Extensive [CI] Mapping toward the Orion-A Molecular Cloud

TL;DR

This study maps [CI] emission across the Orion-A molecular cloud with high resolution, revealing that [CI] traces dense inner regions of the cloud and does not conform to traditional PDR models.

Contribution

First wide-field, high-resolution [CI] mapping of Orion-A, showing [CI] distribution aligns with dense gas tracers and challenges existing PDR models.

Findings

[CI] emission is optically thin with a ratio to CO of 0.05-0.2.

[CI] distribution coincides with dense gas tracers, not just cloud surfaces.

Results are consistent with previous low-resolution observations.

Abstract

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.