Astron. Astrophys. 321, 220-228 (1997)

2. Observations and data reduction

Spatially resolved VRI photometry and optical (600 nm to 720 nm) spectroscopy were obtained at the European Southern Observatory, La Silla, with a CCD camera attached to the Danish 1.54m telescope and with EMMI (ESO Multi Mode Instrument) at the 3.5m New Technology Telescope. All observations were carried out under subarcsec seeing conditions. The logs of the observations are given in Tables 1 & 2.

Table 1. Photometric observations with the Danish 1.54m telescope/CCD camera (2.3.1995)

Table 2. Spectroscopic Observations with NTT/EMMI

The data reduction was carried out using IDL, IRAF, and a stand-alone version of GaussFit (Jeffrys et al. 1991). As the spectra and images of the binary components were blended, we had to apply special techniques in order to deblend the observed intensity distributions. Fitting models using least-squares-fit methods proved to produce more stable and reliable results than using highly non-linear deconvolution techniques.

Fig. 1 shows as an example the 2D long-slit spectrum of the PMS binary star Sz 30. The slit had been oriented along the direction of both components. While the peaks of the intensity distributions for both components are separated, the wings overlap. For even closer binaries the peaks are no longer resolved. This is clearly visible in Fig. 2, which gives a 1D cut in spatial direction of the long-slit spectrum of the binary Sz 20 (crosses). In order to separate the intensity distribution of the spectra of the binary components using IDL we fitted a simple model consisting of two Gaussians (Fig. 2, dashed lines) to each 1D cut in spatial direction of the 2D spectra. The resulting resolved spectra of the components of Sz 24 and Sz 30 are shown in Fig. 3. The spectra of both components of the binary Sz 24 are now clearly separated. Note that the emission lines of OI636.2nm, HeI667.9nm, and HeI706.6nm are present only in the spectrum of the primary. Sz 30 is a visual triple system with the tertiary separated by from the primary. The tertiary was not included in the present survey. The typical error in the spectra of the secondaries of the closest binaries amounts to 10%.

Fig. 1. 2D long-slit spectrum of the pre-main-sequence binary Sz 30 obtained with EMMI at the ESO New Technology Telescope. The strong H emission lines of both components are clearly visible.

Fig. 2. 1D cut in spatial direction of the 2D long-slit spectrum of Sz 20. The observed intensity distribution is indicated by crosses. Two Gaussian (dashed lines) were fitted in order to model the intensity distribution (solid line).

Fig. 3. Resolved spectra of Sz 24 (VW Cha, sep. ) and Sz 30 (sep. 11). All binary components show Lithium absorption at 670.7 nm. For Sz 24, the OI636.2nm, HeI667.9nm, and HeI706.6nm emission lines are present only in the primary.

For the photometric data we first modeled the local point spread function using field stars and then applied this model to the observed intensity distribution of the binaries using GaussFit for the fitting.

The flux calibration was carried out within IRAF by comparison with photometric and spectro-photometric standard stars ¹.

© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998
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