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

Abstract

We analyse synthetic emission maps of the CII 158 mu m 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. 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 C$^ $ inside HII regions, which is crucial to obtain reliable 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 C$^ $ contributes significantly. This leads to a large drop in deficit inside HII regions, with deficit decreasing from 10$^ $ at scales above 10 pc to around 10$^ $ at scales below 2 pc. However, projection effects can significantly affect the radial profile of $I_ CII $, $I_ FIR $, and their ratio, and can create apparent HII regions without any stars. Considering the evolution on MC scales, we show that the luminosity ratio, deficitl , decreases from values of gtrsim 10$^ $ in MCs without star formation to values of around $ $ 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 C$^ $ by stellar radiation. The drop in the deficitl 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 deficitl . (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 deficitl ratio at low values of $