Abstract
The predictions of an approximate model for XUV ionization of He I in the atmospheres of T Tauri stars are compared with observations of the luminosity in He I 5876 A and in soft X-rays. The comparison shows that XUV ionization cannot account for the observations. Collisional processes are then indicated to be the dominant mechanism determining the excitation and ionization of He in T Tauri atmospheres. The absence of He II 1640, therefore, cannot be used as an argument for the lack of coronae in T Tauri stars. The fact that there is no systematic difference in luminosity in He I 5876 between stars with and without X-ray detection argues for a similar chromospheric structure in T Tauri stars up to T ∼ 50 000 K.
