Turbulent dissipation, CH+ abundance, H2 line luminosities, and polarization in the cold neutral medium

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 H-2 line luminosities. We carry out simulations of magnetohydrodynamic (MHD) turbulence in molecular clouds including heating and cooling, and post-process them to study H-2 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 H-2 cooling function for n(H) <= 10(5) cm(-3), T <= 5000 K, and variable H-2 fraction. We make two important simplifying assumptions: (i) the H-2/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 kms(-1). Our models produce H-2 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 (approximate to 10 mu G) 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 similar to 5 mu G, 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.

Automated summary

In the cold neutral medium, intermittent high-temperature excursions are believed to explain the enhanced abundance of CH+ observed along diffuse molecular sightlines, impacting the H-2 line luminosities. Simulation results suggest that for realistic magnetic field strengths and velocity dispersions, the observed CH+ abundances can be reproduced.