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Fig. 6. Spectro-astrometric measurements of the [OI]A6300 forbidden line and the Ha permitted line taken from the continuum contaminated (left) and continuum subtratced (right) spectrum of the T Tauri star V536 Aql. Note how contamination by the continuum emission will tend to drag the position of the line region back toward the source. This effect is much smaller in Ha, as the intensity of the Ha emission is comparable to the continuum intensity. The ratio of the continuum contaminated shifts to those measured from the continuum subtracted spectrum is

before the continuum subtraction is much reduced. If the continuum emission is much stronger than the pure line emission, it is possible that a real displacement in the line region will remain hidden until the continuum contamination is removed. This is especially true where spectro-astrometry has been applied to the study of permitted emission line regions in CTTSs (see Sect. 3). Part of the Ha emission (the line-wings) also originates in the jet and is thus displaced. However, in this case the Ha line is strong compared to the continuum emission, and so the removal of the contamination results in only a small increase in offset.

There have been two approaches to the problem of continuum contamination. First, the continuum can simply be removed to reveal the pure emission line regions. This is shown in Fig. 4. This method has been adopted by [15, 43, 44, 46]. Second, the true line shifts are recovered by calculating the factor by which the offsets are weighted by the continuum and dividing the raw positional displacement by this number. This factor is given above and this method is used by [33, 34]. Both approaches yield similar results.

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