Distribution of the [C I] emission in the ρ Ophiuchi dark cloud

The ³P₁-³P₀ fine-structure line of the neutral carbon atom ([C I]) has been mapped over the 1°.8 × 1°.3 area of the L1688 cloud in the ρ Ophiuchi region with the Mount Fuji submillimeter-wave telescope. The ³P₂- ³P₁ line of [C I] has also been observed toward two representative positions to evaluate the excitation temperature of the [C I] lines. The overall extent of the [C I] distribution generally resembles that of the ¹³CO distribution. The [C I] distribution has two major peaks; one (peak I) is at ρ Oph A, and the other (peak II) is toward the east side of the C¹⁸O core in the southern part of L1688. Peak II is located beyond the C¹⁸O core with respect to the exciting star HD 147889. The C⁰ column density is 5.0 × 10¹⁷ cm^-2 toward peak II. The spatial distribution of the [C I] emission is compared with plane-parallel photodissociation region (PDR) models, which suggest that peak II is associated with a lower density PDR front, adjacent to the dense cloud cores observed in the C¹⁸O line emission. Alternatively, peak II is in the early stage of chemical evolution, where C⁰ has not been completely converted to CO. In this case, the difference in the [C I] and C ¹⁸O distributions represents an evolutionary sequence. This is consistent with a picture of a shock-compressed formation of the dense cores in this region due to influences from the Sco OB2 association.