Chemical Abundances in a Turbulent Medium–H 2 , OH + , H 2 O + , ArH +

Abstract

Supersonic turbulence results in strong density fluctuations in the interstellar medium (ISM), which have a profound effect on the chemical structure. Particularly useful probes of the diffuse ISM are the ArH + , OH + , H 2 O + molecular ions, which are highly sensitive to fluctuations in the density and the H 2 abundance. We use isothermal magnetohydrodynamic simulations of various sonic Mach numbers, , and density decorrelation scales, y dec , to model the turbulent density field. We post process the simulations with chemical models and obtain the probability density functions (PDFs) for the H 2 , ArH + , OH + , and H 2 O + abundances. We find that the PDF dispersions increases with increasing and y dec , as the magnitude of the density fluctuations increases, and as they become more coherent. Turbulence also affects the median abundances: when and y dec are high, low-density regions with low H 2 abundance become prevalent, resulting in an enhancement of ArH + compared to OH + and H 2 O + . We compare our models with Herschel observations. The large scatter in the observed abundances, as well as the high observed ArH + /OH + and ArH + /H 2 O + ratios are naturally reproduced by our supersonic , large decorrelation scale ( y dec = 0.8) model, supporting a scenario of a large-scale turbulence driving. The abundances also depend on the ultraviolet intensity, cosmic-ray ionization rate, and the cloud column density, and the observed scatter may be influenced by fluctuations in these parameters.