T Tauri star

T Tauri star
Drawing of a T-Tauri star with a circumstellar accretion disc

T Tauri stars (TTS) are a class of variable stars named after their prototype – T Tauri. They are found near molecular clouds and identified by their optical variability and strong chromospheric lines.

Characteristics

T Tauri stars are pre–main sequence stars – the youngest visible F, G, K, M spectral type stars (<2 Solar mass). Their surface temperatures are similar to those of 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 hydrogen fusion. Instead, they are powered by gravitational energy released as the stars contract 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 winds. Another source of brightness variability are clumps (protoplanets and planetesimals) in the disk surrounding T Tauri stars.

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 P-P chain, during the last highly convective and unstable stages during the later pre–main sequence phase of the 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

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

It will not occur in stars with less than sixty times the mass of Jupiter. In this way, the rate of lithium depletion can be used to calculate the age of the star.

Protoplanetary discs in the Orion Nebula

Roughly half of T Tauri stars have circumstellar disks, which in this case are called protoplanetary discs because they are probably the progenitors of planetary systems 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 Objects (YSOs). It is thought that the active magnetic fields 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 hypothesised T Tauri stage for our 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, resulting in the theory that before our own Sun matured, it was once a T Tauri star.

Analogs of T Tauri stars in the higher mass range (2–8 solar masses)—A and B spectral type pre–main sequence stars, are called Herbig Ae/Be stars. More massive (>8 Solar mass) 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 are main sequence objects.

See also

  • Orion variables
  • Weak-lined T Tauri star

References


Wikimedia Foundation. 2010.

Игры ⚽ Поможем сделать НИР

Look at other dictionaries:

  • T Tauri star — T Tauri star, any one of a group of very distant, variable stars found between clouds of dust and gas, usually associated with nebulae. ╂[< T Tauri, a star in the constellation Taurus, the prototype of these stars] …   Useful english dictionary

  • T Tauri star — Any of a class of very young stars with masses less than about twice the Sun s. Characterized by unpredictable changes in brightness, they represent an early stage in stellar evolution, having only recently been formed by the gravitational… …   Universalium

  • T Tauri star — noun a class of young visible stars having optical variability; they are often surrounded by a protoplanetary disc See Also: T Tauri …   Wiktionary

  • star cluster — Astron. a number of stars of common origin held together as a group by gravitational attraction. Cf. globular cluster, open cluster, stellar association. * * * ▪ astronomy Introduction  either of two general types of stellar assemblages held… …   Universalium

  • star — starless, adj. /stahr/, n., adj., v., starred, starring. n. 1. any of the heavenly bodies, except the moon, appearing as fixed luminous points in the sky at night. 2. Astron. any of the large, self luminous, heavenly bodies, as the sun, Polaris,… …   Universalium

  • Tauri (disambiguation) — Tauri can refer to *Tauri, the ancient inhabitants of Crimea. *Tau ri, the race in the fictional Stargate series. *The genitive form of taurus (e.g. in star designations) …   Wikipedia

  • Star — For other uses, see Star (disambiguation) …   Wikipedia

  • Star designation — Designations of stars (and other celestial bodies) are done by the International Astronomical Union (IAU). Many of the star names in use today were inherited from the time before the IAU existed. Other names, mainly for variable stars (including… …   Wikipedia

  • Star system — This article is about stars in outer space. For the Hollywood star system, see Star system (film). For a system of planets around a star, see Planetary system. A star system or stellar system is a small number of stars which orbit each other,[1]… …   Wikipedia

  • T Tauri — [ AAVSO visual light curve of T Tau showing a typical 200 day oscillatory interval. The plot is based upon averages of all observations received within one day, and lines connecting the observations have been added as a visual aid. ] Starbox… …   Wikipedia

Share the article and excerpts

Direct link
Do a right-click on the link above
and select “Copy Link”