2013ApJ...775..131G

The He II (1640 Å) line and the resonance doublet of N V (UV1) provide a good diagnostic tool to constrain the excitation mechanism of hot (T_e > 40,000 K) 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 N V profiles in most sources are symmetric and at rest with respect to the star. The velocity dispersion of the profile increases from non-accreting (σ = 40 km/s) to accreting (σ = 120 km/s) TTSs, suggesting that the macroturbulence field in the line formation region decreases as the stars approach the main sequence. Evidence of the N V line being formed in a hot solar-like wind has been found in RW Aur, HN Tau, and AA Tau. The He II profile has a strong narrow component that dominates the line flux; the dispersion of this component ranges from 20 to 60 km/s. Current data suggest that both accretion shocks and atmospheric emission might contribute to the line flux. In some sources, the He II line shows a broad and redward-shifted emission component often accompanied by semiforbidden O III] emission that has a critical electron density of ∼3.4x10¹⁰ cm³. In spite of their different origins (inferred from the kinematics of the line formation region), N V and He II fluxes are strongly correlated, with only the possible exception of some of the heaviest accretors.