Observations of Rotationally Resolved C3in Translucent Sight Lines

The rotationally resolved spectrum of the A 1 Π u ← X 1 Σ 000-000 transition of C 3 , centered at 4051.6 Å, has been observed along 10 translucent lines of sight. To interpret these spectra, a new method for the determination of column densities and analysis of excitation profiles involving the simulation and fitting of observed spectra has been developed. The populations of lower rotational levels ( J ≤ 14) in C 3 are best fitted by thermal distributions that are consistent with the kinetic temperatures determined from the excitation profile of C 2 . Just as in the case of C 2 , higher rotational levels ( J > 14) of C 3 show increased nonthermal population distributions in clouds that have been determined to have total gas densities below ~500 cm -3 .