Observation of the C [CSC]i[/CSC] [TSUP]3[/TSUP][ITAL]P[/ITAL][TINF]2[/TINF]–[TSUP]3[/TSUP][ITAL]P[/ITAL][TINF]1[/TINF] Line toward the Orion Kleinmann-Low Region

The 3P2-3P1fine-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 61×61area centered atOrion KL has been mapped with a grid spacing of 1:5. The intensity distribution of the3P2-3P1 line is found to be similar to that of the 3P1-3P0 line; these lines are rather weak toward Orion KL, while they are both bright at Orion KL’s northern and southern positions. The excitation temperature determined from the intensity ratiobetween the 3P2-3P1 and 3P1-3P0 lines ranges from 40 to 110 K. The excitation temperature is not enhanced toward Orion KL, whereas it tends to be high in the vicinity of θ1 Orionis C. These results indicate that the C I emission arises from a photodissociation surface illuminated by strong UV radiation from θ1 Ori C. The relative reduction in the C Ⅰ intensities toward Orion KL is found to originate from a relatively low excitation temperature rather than from the depletion of the C Ⅰ column density. The origin of the low-excitation temperature of C Ⅰ toward Orion KL is discussed in terms of a radiative transfer effect