EXCITATION OF C2 IN DIFFUSE INTERSTELLAR CLOUDS

We investigate the effects and the implications of incorporating new collision and radiative rates in modeling the excitation of diatomic carbon molecule. The present results suggest that diffuse and translucent interstellar clouds may present a structure in which regions with different densities and kinetic temperatures overlap along the line of sight, such as core-halo clouds, the nested structure of the molecular gas, and clumpiness. Such conclusion reflects the response of the C2 rotational ladder to the interplay of thermal and radiative conditions, with low and high rotational levels tracing different regions of the parameter space. To relieve constraints to the formation and excitation of C2 molecules, we propose a scenario in which the chemistry in diffuse clouds is supplemented by chemistry in many transient and tiny perturbations.