Constraints to the magnetospheric properties of T Tauri stars - II. The Mg II ultraviolet feature

The atmospheric structure of T Tauri stars (TTSs) and its connection with the large-scale outflow is poorly known. Neither the effect of the magnetically mediated interaction between the star and the disc is well understood. The Mg II multiplet is a fundamental tracer of TTSs atmospheres and outflows, and is the strongest feature in the near-ultraviolet spectrum of TTSs. The International Ultraviolet Explorer and Hubble Space Telescope data archives provide a unique set to study the main physical compounds contributing to the line profile and to derive the properties of the line formation region. The MgII profiles of 44 TTSs with resolution 13 000-30 000 are available in these archives. In this work, we use this data set to measure the main observables: flux, broadening, asymmetry, terminal velocity of the outflow, and the velocity of the discrete absorption components. For some few sources repeated observations are available and variability has been studied. There is a warm wind that at sub-au scales absorbs the blue wing of the MgII profiles. The main result found in this work is the correlation between the line broadening, MgII flux, terminal velocity of the flow and accretion rate. Both outflow and magnetospheric plasma contribute to the MgII flux. The flux-flux correlation between MgII and C IV or He II is confirmed; however, no correlation is found between the Mg II flux and the UV continuum or the H-2 emission.