2.3 T Tauri stars
T Tau stars were discovered and recognised as a new type of variables by Joy (1945). They are
associated with interstellar clouds and distinguished by irregular light variations. Emission lines in their
spectra resembling the spectrum of the solar chromosphere are superimposed on photospheric absorption
spectra of G–M types of low luminosity. Strongly enhanced lithium absorption in their spectra indicates the
young age of these stars. T Tau-type stars are, thus, considered as pre-main-sequence stars of
about one solar mass at an age of a few million years, still surrounded by disks of gas and dust
remaining from their formation. It was admitted that magnetic fields play an important role
in the surface activity of T Tau stars similar to sunspots but strongly enhanced in strength
(Petrov and Shcherbakov, 1976; Gershberg, 1982; Appenzeller and Dearborn, 1984), in the flare
activity (Guenther and Ball, 1999), as well as in accretion processes and stellar wind phenomena
(Hartmann, 1990; Königl, 1991).
A subgroup of T Tau stars with weak emission spectra and little, if any, IR excess radiation, called
weak-line T Tau stars, show periodic brightness variations with amplitudes up to 0.5 mag which are caused
by very large cool active regions (Bouvier and Bertout, 1989; Petrov et al., 1994; Hatzes, 1995; Grankin
et al., 1995). Properties of T Tauri type stars were recently reviewed by Petrov (2003).