CO and [C ii] line emission of molecular clouds: the impact of stellar feedback and non-equilibrium chemistry

We analyse synthetic (CO)-C-12,(CO)-C-13, and [C II ] emission maps of molecular cloud (MC) simulations from the SILCC-Zoom project. We present radiation, magnetohydrodynamic zoom-in simulations of individual clouds, both with and without radiative stellar feedback, forming in a turbulent multiphase interstellar medium following on-the-fly the evolution of e.g. H-2 , CO, and C+ . We introduce a no v el post-processing routine based on CLOUDY which accounts for higher ionization states of carbon due to stellar radiation in H II regions. Synthetic emission maps of [CII ] in and around feedback bubbles show that the bubbles are largely devoid of [C II ], as recently found in observations, which we attribute to the further ionization of C + into C-2 +. For both (CO)-C-12 and (CO)-C-13, the cloud-averaged luminosity ratio, L-CO /L-[CII], can neither be used as a reliable measure of the H-2 mass fraction nor of the evolutionary stage of the clouds. We note a relation between the I-CO/I-[C II ] intensity ratio and the H-2 mass fraction for individual pixels of our synthetic maps. The scatter , however , is too large to reliably infer the H-2 mass fraction. Finally, the assumption of chemical equilibrium o v erestimates H-2 and CO masses by up to 150 and 50 per cent, respectively, and L-CO by up to 60 per cent. The masses of H and C (+) would be underestimated by 65 and 30 per cent, respectively, and L([C II ] )by up to 35 per cent. Hence, the assumption of chemical equilibrium in MC simulations introduces intrinsic errors of a factor of 2 in chemical abundances, luminosities, and luminosity ratios.

Automated summary

In this study, synthetic (CO)-C-12, (CO)-C-13, and [C II] emission maps are analysed to study molecular cloud simulations. The findings indicate that feedback bubbles are mostly devoid of [C II] emission, which is attributed to further ionization of C + into C-2 +. Additionally, the assumption of chemical equilibrium leads to overestimation of H-2 and CO masses and underestimation of H and C (+) masses, as well as intrinsic errors in luminosities and luminosity ratios.