Constraints to the magnetospheric properties of T Tauri stars - I. The C ii], Fe ii] and Si ii] ultraviolet features

The C ii] feature at similar to 2325 angstrom is very prominent in the spectra of T Tauri stars (TTSs). This feature is a quintuplet of semiforbidden transitions excited at electron temperatures around 10 000 K that, together with the nearby Si ii] and Fe ii] features, provides a reliable optically thin tracer for accurate measurement of the plasma properties in the magnetospheres of TTSs. The spectra of 20 (out of 27) TTSs observed with the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope have good enough signal-to-noise ratio at the C ii] wavelength. For these stars, we have determined electron densities (n(e)) and temperatures (T-e) in the line emission region as well as the profile broadening (sigma). For most of the stars in the sample (17), we obtain 10(4.1) a parts per thousand(2) T-e a parts per thousand(2) 10(4.5) K and 10(8) a parts per thousand(2) n(e) a parts per thousand(2) 10(12) cm(-3). These stars have suprathermal line broadening (35 a parts per thousand(2) sigma a parts per thousand(2) 165 km s(-1)), except TW Hya and CY Tau with thermal line broadening. Both C ii] line luminosity and broadening are found to correlate with the accretion rate. Line emission seems to be produced in the magnetospheric accretion flow, close to the disc. There are three exceptions: DG Tau, RY Tau and FU Ori. The line centroids are blueshifted indicating that the line emission in these three stars is dominated by the outflo