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.