From Disks to Planets

Weak-lined T Tauri Stars

An X-ray image of T Tauri stars in the L1551 cloud. From the ASCA X-ray satellite.

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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Childhood's End: The Main Sequence

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