T Tauri stars
T Tauri stars(tor -ÿ, -ee) Variable stars of spectral type G or later whose spectra are dominated by strong emission lines, usually attributed to chromospheric activity and stellar winds in these stars. They are invariably embedded in dense patches of gas and dust that may require observations in the infrared: the dust absorbs the visible light of the star and reradiates it at longer wavelengths. The prototype is T Tauri. T Tauri stars are usually found together in groups (T associations) and are the youngest optically observable stages in the life of a star of about the Sun's mass; more massive counterparts are observed as Ae and Be stars. They are frequently associated with Herbig–Haro objects.
There is much evidence that T Tauri stars are young objects, for instance they have a high abundance of lithium, an element destroyed fairly early in a star's life, and they are surrounded by gas and dust. They are thought to be young protostars that have only recently contracted out of the interstellar medium (see star formation). Lying above the main sequence in the Hertzsprung–Russell diagram, they are still contracting and losing mass (see T Tauri wind). Their spectral lines reveal that some are extremely rapid rotators, throwing off material at speeds of up to 300 km s–1. The irregular light variations are believed to arise partly from activity in the chromospheres of these young stars, and partly by the obscuring effect of the patchy dust in the cocoon as it moves in front of the star.
The original class of T Tauri stars, characterized by strong hydrogen emission lines, are often known as classical T Tauri stars (CTTS) to distinguish them from naked or weak-line T Tauri stars (WTTS); the latter have very weak hydrogen emission lines, often discovered in X-rays, and are much less active than the classical type. WTTS may represent a later phase of pre-main-sequence evolution than CTTS or a different evolutionary channel, as may be suggested by the apparently higher frequency of binaries among WTTS than among CTTS and main-sequence stars. See also FU Orionis; YY Orionis stars.