View source for T Tauri star

{{short description|Class of young variable stars}}
{{about|the type of variable star|the particular variable star called "T Tauri"|T Tauri}}
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[[File:TTauLightCurve.png|thumb|A 160 year long [[Photometric system#Photometric letters|visual band]] [[light curve]] for T Tauri, the prototype for the class, plotted from AAVSO data]]
{{Star formation}}

'''T Tauri stars''' ('''TTS''') are a class of [[variable star]]s that are less than about ten million years old.<ref name=appenzeller/>  This class is named after the prototype, [[T Tauri]], a young star in the [[Taurus Molecular Cloud|Taurus star-forming region]]. They are found near [[molecular cloud]]s and identified by their optical [[variable star|variability]] and strong [[chromosphere|chromospheric]] lines.  T Tauri stars are [[pre-main-sequence star]]s in the process of contracting to the main sequence along the [[Hayashi track]], a luminosity–temperature relationship obeyed by infant stars of less than 3 [[solar masses]] ({{Solar mass|link=y}}) in the pre-main-sequence phase of [[stellar evolution]].<ref name="Havard">{{Cite web |title=Stellar Evolution - Cycles of Formation and Destruction; Young Stellar Objects |url=https://chandra.harvard.edu/edu/formal/stellar_ev/story/index5.html |access-date=2023-01-31 |website=[[Harvard University|chandra.harvard.edu]]}}</ref> It ends when a star of {{Solar mass|0.5}} or larger develops a [[radiative zone]], or when a smaller star commences [[nuclear fusion]] on the [[main sequence]].

==History==
While [[T Tauri]] itself was discovered in 1852, the T Tauri class of stars were initially defined by [[Alfred Harrison Joy]] in 1945.<ref name="Joy1945"/>

==Characteristics==
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T Tauri stars comprise the youngest visible F, G, K and M [[spectral type]] stars ({{Solar mass|<2|link=y}}). Their surface temperatures are similar to those of [[main sequence|main-sequence]] stars of the same mass, but they are significantly more luminous because their radii are larger. Their central temperatures are too low for [[proton–proton chain|hydrogen fusion]]. Instead, they are powered by [[gravitational energy]] released as the stars contract, while moving towards the [[main sequence]], which they reach after about 100 million years. They typically rotate with a period between one and twelve days, compared to a month for the Sun, and are very active and variable.

There is evidence of large areas of [[starspot]] coverage, and they have intense and variable [[X-ray]] and [[radio]] emissions (approximately 1000 times that of the Sun). Many have extremely powerful [[stellar wind]]s; some eject gas in high-velocity [[Bipolar outflow|bipolar jets]]. Another source of brightness variability are clumps ([[protoplanet]]s and [[planetesimal]]s) in the disk surrounding T Tauri stars.

[[File:Hs-2000-32-a-animated.gif|thumb|The ejection of a bubble of hot gas from [[XZ Tauri]], a binary system of T Tauri stars. The scale is much larger than that of the Solar System.]]

Their spectra show a higher [[lithium]] abundance than the Sun and other main-sequence stars because lithium is destroyed at temperatures above 2,500,000 K. From a study of lithium abundances in 53 T Tauri stars, it has been found that lithium depletion varies strongly with size, suggesting that "[[lithium burning]]" by the [[proton–proton chain|p-p chain]] during the last highly convective and unstable stages during the later [[pre–main-sequence star|pre–main sequence]] phase of the [[Hayashi track|Hayashi contraction]] may be one of the main sources of energy for T Tauri stars. Rapid rotation tends to improve mixing and increase the transport of lithium into deeper layers where it is destroyed. T Tauri stars generally increase their rotation rates as they age, through contraction and spin-up, as they conserve angular momentum. This causes an increased rate of lithium loss with age. Lithium burning will also increase with higher temperatures and mass, and will last for at most a little over 100 million years.

The p-p chain for lithium burning is as follows

<!-- Autogenerated using Phykiformulae 0.12 [[User:SkyLined#Phykiformulae]]
Li-6 + p+ -> Be-7 (unstable) ->   Li-7 + e+
Li-7 + p+ -> Be-8 (unstable) -> 2 He-4 + {{val|0.92|ul=keV}}
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|- style="height:2em;"
|{{SubatomicParticle|link=yes|Proton}}&nbsp;||+&nbsp;||{{nuclide|link=yes|lithium|6}}&nbsp;||→&nbsp;||{{nuclide|link=yes|beryllium|7}}||&nbsp;||
|- style="height:2em;"
|{{nuclide|link=yes|beryllium|7}}&nbsp;||+&nbsp;||{{SubatomicParticle|link=yes|Electron}}&nbsp;||→&nbsp;||{{nuclide|link=yes|lithium|7}}&nbsp;||+&nbsp;{{SubatomicParticle|link=yes|electron neutrino}}
|- style="height:2em;"
|{{SubatomicParticle|link=yes|Proton}}&nbsp;||+&nbsp;||{{nuclide|link=yes|lithium|7}}&nbsp;||→&nbsp;||{{nuclide|link=yes|beryllium|8}}&nbsp;||&nbsp;||(unstable)
|- style="height:2em;"
|&nbsp;||&nbsp;||{{nuclide|link=yes|beryllium|8}}&nbsp;||→&nbsp;||2&nbsp;{{nuclide|link=yes|helium|4}}&nbsp;||+ energy
|}

It will not occur in stars with less than sixty times the mass of Jupiter ({{Jupiter mass|link=y}}). The rate of lithium depletion can be used to calculate the age of the star.<!--

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==Types ==
Several types of TTSs exist:<ref name=wolk/>
*Classical T Tauri star ('''CTTS''')
*Weak-line T Tauri star ('''WTTS''')
**Naked T Tauri star ('''NTTS'''), which is a subset of WTTS.
Roughly half of T Tauri stars have [[circumstellar disk]]s, which in this case are called [[protoplanetary discs]] because they are probably the progenitors of [[planetary system]]s like the Solar System. Circumstellar discs are estimated to dissipate on timescales of up to 10 million years. Most T Tauri stars are in [[binary star]] systems. In various stages of their life, they are called [[young stellar object]] (YSOs). It is thought that the active [[magnetic field]]s and strong [[solar wind]] of [[Alfvén waves]] of T Tauri stars are one means by which [[angular momentum]] gets transferred from the star to the protoplanetary disc. A T Tauri stage for the [[Solar System]] would be one means by which the angular momentum of the contracting [[Sun]] was transferred to the protoplanetary disc and hence, eventually to the [[planets]].

Analogs of T Tauri stars in the higher mass range (2–8 [[solar mass]]es)—A and B [[spectral type]] [[pre–main-sequence star]]s, are called [[Herbig Ae/Be star|Herbig Ae/Be-type stars]]. More massive (>8 solar masses) [[star]]s in pre–main sequence stage are not observed, because they evolve very quickly: when they become visible (i.e. disperses surrounding circumstellar gas and dust cloud), the hydrogen in the center is already burning and they are [[main sequence]] objects.

==Planets==
Planets around T Tauri stars include: 
*[[1RXS J160929.1−210524]]b around a [[K-type main-sequence star|K-type]] star
*[[PDS 70|PDS 70b]] around a [[K-type main-sequence star|K-type]] star

==See also==
* [[Orion variable]]
* [[EX Lupi]]
* [[P Cygni profile]]

==References==
<references>
<ref name=appenzeller>{{cite journal|doi=10.1007/BF00873081|title=T Tauri stars|journal=The Astronomy and Astrophysics Review|volume=1|issue=3–4|pages=291|year=1989|last1=Appenzeller|first1=I|last2=Mundt|first2=R|bibcode=1989A&ARv...1..291A|s2cid=118324477}}</ref>

<ref name="Joy1945">{{cite journal | title=T Tauri Variable Stars | url=http://articles.adsabs.harvard.edu/full/1945ApJ...102..168J | last1=Joy | first1=Alfred H. | journal=The Astrophysical Journal | volume=102 | pages=168–195 | year=1945 | bibcode=1945ApJ...102..168J | doi=10.1086/144749 }}</ref>

<ref name=wolk>{{cite web|url=http://hea-www.harvard.edu/~swolk/thesis/intro/node2.html |title=T Tauri Stars, Naked and Otherwise |author=Scott J. Wolk |date=1996 |access-date=2018-03-14}}</ref>
</references>
* [https://web.archive.org/web/20110524172358/http://universe.gsfc.nasa.gov/seminars/presentations/walter/toc.html Discussion of V471 Tauri observations and general T-Tauri properties], Frederick M. Walter, Stony Brook University, April 2004
* [https://arxiv.org/abs/astro-ph/0309284 An empirical criterion to classify T Tauri stars and substellar analogs using low-resolution optical spectroscopy], David Barrado y Navascues, 2003

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[[Category:Star types]]
[[Category:T Tauri stars|*]]
[[Category:Star formation]]