Study Turbulence and Probe Magnetic Field Using Gradients Technique: Application to HI-to-H2 Transition Regions

Abstract

The atomic-to-molecular (HI-to-H${}_2$) transition in photodissociation regions (PDRs) has been investigated over the last several decades through analytic and numerical modeling. However, classical PDR models typically assume uniform density gas, ignoring the turbulent nature of the interstellar medium. Recently, Bialy et al. (2017b, 2019) have presented a theoretical framework for studying the HI-to-H${}_2$ in a realistic turbulent medium with a non-homogeneous density structure. Here we extend these turbulent-chemical models to explore the possibility of tracing the magnetic field direction in turbulent PDRs using the Gradients Technique. We utilize both subsonic and supersonic magnetohydrodynamic numerical simulations for chemical HI/H${}_2$ balance calculations. We confirm that the density fluctuations induced by turbulence can disperse the distribution of H${}_2$ and HI fraction. We find that the energy spectrum of moment maps gets shallower when the sonic Mach number MS increases. We explore the ability in magnetic field tracing of gradients of higher-order velocity centroids and compare their performance with that of traditional velocity centroid gradients (VCGs) and with intensity gradients (IGs). We find that the velocity gradients of the second-order centroids (VC${}_2$Gs) are more accurate than VCGs and IGs in probing the magnetic field orientation.

22 pages, 12 figures, accepted for publication in ApJ

References in corpus (25)

• Theory of Star Formation
• Molecular Hydrogen and Global Star Formation Relations in Galaxies
• Density Fluctuations in MHD Turbulence: Spectra, Intermittency and Topology
• The Atomic to Molecular Transition in Galaxies. I: An Analytic Approximation for Photodissociation Fronts in Finite Clouds
• HI-to-H2 Transitions and H I Column Densities in Galaxy Star-Forming Regions
• Velocity centroids as tracers of the turbulent velocity statistics
• Velocity Gradients as a Tracer for Magnetic Fields
• Subsonic Mechanical Alignment of Irregular Grains
• Mapping the Magnetic Interstellar Medium in Three Dimensions Over the Full Sky with Neutral Hydrogen
• Tracing interstellar magnetic field using velocity gradient technique: Application to Atomic Hydrogen data
• The Transition from Atomic to Molecular Hydrogen in Interstellar Clouds: 21cm Signature of the Evolution of Cold Atomic Hydrogen in Dense Clouds
• An application of an optimal statistic for characterising relative orientations
• Magnetic Field Morphology in Interstellar Clouds with the Velocity Gradient Technique
• The Physical Nature of Neutral Hydrogen Intensity Structure
• Studying Turbulence from Doppler-broadened Absorption Lines: Statistics of Logarithms of Intensity
• HI-to-H2 Transition Layers in the Star-Forming Region W43
• Extreme Starlight Polarization in a Region with Highly Polarized Dust Emission
• Extracting the cold neutral medium from HI emission with deep learning: Implications for Galactic foregrounds at high latitude
• Chemical Abundances in a Turbulent Medium -- H${}_2$, OH${}^{+}$, H${}_2$O${}^{+}$, ArH${}^{+}$
• Filaments and striations: anisotropies in observed, supersonic, highly-magnetised turbulent clouds
• Predictions of Cosmic Microwave Background Foregrounds Dust Polarization Using Velocity Gradients
• Probing Magnetic Field Morphology in Galaxy Clusters with the Gradient Technique
• Comment on Clark et al. (2019) "The Physical Nature of Neutral Hydrogen Intensity Structure"
• HD/H${}_2$ ratio in the diffuse interstellar medium
• Obtaining magnetic field strength using differential measure approach and velocity channel maps

Cited by in corpus (16)

• Superdiffusion of Cosmic Rays in Compressible Magnetized Turbulence
• Revealing Gravitational Collapse in Serpens G3-G6 Molecular Cloud using Velocity Gradients
• Turbulent Magnetic Field Amplification by the Interaction of Shock Wave and Inhomogeneous Medium
• Anisotropies in Compressible MHD Turbulence: Probing Magnetic Fields and Measuring Magnetization
• A multi-shock model for the density variance of anisotropic, highly-magnetised, supersonic turbulence
• Multi-scale Magnetic Fields in the Central Molecular Zone: Inference from the Gradient Technique
• The Velocity Statistics of Turbulent Clouds in the Presence of Gravity, Magnetic fields, Radiation, and Outflow Feedback
• Role of magnetic fields in fueling Seyfert nuclei
• Measuring magnetization with rotation measures and velocity centroids in supersonic MHD turbulence
• Anisotropic Turbulence in Position-Position-Velocity Space: Probing Three-Dimensional Magnetic Fields
• Nature of Striation in 21 cm Channel Maps: Velocity Caustics
• Decomposing Magnetic Fields in Three Dimensions over the Central Molecular Zone
• Probing Three-Dimensional Magnetic Fields: I -- Polarized Dust Emission
• Mapping the Galactic Magnetic Field Orientation and Strength in Three Dimensions
• Multi-Phase Magnetic Fields in the Galaxy NGC 3627
• Characterizing three-dimensional magnetic field, turbulence, and self-gravity in the star-forming region L1688