Evidence of hot high velocity photoionized plasma falling on actively accreting T-Tauri Stars

Abstract: The HeII (1640 A) line and the resonance doublet of NV (UV1) provide a good diagnostic tool to constrain the excitation mechanism of hot (Te>40,000K) atmospheric/magnetospheric plasmas in T Tauri stars (TTSs). Making use of the data available in the Hubble Space Telescope Archive, this work shows that there are, at least, two distinct physical components contributing to the radiation in these tracers: the accretion flow sliding on the magnetosphere and the atmosphere. The NV profiles are symmetric and at rest with respect to the star in most sources. The velocity dispersion of the profile increases from non-accreting (40 km/s) to accreting (120 km/s) TTSs suggesting that the macroturbulence in the line formation region decreases as the stars approach the main sequence. Evidence of the NV line being formed in a hot solar-like wind has been found in RW Aur, HN Tau and AA Tau. The HeII profile has a strong narrow component that dominates the line flux; the dispersion of this component ranges from 20 to 60 km/s. The sources of this radiation is uncertain. Current data suggest that both accretion shocks and atmospheric emission might contribute to the line flux. In some sources de HeII line shows a broad and rewards shifted emission component often accompanied by semiforbidden OIII emission that has a critical density of about 3.4e10 cm-3. In spite of the different origins (inferred from the kinematics of the line formation region), NV and HeII fluxes are strongly correlated, with the possible exception of the heaviest accretors.