T-Tauri star

A T-Tauri star (TTS) is a type of variable star showing substantial irregular day-to-day variation, and often showing certain specific emission lines (H-alpha, Ca II, Fe, O I, Si II) and often a lithium absorption line. They are judged to be very young (pre-main-sequence) dwarfs of about 2.5 solar masses or less. They often have protoplanetary disks and often are in the vicinity of molecular clouds. They often have strong stellar winds and/or jets, both ascribed to accretion from the disk. Their energy/luminosity is presumed to be from gravitation (Kelvin-Helmholtz mechanism) rather than fusion, the latter not yet triggered. They generally have a short rotation period in the range of 1-12 days. Some subtypes:

• CTTS (classical T-Tauri star) extensive disk indicated by strong (broad) emission lines.
• WTTS (weak-line T-Tauri star) weak or non-existent signs of a disk.
• NTTS (naked T-Tauri star) sometimes used synonymously with WTTS, but the two terms are sometimes used to distinguish between "weak" and "non-existent".

The name comes from T Tauri, a star first discerned to have these characteristics, which serves as a prototype/example.