tokens: (see Section 2.4 ). For solar metallicity μ converges to ∼2.4 in the CNM, to ∼1.2 in the WIM, and to ∼0.6 in the CGM. As we deal with only low-density environments in the simulations the assumption of an ideal gas is legitimated. Hence, γ = 5/3 in all simulations. 2.2 Self-gravity The effect of self-gravity for an arbitrary matter distribution is described by solving the well-known Poisson’s Equation for the gravitational potential ( G denotes Newton’s gravitational constant). Each mass element of contributes to . If the total mass becomes a significant fraction of the (temperature-dependent) Jeans mass M J for a particular , self-gravity cannot be neglected as it relevantly affects the dynamics of all mass elements. However, it has been shown in Sander Hensler ( 2019 ) that self-gravity has a measurable effect on the evolution of even clouds with masses well below their Jeans mass. If self-gravity is stronger than the acceleration of gas in the opposite direction (e.g. due to evaporation or advection by a streaming CGM), the net acceleration at the cloud surface points in the same direction like the density gradient. Hence, the condition for Rayleigh–Taylor instability (RTI) is not given. So, it is expected that self-gravity significantly suppresses RTI at the cloud surface (see Murray et al. 1993 ) and the typical finger-like structures do not form. According to Murray et al. ( 1993 ), the minimum mass of self-gravitating clouds above which they are stable against Kelvin–Helmholtz instability (KHI) when moving with a certain speed v rel through an ambient medium with density ρ CGM reads with being the average mass density in the cloud. To ensure that self-gravitating clouds, which are embedded in a medium with pressure P CGM , are gravitationally stable initially, their total mass must be below their Bonnor–Ebert mass (Ebert 1955 ; Bonnor 1956 ) Here, and are the average temperature and mean molecular weight in the cloud, respectively. 2.3 Metallicity Observations indicate a common metallicity for CHVCs of 0.1 to (Wakker 2001 ). In order to study extreme cases, we consider two different initial cloud metallicities of and . We note that the CGM has constant metallicity in all runs (see Section 4.3 ). The ratio of the abundances of hydrogen and helium is assumed solar and so are the ratios of metal abundances. 2.4 Dissociation and ionization of elements The simulations contain cool clouds and hot plasma, so a temperature range from a few hundreds up to several 10 6 K is covered. For T 900 K molecules exist, but they successively dissociate and finally become totally ionized with rising temperature. In our simulations, we semi-analytically calculate both dissociation and ionization for hydrogen in thermodynamic equilibrium. The fraction of H ii is then related to He and metals. We note that we do not trace single species but rather have one fluid for which the composition is calculated based upon a given metallicity. The fractions of
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