The origin and evolution of the [CII] deficit in HII regions and star-forming molecular clouds

Abstract

We analyse synthetic maps of the [CII] 158 $\mu$m line and FIR continuum of simulated molecular clouds (MCs) within the SILCC-Zoom project to study the origin of the [CII] deficit, i.e., the drop in the [CII]/FIR intensity ratio. All simulations include stellar radiative feedback and account for further ionisation of C${}^{+}$ into C${}^{2+}$ inside HII regions. For individual HII regions, $I_{\mathrm{FIR}}$ is initially high in the vicinity of young stars, and then moderately decreases as the gas is compressed into shells. In contrast, $I_{\mathrm{CII}}$ drops strongly over time, to which the second ionisation of C${}^{+}$ into C${}^{2+}$ contributes. This leads to a large drop in $I_{[\mathrm{CII}]}/I_{\mathrm{FIR}}$ inside HII regions, decreasing from 10${}^{-3}$-10${}^{-2}$ at scales above 10 pc to 10${}^{-6}$-10${}^{-4}$ at scales below 2pc. However, projection effects can affect the radial profile of $I_{[\mathrm{CII}]}$ and $I_{\mathrm{FIR}}$ and create apparent HII regions without any stars. On MC scales, $L_{[\mathrm{CII}]}/L_{\mathrm{FIR}}$ decreases from values $\gtrsim$10${}^{-2}$ in MCs without star formation to values around $\sim 10^{-3}$ in MCs with star formation. We attribute this and the origin of the [CII] deficit to two main contributors: (i) the saturation of the [CII] line and (ii) the conversion of C${}^{+}$ into C${}^{2+}$ by stellar radiation. The drop in $L_{[\mathrm{CII}]}/L_{\mathrm{FIR}}$ can be divided into two phases: (i) early on, the saturation of [CII] and the further ionisation of C${}^{+}$ limit the increase in $L_{[\mathrm{CII}]}$, while $L_{\mathrm{FIR}}$ increases rapidly, leading to the initial decline of $L_{[\mathrm{CII}]}/L_{\mathrm{FIR}}$. (ii) In more evolved HII regions, $L_{\mathrm{CII}}$ stagnates and even partially drops due to the aforementioned reasons. $L_{\mathrm{FIR}}$ stagnates as the gas gets pushed into the cooler shells keeping $L_{[\mathrm{CII}]}/L_{\mathrm{FIR}}$ at low values of $\sim 10^{-3}$.

18 pages, 18 figures including appendix. Accepted for publication in A&A, updated to final version

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