Classical T Tauri stars are easily identified by their strong emission lines produced by the disk/star interaction. However, once the disk has dissipated enough so that it no longer interacts with the star, these lines are no longer present or are very weak. These "Weak-lined" T Tauri stars are primarily found because they are bright X-ray sources. T Tauri stars produce X-rays in hot plasma trapped in magnetic fields above the stellar surface. This is similar to the process in which the Sun produces bright flares but 100-1000 times more powerful. X-ray imaging satellites, such as EINSTEIN, ROSAT and ASCA have discovered hundreds of Weak-lined T Tauri stars.
The primary difference between the Classical and Weak-lined T Tauri
stars is their disk properties. By the time the star has become a
Weak-lined T Tauri star the disk is very weak or no longer present. But
where had the disk material gone? The simplest explanation is
that it has formed into planets. First, the dust grains
form small bodies about 1 kilometer in size, called
planetesimals, and then the planetesimals in turn collide together to form
planets. Thus, Weak-lined T Tauri stars may
harbor very young planetary systems.
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