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 (MHD) simulations of various sonic Mach numbers, ${\mathcal{M}}_s$, and density decorrelation scales, $y_{\mathrm{dec}}$, 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 ${\mathcal{M}}_s$ and $y_{\mathrm{dec}}$, as the magnitude of the density fluctuations increases, and as they become more coherent. Turbulence also affects the median abundances: when ${\mathcal{M}}_s$ and $y_{\mathrm{dec}}$ 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 $({\mathcal{M}}_s=4.5)$, large decorrelation scale $(y_{\mathrm{dec}}=0.8)$ model, supporting a scenario of a large-scale turbulence driving. The abundances also depend on the UV intensity, CR ionization rate, and the cloud column density, and the observed scatter may be influenced by fluctuations in these parameters.

Accepted for publication in the Astrophysical Journal

References in corpus (14)

• Theory of Star Formation
• The Density Probability Distribution in Compressible Isothermal Turbulence: Solenoidal versus Compressive Forcing
• Herschel Survey of Galactic OH+, H2O+, and H3O+: Probing the Molecular Hydrogen Fraction and Cosmic-Ray Ionization Rate
• Density Fluctuations in MHD Turbulence: Spectra, Intermittency and Topology
• HI-to-H2 Transitions and H I Column Densities in Galaxy Star-Forming Regions
• [CII] 158$\mu$m Emission and Metallicity in PDRs
• Ubiquitous argonium (ArH${}^{+}$) in the diffuse interstellar medium -- a molecular tracer of almost purely atomic gas
• Molecular line intensities as measures of cloud masses - I. Sensitivity of CO emissions to physical parameter variations
• The Turbulent Warm Ionized Medium: Emission Measure Distribution and MHD Simulations
• The chemistry of interstellar argonium and other probes of the molecular fraction in diffuse clouds
• The HI Probability Distribution Function and the Atomic-to-Molecular Transition in Molecular Clouds
• HI-to-H2 Transition Layers in the Star-Forming Region W43
• Cloud coalescence: a dynamical instability affecting multiphase environments
• A Herschel/HIFI Legacy Survey of HF and H2O in the Galaxy: Probing Diffuse Molecular Cloud Chemistry

Cited by in corpus (1)

• CO emission and CO hotspots in diffuse molecular gas