T Tauri star
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Image:TTauriStarDrawing.jpg T Tauri stars 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.
T Tauri stars are the youngest visible stars. 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 burning. 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.
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 pre main sequence later 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 slow the transport of lithium into deeper layers where it is destroyed. Stars generally slow their rotation rates as they age, through the transfer of angular momentum to the protoplanetary disc. 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 Myr (megayears = 100,000,000 years).
The P-P chain for Lithium burning is as follows
- p+ + 6Li -> 7Li
- p+ + 7Li -> 8Be (unstable)
- 8Be -> 4He + 4He + energy
It will not occur in stars with a mass of less than 60 Jupiters. In this way, the rate of lithium depletion can be used to calculate the age of the star.
Roughly half of T Tauri stars have circumstellar disks, which in this case are called protoplanetary disc because they are probably the progenitors of planetary systems like the solar system. 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 Alfven 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.
References
es:Estrella T Tauri fr:Variable de type T Tauri hr:T Tauri zvijezda it:Stelle T Tauri sk:Premenná hviezda typu T Tauri