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, 1976Gershberg, 1982Appenzeller and Dearborn, 1984), in the flare activity (Guenther and Ball, 1999), as well as in accretion processes and stellar wind phenomena (Hartmann, 1990Königl, 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, 1989Petrov et al., 1994Jump To The Next Citation PointHatzes, 1995Jump To The Next Citation PointGrankin et al., 1995). Properties of T Tauri type stars were recently reviewed by Petrov (2003).

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