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

¹ Universität zu Köln, I. Physikalisches Institut, Zülpicher Str. 77, 50937 Köln, Germany
² Center for Data and Simulation Science, University of Cologne, 50923 Köln, Germany
³ Research Center for Astronomical Computing, Zhejiang Laboratory, Hangzhou 311100, China

^★ Corresponding author; seifried@ph1.uni-koeln.de

Received: 21 February 2024
Accepted: 23 October 2024

Abstract

Aims. We analyse synthetic emission maps of the [CII] 158 µm line and far-infrared (FIR) continuum of simulated molecular clouds (MCs) within the SILCC-Zoom project to study the origin of the observed [CII] deficit, that is, the drop in the [CII]/FIR intensity ratio caused by stellar activity.

Methods. All simulations include stellar radiative feedback and the on-the-fly chemical evolution of hydrogen species, CO, and C⁺. We also account for further ionisation of C⁺ into C²⁺ inside HII regions, which is crucial to obtain reliable results.

Results. Studying individual HII regions, we show that I_FIR is initially high in the vicinity of newly born stars, and then moderately decreases over time as the gas is compressed into dense and cool shells. In contrast, there is a large drop in I_CII over time, to which the second ionisation of C⁺ into C²⁺ contributes significantly. This leads to a large drop in I_[CII] /I_FIR inside HII regions, with I_[CII] /I_FIR decreasing from 10⁻³−10⁻² at scales above 10 pc to around 10⁻⁶−10⁻⁴ at scales below 2 pc. However, projection effects can significantly affect the radial profile of I_[CII]/I_FIR, and their ratio, and can create apparent HII regions without any stars. Considering the evolution on MC scales, we show that the luminosity ratio, L_[CII]/L_FIR, decreases from values of ≳10⁻² in MCs without star formation to values of around ~10⁻³ in MCs with star formation. We attribute this decrease and thus the origin of the [CII] deficit to two main contributors: (i) the saturation of the [CII] line and (ii) the conversion of C⁺ into C²⁺ by stellar radiation. The drop in the L_[CII]/L_FIR ratio can be divided into two phases: (i) During the early evolution of HII regions, the saturation of [CII] and the further ionisation of C⁺ limit the increase in L_[CII], while L_FIR increases rapidly, leading to the initial decline of L_[CII]/L_FIR. (ii) In more evolved HII regions, L_CII stagnates and even partially drops over time due to the aforementioned reasons. L_FIR also stagnates as the gas gets pushed into the cooler shells surrounding the HII region. In combination, this keeps the global L_[CII]/L_FIR ratio at low values of ~10⁻³.