2. Observations and data reduction

For our long-slit spectroscopic survey of the FELs of TTSs we have selected a total of 38 objects which cover a wide range of equivalent widths of the [OI] line in the catalogue of Cohen & Kuhi (1979).

 

campaign telescope period instrument

1 CA 3.5 m 02.12.88 - 10.12.88 T

2 CA 3.5 m 14.12.88 - 22.12.88 T

3 CA 2.2 m 07.09.92 - 16.09.92 c

4 CA 3.5 m 14.12.92 - 21.12.92 T

5 CA 2.2 m 14.12.92 - 21.12.92 c

6 CA 2.2 m 24.08.93 - 30.08.93 c

7 WHT 4.2 m 31.08.93 - 05.09.93 I

8 CA 2.2 m 01.09.93 - 30.09.93 c

9 CA 3.5 m 17.11.93 - 21.11.93 T

Table 1: List of observing periods at the Calar Alto Observatory (CA) and the Observatorio Roque de los Muchachos (WHT). (c = 2.2 m coudé spectrograph, T = 3.5 m Cassegrain twin spectrograph, I = 4.2 m ISIS Cassegrain spectrograph)

 

For many of these objects we have have taken long-slit spectra at various (usually 4-5) position angles. Note that for several TTSs of our sample spectrograms at only one position angle have been secured either because the outflow direction was known from imaging studies or because the spectrogram served only as a pre-investigation for possible future studies. The data were collected during several observing runs between 1988 and 1993 using the coudé spectrograph at the 2.2 m telescope and the Cassegrain twin spectrograph at the 3.5 m telescope of the Calar Alto (CA) Observatory in Spain. The spectral resolution was and , respectively. The spatial resolution, determined from the spatial width of the stellar continuum (), varied in general between 1.4'' and 2.0''. In 1993 we used, in addition, the ISIS spectrograph on the 4.2 m William Herschel Telescope (WHT) providing a spectral and spatial resolution of and 1'' - 1.3'', respectively. The wavelength coverage at the WHT and CA 3.5 m telescope was from 6250 to 6750 Å. At the CA 2.2 m telescope the wavelength coverage was from 6250 to 6450 Å or from 6535 to 6735 Å per frame. The individual observing periods are listed in Table 1 (click here).

The spectra were reduced by a program developed by one of us (J.S.). Following the standard data reduction, a special continuum subtraction procedure was performed in order to reveal the relatively weak FEL regions near the source. This step is essential for deriving information about the spatial and kinematic properties of the outflow in the immediate vicinity of the YSO and is discussed in full detail elsewhere (Hirth et al. 1994a).

From the continuum-subtracted long-slit spectrograms position-velocity (PV) maps of the individual FELs have been derived. From these PV maps two important spatial quantities of the FELs as a function of the radial velocity have been extracted: the corrected spatial width () and the offset y of the position of the FEL centroid relative to the stellar position.

In a first step using the stellar continuum spectrum recorded on the original spectrogram the stellar position and the actual spatial resolution () have been derived by means of a Gaussian line fit in the spatial direction of the stellar continuum in the immediate vicinity of the FELs (i.e. along the direction of the slit). In a second step, using the recorded FELs on the continuum-subtracted long-slit spectrogram, the spatially integrated intensity I, the centroid position y', and the (uncorrected) spatial width (FWHM) of the FEL distribution along the direction of the slit have been derived as a function of radial velocity by means of a Gaussian line fit to the FEL distribution recorded in each wavelength bin. Finally the spatial offset of the centroid and the corrected spatial width of the FEL as a function of radial velocity have been calculated. Due to the differential nature of that procedure a relatively high accuracy of the offset y of has been generally achieved. Evidently, due to the deconvolution procedure applied, the deduced spatial widths are of lower accuracy. Depending on the S/N ratio of the data in our spectrograms, reliable corrected spatial widths can only be deduced for larger than . This means that is required to deduce reliable changes in as a function of radial velocity (e.g. the differences between the LVC and HVC). Although our method is relatively sensitive to derive relative changes in the spatial width the absolute values for the corrected width may be systematically wrong due to the assumption of Gaussian profiles for both the emission region and the point spread function.

The outflow direction has been determined by comparing long-slit spectra centered on the star and obtained at different position angles in steps of to . Spectra taken with a slit orientation parallel to the outflow show the maximum spatial offset of the emission centroid of the FEL from the stellar position (for more details see Hirth et al. 1994a). Spectra taken perpendicular to the determined outflow direction show no significant spatial offset of the FEL (i.e. less than 0.05'' - 0.1''). This is an important criterion for the validity of the determined outflow direction. Of course, there are several cases in which the outflow direction was known already from direct imaging in the [SII] , 6731 lines or in .

The kinematical properties of the forbidden line emission were derived by Gaussian fits in the PV maps along the radial velocity axis. The typical error in radial velocity space was for the Cassegrain twin and ISIS spectra and for the coudé spectra. The velocity width (FWHM) of the individual components of the FEL was again determined by quadratic subtraction of the instrumental profile (FWHM) from the measured width (FWHM), i.e. .