Large scale mapping of Ci and the Ci-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 conducted 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 Ci column densities, N(C i), range from 4.85 × 1014 cm−2 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 Ci abundance varies less than CO in regions with UV 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 Ci-to-CO transition occurs. With current observation sensitivity, [C i] emission is detected toward 3.5% pixels of CO-dark gas around the cloud edge. Utilizing [C I] as 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 calculation 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.