2 The observational data

The observational material has been collected at ESO with the aid of the CES (Coude Echelle Spectrograph) fed by the fiber link with the Cassegrain focus of the 3.6 m telescope of the La Silla Observatory. All the stars have been observed with the highest resolving power, using the Very Long Camera in the spectral range from 4040 to 4066 Å. Centering CES at a wavelength of 4053 Å we determined $R \sim 220~000$ by the full width at half maximum of $\sim$2.7 pixel for a narrow line in the spectrum. The instrument is equipped with an image slicer which splits the starlight into a dozen of well-illuminated slices. The detector is an EEV 2K$\times$4K CCD (pixel size 15$\times$15 $\mu$m) with 80% quantum efficiency in the domain of interest.

The objects for this project were chosen using the existing sample of McDonald spectra (Kreowski & Sneden 1993) which includes the NaI D₁ and D₂ lines as well as the major 5780 and 5797 DIBs. Spectra of some objects have been acquired also using the high resolution (R=120 000) echelle spectrometer fed with the 2 m. telescope of the Terskol Observatory (Northern Caucasia) - in this case the KI line at $\sim$7700 Å was observed. The chosen targets are listed in Table 1 where HD numbers, spectral types, luminosity classes, colour excesses and rotational velocities are given. We also added some other interstellar data such as intensities of the 5780 and 5797 Å major DIBs (measured in the McDonald spectra) and the column densities of the C₂ molecule (found in publications). The targets were selected using the profiles of atomic interstellar lines i.e. they do not show Doppler splitting (Fig. 1). Such a choice of targets is necessary to see the closely packed rotational features of the C₃ band resolved. We have added HD 152236, a relatively bright, reddened southern star. This star is known to have relatively strong CH, CN and C₂ interstellar lines (Crawford 1990; Crawford 1995), what leads, together with the enormous strength of the 5850 Å diffuse band, usually well correlated with the 5797 Å DIB (Jenniskens et al. 1996), to the conclusion that it is the "zeta'' type object, although the behaviour of major DIBs is not known directly. The brightness of all our targets was crucial to allow the achievement of high S/N ratio.

Figure 1 clearly demonstrates that some of the targets, selected for the project and listed in Table 1, show very strong KI lines while the others - very weak. The same division can be made using the intensity ratio of the major DIBs: 5780 and 5797 as a criterion - see Fig. 2. Apparently the 5797/5780 ratio grows together with that of KI/E(B-V) i.e the band 5797 seems to be correlated with interstellar atomic lines as suggested by Kreowski et al. (1997). Narrow DIBs generally look stronger in "zeta'' type interstellar clouds than in "sigma'' ones as already suggested by Kreowski & Sneden (1995). Our sample deliberately contains both "sigma'' and "zeta'' type objects to make a comparison of the behaviour of the C₃ chain in both environments possible.

Every target, listed in the lower part of Table 1, was observed at least twice to make a successful removal of cosmic ray spikes possible. The exposure times were selected to achieve the S/N ratio higher than 500 as the expected depths of the C₃ features are of the order of 1% of the continua. Our target list contains HD 149757, observed also by Maier et al. (2001) to allow a direct comparison between the existing and new measurements.