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
  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.
