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

Abstract

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 2.6 arcsec (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 (1'6 × 1'6). 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.