Simulations of Molecular Clouds:Resolution Requirements and Core Formation

The formation of molecular hydrogen (H$_2$) and carbon monoxide (CO) is sensitive to the volume and column density distribution of the turbulent interstellar medium. In order to obtain correct numerical approximations of the molecular cloud formation in nature, simulations that couple the gas dynamics and chemical evolution are gaining popularity over the last decade. However, a comprehensive study on the spatial resolution required to model different molecules is missing. The simulations presented in this thesis are designed to investigate the resolution requirements for a converged formation history of H$_2$ and CO molecules and serve to indicate whether such requirements have been met in existing studies. For this purpose, the \textsc{Flash} code is used to study H$_2$ and CO formation in a large set of hydrodynamical simulations of periodic boxes with driven supersonic turbulence and of colliding flows. The simulations include a non-equilibrium chemistry network, gas self-gravity, and diffuse radiative transfer. The resolution requirements are determined by identifying two critical conditions for numerical modelling: the simulation has to at least resolve the densities at which (1) the molecule formation time in each cell in the computational domain is equal to the dissociation time, and (2) the formation time is equal to the typical cell-crossing time. These requirements are affected by the composition of the gas as well as by the strength of turbulence and interstellar radiation field in the molecule forming regions. For the solar metallicity gas, which is subject to a solar neighbourhood interstellar radiation field and typical velocity dispersion observed in molecular clouds, the second criterion is found to be more restrictive, for both H$_2$ and CO formation. The numerical results and derived resolution criteria indicate that a spatial resolution of $\lesssim 0.2$~pc is sufficient for converged H$_2$ formation; the required resolution for CO convergence is $\lesssim 0.04$~pc. The expressions for the resolution requirements derived in this thesis can be used to check whether molecule formation is converged in any given simulation. Finally, the chemically and dynamically resolved simulation of molecular clouds is used to investigate the formation of massive molecular cloud cores. The cores are found to accrete gas from the parent cloud primarily along designated channels defined by the filamentary structures in the molecular clouds