2021MNRAS.500.3290M

In the cold neutral medium, high out-of-equilibrium temperatures are created by intermittent dissipation processes, including shocks, viscous heating, and ambipolar diffusion. The high-temperature excursions are thought to explain the enhanced abundance of CH⁺ observed along diffuse molecular sightlines. Intermittent high temperatures should also have an impact on H₂ line luminosities. We carry out simulations of magnetohydrodynamic (MHD) turbulence in molecular clouds including heating and cooling, and post-process them to study H₂ line emission and hot-gas chemistry, particularly the formation of CH⁺. We explore multiple magnetic field strengths and equations of state. We use a new H₂ cooling function for n_H ≤10⁵ cm^–3, T ≤5000 K, and variable H₂ fraction. We make two important simplifying assumptions: (i) the H₂/H fraction is fixed everywhere and (ii) we exclude from our analysis regions where the ion-neutral drift velocity is calculated to be greater than 5 km s^–1. Our models produce H₂ emission lines in accord with many observations, although extra excitation mechanisms are required in some clouds. For realistic root-mean-square (rms) magnetic field strengths (≃10 µG) and velocity dispersions, we reproduce observed CH⁺ abundances. These findings contrast with those of Valdivia et al. (2017) Comparison of predicted dust polarization with observations by Planck suggests that the mean field is >=5 uG, so that the turbulence is sub-Alfvenic. We recommend future work treating ions and neutrals as separate fluids to more accurately capture the effects of ambipolar diffusion on CH⁺ abundance.