Highly excited-carbon level population and nature of the low-frequency radio line forming regions toward Cassiopeia A.

Observations of the C552α (39 MHz), C486α/C612β (57 MHz), C421α/C530β (88 MHz) and C382α/C481β (118 MHz) carbon radio recombination lines toward Cas A indicate significant deviation of the corresponding level populations from the equilibrium values. The radio lines have Doppler width of 15.9±1.5 km/s and most likely are the result of synthesis of the narrow lines originating in the outer envelopes of molecular clouds having different radial velocities. Carbon in the envelopes is ionized by diffuse ultraviolet radiation 912 Å < λ < 1101 Å.