV-14 116]], and [[HE 1256-2738]].<ref>{{cite news |title=Astronomers discover two heavy metal stars |date=2 August 2013 |department=Astronomy |website=Sci-News.com |url=http://www.sci-news.com/astronomy/science-heavy-metal-stars-01283.html |access-date=5 November 2016}}</ref>

== Footnotes ==
{{notelist}}

== References ==
{{reflist|25em}}

{{Star}}

[[Category:Star types]]
[[Category:Subdwarfs| ]]