Observations of neutral atomic carbon at 809 GHz

We have detected the 809 GHz 3P2-3P1 fine-structure line of neutral atomic carbon in four dense molecular clouds: M17, W51, W3, and DR 21(OH). These observations complement the published observations of the 492 GHz 3P1-3P0 line and allow the excitation temperature of the 3P levels along with the line optical depths to be determined. The results indicate excitation temperatures Tx approximately 30-60 K and optical depths of tau 10 < or approximately 1. This implies that the approximately 10(18) cm-2 lower limit to the C I abundance derived from 492 GHz observations is probably the actual abundance, which gives C I/CO approximately 0.1 in dense molecular clouds.