ALMA Observations of Atomic Carbon [C I]
  (${^3\mathrm{P}}_1\rightarrow{^3\mathrm{P}}_0$) and Low-$J$ CO Lines in the
  Starburst Galaxy NGC 1808
  We present [C I] (${^3\mathrm{P}}_1\rightarrow{^3\mathrm{P}}_0$), $^{12}$CO,
$^{13}$CO, and C$^{18}$O ($J=2\rightarrow1$) observations of the central region
(radius 1 kpc) of the starburst galaxy NGC 1808 at 30-50 pc resolution
conducted with Atacama Large Millimeter/submillimeter Array. Radiative transfer
analysis of multiline data indicates warm ($T_\mathrm{k}\sim40\mathrm{-}80$ K)
and dense ($n_\mathrm{H_2}\sim10^{3\mathrm{-}4}$ cm$^{-3}$) molecular gas with
high column density of atomic carbon ($N_\mathrm{CI}\sim3\times10^{18}$
cm$^{-2}$) in the circumnuclear disk (central 100 pc). The C I/H$_2$ abundance
in the central 1 kpc is $\sim3-7\times10^{-5}$, consistent with the values in
luminous infrared galaxies. The intensity ratios of [C I]/CO(1-0) and [C
I]/CO(3-2), respectively, decrease and increase with radius in the central 1
kpc, whereas [C I]/CO(2-1) is uniform within statistical errors. The result can
be explained by excitation and optical depth effects, since the effective
critical density of CO (2-1) is comparable to that of [C I]. The distribution
of [C I] is similar to that of $^{13}$CO (2-1), and the ratios of [C I] to
$^{13}$CO (2-1) and C$^{18}$O (2-1) are uniform within $\sim30\%$ in the
central $<400$ pc starburst disk. The results suggest that [C I]
(${^3\mathrm{P}}_1\rightarrow{^3\mathrm{P}}_0$) luminosity can be used as a
CO-equivalent tracer of molecular gas mass, although caution is needed when
applied in resolved starburst nuclei (e.g., circumnuclear disk), where the [C
I]/CO(1-0) luminosity ratio is enhanced due to high excitation and atomic
carbon abundance. The [C I]/CO(1-0) intensity ratio toward the base of the
starburst-driven outflow is $\lesssim0.15$, and the upper limits of the mass
and kinetic energy of the atomic carbon outflow are $\sim1\times10^4~M_\odot$
and $\sim3\times10^{51}$ erg, respectively.
