Official URL: https://doi.org/10.1051/0004-6361:20034381

The Paschen beta (1.2822 μm) emission line found in the near-infrared spectra of T-Tauri stars (TTSs) is believed to trace the accretion of material onto the central star. We present spectroscopic data which suggests that this may not always be the case. The technique of spectro-astrometry is used by us to measure positional displacements in the Paβ emission from four T-Tauri stars, namely DG Tau, V536 Aql, LkHα 321 and RW Aur. We also observed the optical forbidden emission from these sources, for example the [SII]6716, 6731, [OI]6300, 6363 and [NII]6548, 6583 lines. Forbidden emission lines are formed in the outflows that accompany the evolution of protostars and so are ideal to use as a comparison to confirm that the measured offsets in the Paβ emission are indeed due to outflowing material. Models based on the magnetospheric accretion theory have been the most successful to date in explaining the origin of atomic hydrogen emission lines. Yet we see that the line profiles of the sources showing displacement in their Paβ emission all have features that the magnetospheric accretion model has so far failed to explain, such as broad full width half maxima, large wings and an absence of red shifted absorption features. The failure of the models to explain the presence of large extended wings in the line profiles is particularly interesting in the context of this study as in all cases it is in the extended wings that we measure offsets in position with respect to the source.