The EDIBLES survey VII. A survey of C₂ and C₃ in interstellar clouds

Context. Small linear carbon chain radicals such as C-2 and C-3 act as both the building blocks and dissociation fragments of larger carbonaceous species. Their rotational excitation traces the temperature and density of local environments. However, these homo-nuclear di- and triatomic species are only accessible through their electronic and vibrational features because they lack a permanent dipole moment, and high signal-to-noise ratio data are necessary as the result of their generally low abundances in the interstellar medium (ISM).Aims. In order to improve our understanding of small carbonaceous species in the ISM, we carried out a sensitive survey of C-2 and C-3 using the ESO Diffuse Interstellar Bands Large Exploration Survey (EDIBLES) dataset. We also expanded our searches to C-4, C-5, and the (CC)-C-13-C-12 isotopologue in the most molecule-rich sightlines.Methods. We fitted synthetic spectra generated following a physical excitation model to the C-2 (2-0) Phillips band to obtain the C-2 column density (N) as well as the kinetic temperature (T-kin) and number density (n) of the host cloud. The C-3 molecule was measured through its & Atilde; - (000-000) electronic origin band system. We simulated the excitation of this band with a double-temperature Boltzmann distribution.Results. We present the largest combined survey of C-2 and C-3 to date in which the individual transitions can be resolved. In total, we detected C-2 in 51 velocity components along 40 sightlines, and C-3 in 31 velocity components along 27 sightlines. Further analysis confirms the two molecules are detected in the same velocity components. We find a very good correlation between N(C-2) and N(C-3) with a Pearson correlation coefficient r = 0.93 and an average N(C-2)/N(C-3) ratio of 15.5 +/- 1.4. A comparison with the behaviour of the C-2 diffuse interstellar bands (DIBs) shows that there are no clear differences among sightlines with and without detections of C-2 and C-3. This is in direct contrast to the better-studied non-C-2 DIBs, which have reduced strengths in molecule-rich environments, consistent with the idea that the C-2 DIBs are indeed a distinguishable DIB family. We also identify, for the first time, the Q(2), Q(3), and Q(4) transitions of the (CC)-C-13-C-12 (2-0) Phillips band in the stacked average spectrum of molecule-rich sightlines, and estimate the isotopic ratio of carbon C-12/C-13 to be 79 +/- 8, consistent with literature results. At this stage it is not yet possible to identify these transitions in individual sightlines. Our search for the C-4 and C-5 optical bands was unsuccessful; even in stacked spectra no unambiguous identification could be made.