2014MNRAS.442.2951L

The Cii] feature at ∼ 2325 Å 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 Siii] and Feii] 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 Cii] wavelength. For these stars, we have determined electron densities (ne) and temperatures (Te) in the line emission region as well as the profile broadening (σ). For most of the stars in the sample (17), we obtain 10^4.1 ≲ Te ≲ 10^4.5 K and 10⁸ ≲ ne ≲ 10¹²/cm3. These stars have suprathermal line broadening (35 ≲ σ ≲ 165 km/s), except TW Hya and CY Tau with thermal line broadening. Both Cii] 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 outflow.