3 Results

Our reduction of the spectra was made using the DECH code (Galazutdinov 1992). This program allows a flat-field division, bias/background subtraction, one-dimensional spectrum extraction from the 2-dimensional images, correction for the diffuse light, spectrum addition, excision of cosmic ray features, etc. The DECH code also allows location of a fiducial continuum, measurements of the line equivalent widths, line positions and shifts, etc.

Figure 3 presents the six reduced spectra. Our very high resolution and S/N ratio makes the very narrow C₃ features detectable. The C₃ band is evidently seen in three of spectra - they are all "zeta'' type objects in which the bands of C₂ are also quite strong. The C₃ bands are below the level of detection towards the remaining three "sigma'' type objects. Only the upper limits can be estimated which makes a measurement of C₂ to C₃ abundance ratio impossible in these environments.

It seems of importance to compare the intensities of C₃ transitions, measured in our spectra with those from the publication of Maier et al. (2001). The comparison is given in Fig. 5. It is evident that the observed features are usually of the same equivalent width, especially the strongest ones. The weaker features differ sometimes between the two sets of data. Thus it seems well-advised to use only the strongest C₃ features to estimate the abundance of this species towards some chosen targets. It may be also concluded that the resolution of R=120 000is high enough to allow precise measurements of the narrow C₃ features.

To compare C₃ and C₂ abundance ratios we preferred to use direct results of observations (equivalent widths) instead of published column density values, calculated using various oscillator strengths. Ratio of equivalent widths of C₃ and C₂ absorption bands may be a very sensitive parameter, characterizing individual interstellar clouds. Figure 4 demonstrates a comparison between the intensities of single transitions inside C₂ and C₃ bands. In the case of C₃ it is quite evident that the column density of this species can be estimated using the four strongest transitions (Q6 till Q12). Their equivalent widths (listed in Table 2) are similar and thus the level, based on the intensities of these features is a good measurement of the C₃ column density towards a chosen target. In the case of C₂ we used Q(2) and Q(4) transitions as they have been measured in all targets of Table 1. One can see from the Fig. 4 that the abundance ratio of the two bare carbon species: C₂ and C₃ is variable; it is very high towards the HD 179406 star which is known to be shining through a very special cloud. The latter produces very narrow but also very strong DIBs (Walker et al. 2001). We would like to emphasize that a C₃ abundance estimate should be rather uncertain in the case of HD 210121 (Roueff et al. 2002) due to relatively low spectral resolution evidently not enough to make reliable measurements of equivalent widths.

The plotted spectra suggest that C₃ molecule, as well as C₂, is hardly detectable in the observed "sigma'' type clouds. It generally behaves in a way very similar to that of the neighbour KI (4044.136 and 4047.206 Å) lines (see also Fig. 1). This observation confirms that the C₂ and C₃ abundances depend on the ionization level of elements characterized by the ionization potential lower than that of hydrogen. Apparently the molecular abundances depend on the density of high frequency photons originating in neighbour, hot stars.

Figure 5: A comparison of the equivalent widths of subsequent features of the C3 band in the spectrum of HD 149757 observed by Maier et al. (2001) with those from the present work.

Figure 6: Illustration of differences in C3 abundance in "zeta'' (HD 149757) and "sigma'' (HD 147165 and HD 144217) objects. Demonstrated spectra were achieved by shifting all rotational lines to point zero at radial velocity scale and combining them to get as a result one narrow structure that represents average absorption of all rotational levels.

The proximity of KI lines allows an additional test of the presence of C₃ features in "sigma'' type objects. While the wavelengths of potassium lines are shifted to the rest wavelength frame, C₃ transitions coincide with those, published by Roueff et al. (2002) up to the third decimal point! In this case the identification of the molecular band leaves no doubt. The high precision of the wavelengths of different transitions inside the C₃ band allows us to add the same spectrum up to $\sim$40 times in the scale of radial velocities. The spectrum is being shifted to zero radial velocity of subsequent transitions and then all spectra prepared in this way are combined. This creates just one strong feature at the zero radial velocity which may represent the total strength of the band. The S/N ratio of the spectrum prepared in this way is 5-6 times higher than the original one (Fig. 6). However, even this procedure has not allowed to measure directly the band intensity towards "sigma'' type objects; we can only make a better estimate of their upper limits. Table 1 gives the observed column densities of C₃ molecule and 3$\sigma$ upper limits for "sigma'' type stars derived from

$$\begin{eqnarray*}W_{3\sigma}=3 \times (wd)^{1/2}(S/N)^{-1}, \end{eqnarray*}$$

where $W_{3\sigma}$ is 3$\sigma$ limiting equivalent width, w is FWHM of the feature presented in Fig. 6, estimated as 0.03 Å, d is the spectrograph dispersion, in Å/pixel, and S/N is signal-to-noise level of spectra presented in Fig. 6.

C₃ molecule was thus proved to be below the level of detection towards "sigma'' type targets as well as the shorter C₂ molecule (Lambert et al. 1995). Apparently the environments, characterized by relatively strong broad diffuse bands and weak polar molecules do not contain short, bare carbon chain species.

C₂ and C₃ molecules are much more likely observed towards targets characterized by strong narrow DIBs and the features of simplest polar molecules as well as high far-UV extinction ("zeta'' type objects). In these cases both carbon species are usually observable but their mutual abundance ratio may be variable.

As it is clearly seen in Fig. 4 the C₂ and C₃ abundances are not simply related to E_B-V; they must depend on other parameters of intervening clouds as well.

Neither C₂ nor C₃ can be considered as carriers of any of the diffuse interstellar bands. However, they are likely precursors of the species in which narrow DIBs (such as 5797) are originated.

An extension of the sample of C₃ observations towards reddened stars seems highly desirable as the above conclusions are inferred from extremely scarce samples of reddened stars.

Acknowledgements

The authors want to express their gratitude to the staff of the ESO for the technical help during the observations. JK acknowledges the financial support of the French-Polish project JUMELAGE. GAG wants to express his thanks to Russian Foundation for Basic Research for financial support under the grant No 02-02-174423.