Context. Small linear carbon chain radicals such as C₂ and C₃ 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 C₂ and C₃ using the ESO Diffuse Interstellar Bands Large Exploration Survey (EDIBLES) dataset. We also expanded our searches to C₄, C₅, and the ¹³C¹²C isotopologue in the most molecule-rich sightlines.
Methods: We fitted synthetic spectra generated following a physical excitation model to the C₂ (2-0) Phillips band to obtain the C₂ column density (N) as well as the kinetic temperature (T_kin) and number density (n) of the host cloud. The C₃ molecule was measured through its à − \tilde X (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 C₂ and C₃ to date in which the individual transitions can be resolved. In total, we detected C₂ in 51 velocity components along 40 sightlines, and C₃ 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(C₂) and N(C₃) with a Pearson correlation coefficient r = 0.93 and an average N(C₂)/N(C₃) ratio of 15.5± 1.4. A comparison with the behaviour of the C₂ diffuse interstellar bands (DIBs) shows that there are no clear differences among sightlines with and without detections of C₂ and C₃. This is in direct contrast to the better-studied non-C₂ DIBs, which have reduced strengths in molecule-rich environments, consistent with the idea that the C₂ 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 ¹³C¹²C (2-0) Phillips band in the stacked average spectrum of molecule-rich sightlines, and estimate the isotopic ratio of carbon ¹²C/¹³C 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 C₄ and C₅ optical bands was unsuccessful; even in stacked spectra no unambiguous identification could be made.