T Tauri stars (named for the class prototype) are pre-main sequence stars. They usually show both strong infrared and ultraviolet continuum emission in addition to optical continuum and emission lines. They are associated with star forming regions from which they themselves have recently formed (within the past few million years).

T Tauri stars are in the short phase of evolution between protostellar collapse and the main sequence. The star shines due to left-over heat from the conversion of gravitational potential energy to heat. They lie on or near the Hayashi track - a region of the Hertzsprung-Russell Diagram where stars are completely convective. T Tauri stars are not yet capable of fusing hydrogen to helium in their cores, but they may burn deuterium since doing so requires much less heat and pressure. They frequently contain lithium in their spectra, which is also an indicator that they are not yet capable of sustaining fusion reactions; lithium burns very easily in stars, and it is destroyed very early in a star's lifetime if the star is fully convective.

As mentioned above, T Tauri stars are found in or near star-forming regions, so they are also associated with far-infrared emission of the parent region. They tend to be variable, due both to accretion instabilities and to magnetic activity. Some of them are embedded deep within molecular clouds, making them invisible to optical telescopes. In these cases, they are often bright infrared or radio sources.

There are three types of T Tauri stars: Classical T Tauri stars (CTTS), Weak-line T Tauri stars (WTTS), and Naked T Tauri stars (NTTS).

Classical T Tauri stars (like T Tauri itself) have accretion disks on the equatorial plane, along with a strong stellar wind directed along the polar axis. The accretion disk comes from the nebular gas from which the star itself formed. After the central protostar collapses to a sphere, gas continues to fall onto it from the nebula, via the accretion disk. The disk itself is luminous in the infrared, and the boundary layer where the disk meets the surface of the star gets quite hot, emitting continuum and line emission at optical wavelengths. The H-alpha emission line at 6563 angstroms is particularly strong as are emission lines of ionized calcium. These are indicators of magnetic activity in normal, main-sequence stars, and are generated by the same process here. The magnetic fields in these stars can be quite strong, because like all early pre-main sequence stars they are fully convective. Convection, coupled with the high rotation speeds of these young stars, acts to amplify magnetic fields already present via a dynamo process. These magnetic fields also increase the amount of activity on the stellar surface, and are responsible for the elevated ultraviolet and X-ray emission occasionally observed. The wind blows in both (polar) directions normal to the accretion plane. At one time, the wind was believed to be driven by Alfven waves, but this is probably not the case, since the winds appear to be electrically neutral rather than ionized.

Weak-line and Naked T Tauri stars are believed to lack both an accretion disk and a strong wind. Weak-lined T Tauri stars are so named because we only see weak emission lines from them, and Naked T Tauri stars have little or no excess continuum or line emission at all. WTTS and NTTS are commonly observed in binary stars, so it was believed that perhaps the binary companions disrupt and dissipate each other's accretion disks. It may be that CTTS evolve into WTTS and NTTS, but there is evidence of CTTS that are older than some WTTS in the same star-forming region, so this may not be the case. Accretion may be sporadic, and stars may alternate between the CTTS, WTTS, and NTTS stages of their evolution.

Sources: old class notes, and the excellent Protostars and Planets III review book, ed. by E.H. Levy and J.I. Lunine, University of Arizona Press, and the reviews by Shu et al., and by Basri and Bertout it contains.

Log in or register to write something here or to contact authors.