Large scale mapping of [CI] and the [CI]-to-CO transition in $ρ$ 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 $ρ$ 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.

Figures (12)

• Figure 1: Sky coverage of multi-wavelength observations toward the $\rho$ Ophiuchi cloud. The background is the integrated intensity of [C i]Ṫhe line in brown is the contour of the data coverage of $^{12}$CO and $^{13}$CO by FCRAO. The line in light blue is the contour of the data coverage of C$^{18}$O by FCRAO. The line in yellow is the contour of the data coverage of [C i] by SWAS. The line in purple is the contour of the data coverage of dust emission observed by Herschel and used to derive the H$_2$ column density.
• Figure 2: The contours of the total integrated intensity maps of [C i] ($^3$P$_1$ -$^3$P$_0$) emission overlaid on images of four different tracers: $^{12}$CO (integrated from -15 km s$^{-1}$ to 20 km s$^{-1}$), $^{13}$CO (integrated from -15 km s$^{-1}$ to 20 km s$^{-1}$), [C i] (integrated from -15 km s$^{-1}$ to 20 km s$^{-1}$), and H$_2$ column density map. The white contour levels are 0, 10, and 20 K km s$^{-1}$. The B2V star HD 147889 is indicated by a red star symbol in all panels.
• Figure 3: Spectra of [C i] $^{12}$CO, $^{13}$CO, and C$^{18}$O emission with initial velocity resolution toward three selected positions: Peak 1, Peak 2 and Peak 3.
• Figure 4: (a): Spatial distribution of different mask regions based on [C i] and CO(1-0) emission. The edge of CO data is defined by a solid chocolate-colored line. Regions where [C i] intensity is weaker than CO(1-0) intensity (W([C i]) $<$ W(CO(1-0))) are shown in gray, while stronger [C i] regions (W([C i]) $>$ W(CO(1-0))) are shown in red and cyan. Among the latter, the cyan pixels represent areas that would be excluded by applying a two-pixel buffer beyond the data boundary, a threshold implemented to mitigate convolution artifacts from external null values. (b) and (c): The spectra of two representative positions, A and B, where the [C i] emission is stronger than that of $^{12}$CO.
• Figure 5: Spatial distribution of column densities N([C i]) (Panel a) and N($^{13}$CO) (Panel b). The three peak of W([C i]) are marked on the N($^{13}$CO) map.