Extensive [CI] Mapping toward the Orion-A Molecular Cloud
  We have carried out wide-field (0.17 degree^2) and high-angular resolution
(21.3 arcsec ~ 0.04 pc) observations in [CI] line toward the Orion-A giant
molecular cloud with the Atacama Submillimeter Telescope Experiment (ASTE) 10 m
telescope in the On-The-Fly (OTF) mode. Overall features of the [CI] emission
are similar to those of the CO (1--0) emission in Shimajiri et al. (2011); the
total intensity ratio of the [CI] to CO emission ranges from 0.05 to 0.2. The
optical depth of the [CI] emission is found to be 0.1 -- 0.75, suggesting
optically thin emission. The column density of the [CI] emission is estimated
to be (1.0 -- 19) x 10^17 cm^-2. These results are consistent with the results
of the previous [CI] observations with a low-angular resolution of 2.2 arcmin
(e.g. Ikeda et al. 1999). In the nearly edge-on PDRs and their candidates of
the Orion Bar, DLSF, M 43 Shell, and Region D, the distributions of the [CI]
emission coincide with those of the CO emission, inconsistent with the
prediction by the plane-parallel PDR model (Hollenbach & Tielens 1999). In
addition, the [CI] distribution in the Orion A cloud is found to be more
similar to those of the ^{13}CO (1--0), C^{18}O (1--0), and H^{13}CO^+ (1--0)
lines than that of the CO (1--0) line, suggesting that the [CI] emission is not
limited to the cloud surface, but is tracing the dense, inner parts of the
cloud.
