http://arxiv.org/abs/1704.06487
We present 3D simulations of the formation process of two molecular clouds (MCs) within their larger-scale galactic environment. Using adaptive mesh refinement, we model the two MCs within the SILCC project with an unprecedented resolution of 0.06 pc combined with a chemical network for the formation of H$2$ and CO including (self-) shielding and important thermal processes. The MCs form within a few Myr with mass growth rates of up to 10$^{-2}$ M$\rm{sun}$ yr$^{-1}$ and final masses of $\sim$ 50000 M$\rm{sun}$. We show that the usage of different definitions for MCs by thresholds in density, H$_2$ or CO mass fraction significantly change the inferred cloud properties. While CO traces well the evolution of dense gas with $n \geq$ 300 cm$^{-3}$, H$_2$ is also found in gas with lower number density ($n \lesssim$ 30 cm$^{-3}$) due to turbulent mixing. The CO-to-H$_2$ ratio increases within the first 2 Myr reaching a value of $\sim$ 1.8 $\times$ 10$^{-4}$ at later stages. The $X\rm{CO}$ factor, however, is rather time-independent with values of 1 – 4 $\times$ 10$^{20}$ cm$^{-2}$ (K km s$^{-1}$)$^{-1}$. We show that a spatial resolution of $\sim$ 0.1 pc is required to accurately model the chemical, dynamical, and structural evolution of MCs. At a coarser resolution the mass, velocity dispersion, and chemical abundances of the clouds are underestimated. Furthermore, we show that the progressive increase of resolution has to occur over a time of 1 – 1.5 Myr. This ensures that the maximum refinement level is reached within the free-fall time of the densest structures and avoids the spurious formation large-scale, rotating objects by unresolved turbulent flows. In addition, the accelerated formation of chemical species in dense, turbulent environments is captured properly. Finally, we demonstrate that $\gtrsim$ 200 time steps should be spent on each refinement level to avoid grid artefacts.
D. Seifried, S. Walch, P. Girichidis, et. al.
Mon, 24 Apr 17
4/54
Comments: 23 pages, 21 figures, submitted to MNRAS, comments are welcome