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Within the SILCC-Zoom project, we study the impact of magnetic fields and radiative stellar feedback on the emission of CO using the CO, J=1-0 line. In Seifried et al., 2019 ( arxiv:1906.01015 ), we present a detailed analysis of the CO emission and its dependence on the local cloud properties. 
We show that CO-bright gas is associated with gas below a temperature of ~50 K and above a density of ~300cm−3. The amount of CO-dark gas scales with the amount of gas at low visual extinction (AV<1.5) and a significant fraction (up to 65%) of the H2 can be CO-dark. We also show that the large range of cloud conditions causes the XCO factors of the various clouds to have a large spread of up to a factor of 4 around a fiducial value of about 1.5×1020cm−2(Kkms−1)−1. This significantly affects the accuracy of determining the H2 mass of molecular clouds and their virial state. 
In order to overcome this problem, we suggest a new method to determine the H2 mass of molecular clouds. It relies on the availability of CO, J=1-0 observations and AV maps. It reduces the uncertainty of the clouds’ overall H2 mass to a factor of ~1.8 and is also applicable for sub-pc-scaled structures of the clouds. 

Column density of hydrogen nuclei (top) and CO (bottom) for two runs without magnetic fields without (left) and with feedback (middle), and one run with magnetic fields (right). CO shows a significantly more compact distribution than that of the total gas leading to a significant amount of CO-dark gas, in particular in the outer regions of the clouds.