ALMA View of the ρ Ophiuchi A PDR with a 360 au Beam: The [C i] Emission Originates from the Plane-parallel PDR and Extended Gas

We present the results of data analysis of the [C i] (3P1–3P0) emission from the ρ Ophiuchi A photon-dominated region (PDR) obtained in the ALMA ACA standalone mode with a spatial resolution of 2farcs6 (360 au). The [C i] emission shows filamentary structures with a width of ∼1000 au, which are adjacent to the shell structure seen in the 4.5 μm map. We found that the 4.5 μm emission, C0, and CO are distributed in this order from the excitation star (S1) in a complementary pattern. These results indicate that [C i] is emitted from a thin layer in the PDR generated by the excitation star, as predicted in the plane-parallel PDR model. In addition, extended [C i] emission was also detected, which shows nearly uniform integrated intensity over the entire field of view (1farcm6 × 1farcm6). The line profile of the extended component is different from that of the above shell component. The column density ratio of C0 to CO in the extended component was ∼2, which is significantly higher than those of Galactic massive star-forming regions (0.1–0.2). These results suggest that [C i] is emitted also from the extended gas with a density of ${n}_{{{\rm{H}}}_{2}}\sim {10}^{3}$ cm−3, which is not greatly affected by the excitation star.