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T Tauri star

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Class of young variable stars

This article is about the type of variable star. For the particular variable star called "T Tauri", seeT Tauri.

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Artist's impression of a T Tauri star with a circumstellar accretion disc

Star formation

Object classes
Interstellar medium Molecular cloud Bok globule Dark nebula Young stellar object Protostar Pre-main-sequence star T Tauri star Herbig Ae/Be star Herbig–Haro object
Theoretical concepts
Accretion Initial mass function Jeans instability Kelvin–Helmholtz mechanism Nebular hypothesis Planetary migration
v t e

T Tauri stars (TTS) are a class ofvariable stars that are less than about ten million years old.^[1] This class is named after the prototype,T Tauri, a young star in theTaurus star-forming region. They are found nearmolecular clouds and identified by their opticalvariability and strongchromospheric lines. T Tauri stars arepre-main-sequence stars in the process of contracting to the main sequence along theHayashi track, a luminosity–temperature relationship obeyed by infant stars of less than 3solar masses (M_☉) in the pre-main-sequence phase ofstellar evolution.^[2] It ends when a star of 0.5 M_☉ or larger develops aradiative zone, or when a smaller star commencesnuclear fusion on themain sequence.

History

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WhileT Tauri itself was discovered in 1852, the T Tauri class of stars were initially defined byAlfred Harrison Joy in 1945.^[3]

Characteristics

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T Tauri stars comprise the youngest visible F, G, K and Mspectral type stars (<2 M_☉). Their surface temperatures are similar to those ofmain-sequence stars of the same mass, but they are significantly more luminous because their radii are larger. Their central temperatures are too low forhydrogen fusion. Instead, they are powered bygravitational energy released as the stars contract, while moving towards themain 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 ofstarspot coverage, and they have intense and variableX-ray andradio emissions (approximately 1000 times that of the Sun). Many have extremely powerfulstellar winds; some eject gas in high-velocitybipolar jets. Another source of brightness variability are clumps (protoplanets andplanetesimals) in the disk surrounding T Tauri stars.

The ejection of a bubble of hot gas fromXZ Tauri, a binary system of T Tauri stars. The scale is much larger than that of the Solar System.

Their spectra show a higherlithium 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 thep-p chain during the last highly convective and unstable stages during the laterpre–main sequence phase of theHayashi 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

p	+	⁶ ₃Li	→	⁷ ₄Be
⁷ ₄Be	+	e⁻	→	⁷ ₃Li	+ ν _e
p	+	⁷ ₃Li	→	⁸ ₄Be		(unstable)
		⁸ ₄Be	→	2 ⁴ ₂He	+ energy

It will not occur in stars with less than sixty times the mass of Jupiter (M_J). The rate of lithium depletion can be used to calculate the age of the star.

Types

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Several types of TTSs exist:^[4]

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 havecircumstellar disks, which in this case are calledprotoplanetary discs because they are probably the progenitors ofplanetary systems like the Solar System. Circumstellar discs are estimated to dissipate on timescales of up to 10 million years. Most T Tauri stars are inbinary star systems. In various stages of their life, they are calledyoung stellar object (YSOs). It is thought that the activemagnetic fields and strongsolar wind ofAlfvén waves of T Tauri stars are one means by whichangular momentum gets transferred from the star to the protoplanetary disc. A T Tauri stage for theSolar System would be one means by which the angular momentum of the contractingSun was transferred to the protoplanetary disc and hence, eventually to theplanets.

Analogs of T Tauri stars in the higher mass range (2–8solar masses)—A and Bspectral type pre–main-sequence stars, are calledHerbig Ae/Be-type stars. More massive (>8 solar masses)stars 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 aremain sequence objects.

Planets

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Planets around T Tauri stars include:

1RXS J160929.1−210524b around aK-type star
PDS 70b around aK-type star

References

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^Appenzeller, I; Mundt, R (1989). "T Tauri stars".The Astronomy and Astrophysics Review.1 (3–4): 291.Bibcode:1989A&ARv...1..291A.doi:10.1007/BF00873081.S2CID 118324477.
^"Stellar Evolution - Cycles of Formation and Destruction; Young Stellar Objects".chandra.harvard.edu. Retrieved2023-01-31.
^Joy, Alfred H. (1945)."T Tauri Variable Stars".The Astrophysical Journal.102:168–195.Bibcode:1945ApJ...102..168J.doi:10.1086/144749.
^Scott J. Wolk (1996)."T Tauri Stars, Naked and Otherwise". Retrieved2018-03-14.

Discussion of V471 Tauri observations and general T-Tauri properties, Frederick M. Walter, Stony Brook University, April 2004
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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Lists

v t e Star formation
Object classes	Interstellar medium Molecular cloud Bok globule Dark nebula Young stellar object Protostar T Tauri star Pre-main-sequence star Herbig Ae/Be star Herbig–Haro object
Theoretical concepts	Initial mass function Jeans instability Kelvin–Helmholtz mechanism Nebular hypothesis Planetary migration
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