SILCC-Zoom: the dynamic and chemical evolution of molecular clouds

We present 3D 'zoom-in' simulations of the formation of two molecular clouds out of the galactic interstellar medium. We model the clouds-identified from the SILCC simulations with a resolution of up to 0.06 pc using adaptive mesh refinement in combination with a chemical network to follow heating, cooling and the formation of H-2 and CO including (self-) shielding. The two clouds are assembled within a few million years with mass growth rates of up to similar to 10(-2) M-circle dot yr(-1) and final masses of similar to 50 000 M-circle dot. A spatial resolution of less than or similar to 0.1 pc is required for convergence with respect to the mass, velocity dispersion and chemical abundances of the clouds, although these properties also depend on the cloud definition such as based on density thresholds, H-2 or CO mass fraction. To avoid grid artefacts, the progressive increase of resolution has to occur within the free-fall time of the densest structures (1-1.5Myr) and greater than or similar to 200 time-steps should be spent on each refinement level before the resolution is progressively increased further. This avoids the formation of spurious, large-scale, rotating clumps from unresolved turbulent flows. While CO is a good tracer for the evolution of dense gas with number densities n >= 300 cm(-3), H-2 is also found for n less than or similar to 30 cm(-3) due to turbulent mixing and becomes dominant at column densities around 30-50 M-circle dot pc(-2). The CO-to-H-2 ratio steadily increases within the first 2 Myr, whereas X-CO similar or equal to 1-4 x 10(20) cm(-2) (K km s(-1))(-1) is approximately constant since the CO(1-0) line quickly becomes optically thick