Large scale mapping of [C_I] and the [C_I]-to-CO transition in ρ Ophiuchus molecular cloud

Atomic carbon ([C_I]) is a key species in the carbon chemistry of the interstellar medium (ISM). Using the Submillimeter Wave Astronomy Satellite (SWAS), we conduct a [C_I](³P₁ → ³P₀) 492 GHz survey covering approximately 4 deg² of the L1688 and L1689 regions in the ρ Oph molecular cloud, achieving a spatial resolution of 4.25′. The derived [C_I] column densities, N([C_I), range from 4.85 × 10¹⁴ to 6.29 × 10¹⁷ cm⁻², corresponding to an abundance ratio N([C_I])/N(H₂) of 2.24 × 10⁻⁷ to 2.39 × 10⁻⁴, with a median value of 1.8 × 10⁻⁵. Combining observations with photodissociation region (PDR) modeling, we find that [C_I] abundance varies less than that of CO in regions with Ultraviolet intensity G₀ > 16 and N(H₂) < 4.6 × 10²¹ cm⁻², suggesting [C_I] is a more reliable tracer of molecular hydrogen in low-density, high-radiation environments where the [C_I]-to-CO transition occurs. Utilizing [C_I] as a direct H₂ tracer, the CO-dark gas fraction is estimated to be 0.43, meaning that 43% of the total cloud mass will be missed by conventional calculations based on CO observations but can be calibrated by [C_I] emission. The [C_I] line widths are systematically broader than those of ¹³CO, possibly due to contributions from atomic carbon. These findings provide key insights into Galactic [C_I] emission and the carbon cycle evolution in the interstellar medium. Future high-sensitivity [C_I](³P₁ → ³P₀) surveys with the Chinese Survey Space Telescope (CSST) will significantly advance our understanding of the carbon cycle evolution.