Large scale mapping of [CI] and the [CI]-to-CO transition in $\rho$ Ophiuchus molecular cloud

Abstract

Atomic carbon ([CI]) is a key species in the carbon chemistry of the interstellar medium (ISM). Using the Submillimeter Wave Astronomy Satellite (SWAS), we conducted a [CI](${}^3$P${}_1$--${}^3$P${}_0$) 492 GHz survey covering approximately 4 deg${}^2$ of the L1688 and L1689 regions in the $\rho$ Oph molecular cloud, achieving a spatial resolution of 4.25$\hbox{$^{\prime}$}$. The derived [CI] column densities, N([CI]), range from 4.85 $\times$ 10${}^{14}$ cm${}^{-2}$ to 6.29 $\times$ 10${}^{17}$ cm${}^{-2}$, corresponding to an abundance ratio N([CI])/N($H_2$) of 2.24$\times$ 10${}^{-7}$ to 2.39$\times$ 10${}^{-4}$, with a median value of 1.8$\times$ 10${}^{-5}$. Combining observations with photodissociation region (PDR) modeling, we find that [CI] abundance varies less than CO in regions with UV intensity G${}_0$ $>16$ and N(H${}_2$) $<$ 4.6 $\times$ 10${}^{21}$ cm${}^{-2}$, suggesting [CI] is a more reliable tracer of molecular hydrogen in low-density, high-radiation environments where the [CI]-to-CO transition occurs. Utilizing [CI] as direct H${}_2$ 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 [CI] emission. The [CI] line widths are systematically broader than those of ${}^{13}$CO, possibly due to contributions from atomic carbon. These findings provide key insights into Galactic [CI] emission and the carbon cycle evolution in the interstellar medium. Future high-sensitivity [CI] (${}^3$P${}_1$--${}^3$P${}_0$) surveys with the Chinese Survey Space Telescope (CSST) will significantly advance our understanding of the carbon cycle evolution.

16 pages, 12 figures. Accepted for publication in Science China Physics, Mechanics & Astronomy