Simulating the formation of molecular clouds. I. Slow formation by gravitational collapse from static initial conditions

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Abstract
We study the formation of H2 in the ISM, using a modified version of the astrophysical magnetohydrodynamical code ZEUS-MP that includes a non-equilibrium treatment of the formation and destruction of H2. We use a local approximation to compute the shielding of H2 against photodissociation, making three-dimensional high-resolution simulations of cloud formation feasible with modest computational resources. Our modification to ZEUS-MP also includes a detailed treatment of the thermal behaviour of the gas. In this paper, we focus on the problem of molecular cloud formation in gravitationally unstable, initially static gas. (In a subsequent paper, we consider turbulent flow). We show that in these conditions, H2 formation is driven by gravitational collapse, and so occurs over one or more gravitational free-fall timescales. For initial densities consistent with those observed in the cold, neutral atomic phase of the interstellar medium, this corresponds to a cloud formation timescale t > 10 Myr. We also show that the collapsing gas very quickly reaches thermal equilibrium and that the equation of state of the thermal equilibrium gas is softer than isothermal, with an effective polytropic index in the range gamma_eff = 0.7 -- 0.8. Finally, we demonstrate that although these results show little sensitivity to variations in most of our simulation parameters, they are highly sensitive to the assumed initial density n_i. Reducing n_i significantly increases the cloud formation timescale and decreases the amount of hydrogen ultimately converted to H2.

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