ALMA view of the $\rho$ Ophiuchi A PDR with a 360-au beam: the [CI] emission originates from the plane-parallel PDR and extended gas

TL;DR

This study uses ALMA observations to reveal that [CI] emission in the $ ho$ Ophiuchi A PDR originates from both a thin, star-illuminated layer and extended, diffuse gas, challenging previous assumptions about its distribution.

Contribution

It provides high-resolution evidence that [CI] emission comes from both a plane-parallel PDR layer and extended gas, offering new insights into the structure of PDRs.

Findings

[CI] emission forms filamentary structures near the star

Extended [CI] emission shows uniform intensity across the field

The C$^0$ to CO ratio in extended gas is higher than in typical star-forming regions

Abstract

We present the results of data analysis of the [CI] ($^{3}P_{1}$-$^{3}P_{0}$) emission from the $\rho$ Ophiuchi A photon-dominated region (PDR) obtained in the ALMA ACA stand-alone mode with a spatial resolution of 2.''6 (360 au). The [CI] emission shows filamentary structures with a width of $\sim$1000 au, which are adjacent to the shell structure seen in the 4.5 $\mu$m map. We found that the 4.5 $\mu$m emission, C$^0$, and CO are distributed in this order from the excitation star (S1) in a complementary pattern. These results indicate that [CI] 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 [CI] emission was also detected, which shows nearly uniform integrated intensity over the entire field-of-view (1.'6$\times$1.'6). The line profile of the extended component is different from that of the above shell component. The column density ratio of C$^0$ to CO in the extended component was $\sim$2, which is significantly higher than those of Galactic massive star-forming regions (0.1-0.2). These results suggest that [CI] is emitted also from the extended gas with a density of $n_\mathrm{H_2} \sim 10^3$ cm$^{-3}$, which is not greatly affected by the excitation star.