Dicarbon molecule in the interstellar clouds†

We present high-resolution and high signal-to-noise spectroscopic observations of interstellar molecular lines of C₂ towards early-type stars. C₂ is particularly interesting because it is the simplest multicarbon molecule and its abundances give information on the chemistry of interstellar clouds, especially on the pathway of formation of (hydro)carbon chains and PAHs which may be considered as possible carriers of diffuse interstellar bands (DIBs). Homonuclear diatomic molecules have negligible dipol moments and hence radiative cooling of excited rotational levels may go only trough the slow quadrupole transitions (van Dishoeck & Black 1982). In C₂, pumped by galactic average interstellar field rotational levels are excited effectively much above the gas kinetic temperature and a rotational ladder of electronic transitions is usually observed from high rotational levels. Relations between abundances of the dicarbon and other simple interstellar molecules are considered as well.