Observation of the (CIP2)-P-3-P-3(1) line toward the Orion Kleinmann-Low region

The P-3(2)-P-3(1) fine-structure line of the neutral carbon atom (809 GHz) has been observed toward the Orion Kleinmann-Low (KL) region with the Mount Fuji submillimeter-wave telescope. The 6' x 6' area centered at Orion KL h as been mapped with a grid spacing of 1'.5. The intensity distribution of t he P-3(2)-P-3(1) line is found to be similar to that of the P-3(1)-P-3(0) l ine; these lines are rather weak toward Orion KL, while they are both brigh t at Orion KL's northern and southern positions. The excitation temperature determined from the intensity ratio between the P-3(2)-P-3(1) and P-3(1)-P -3(0) lines ranges from 40 to 110 K. The excitation temperature is not enha nced toward Orion KL, whereas it tends to be high in the vicinity of theta (1) Orionis C. These results indicate that the C I emission arises from a p hotodissociation surface illuminated by strong UV radiation from theta (1) Ori C. The relative reduction in the C I intensities toward Orion KL is fou nd to originate from a relatively low excitation temperature rather than fr om the depletion of the C I column density. The origin of the low-excitatio n temperature of C I toward Orion KL is discussed in terms of a radiative t ransfer effect.