2. Observations and data reduction
The data presented in this paper were taken in December 1994 at La Palma, using the Intermediate Dispersion Spectrograph (IDS) on the Isaac Newton Telescope. The TEK 3 1280x1180 pixel CCD was employed as the detector and the R1200Y grating used as our dispersing element. The spectral resolution obtained was 0.39 Å/pixel and the spatial resolution /pixel. Additional data used in the plot of the extent of the forbidden [SII]6716/6731 emission (position angle of 226o in Fig. 1) are from a large sample of HAEBES spectra described in Corcoran & Ray (1997).
The long-slit data were reduced using the IRAF 1 long-slit package with bias subtraction, flat field correction, sky subtraction and wavelength calibration applied. Additionally the stellar continuum was subtracted from the region of interest (here the wavelength range about the [SII]6716/6731 lines). This subtraction is necessary to increase the contrast of the forbidden emission line region with respect to the background thus providing greater detail close to the star. J. Solf (see, e.g., Böhm & Solf 1994) developed a method of subtracting a model continuum using an appropriate standard star rotationally broadened to match the rotation of the TTS, as used in the observations of TTS by Hirth et al. (1994). Examining a number of standard stars of spectral types A taken at the same time as the HAEBES observations, we note that the photosphere of LkH 233 should be essentially featureless at 6716/6731Å and thus no significant contribution from photospheric absorption lines is expected. Therefore, we found it satisfactory to use a simple continuum subtraction method. We fitted each row in the wavelength domain with a low order polynomial, interpolating across the forbidden line region. The continuum image was constructed from the resulting fitted rows. Difference frames (e.g. Fig. 2) produced by subtracting the continuum frame from the original frame, then contain only line emission.
All velocities indicated (e.g. Fig. 2) are with respect to the stellar rest velocity as estimated using the sodium (Na D) interstellar absorption lines (see Corcoran & Ray 1997), and from photospheric iron lines. The typical error in velocity is 8 kms-1. The positional error is . The results of the observations at various position angles are presented in Table 1, with velocities and extent of the high- and low-velocity components averaged over the two lines of the [SII]6716/6731 doublet. and are the average velocities, relative to the stellar photosphere, of the centroids of the HVC and LVC of the [SII]6716/6731 lines, respectively. and are the full widths half maximum of the HVC and LVC, corrected for instrumental width, in kms , as determined by a gaussian fit to each component. The error in the FWHM measurements is 20 kms . Yf - Yc is the extent of the forbidden line from the stellar photo-centre measured from the first contour of the forbidden emission above the background 3 level.
Table 1. Geometry and velocities of the jet/counter-jet from LkH 233.
© European Southern Observatory (ESO) 1998
Online publication: July 20, 1998