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

The H I-to-H2 transition in photodissociation regions has been investigated over the last several decades through analytic and numerical modeling. Nevertheless, the little model includes realistic turbulent fluctuations and the contribution from the magnetic field. In this work, we consider the effect of turbulence on the HI and H2 transition and explore the possibility of tracing the magnetic field direction in photodissociation regions using the Gradient Technique. We utilize both subsonic and supersonic magnetohydrodynamic numerical simulations for the chemical HI/H2 balance calculations. We show that the density fluctuation induced by turbulence can disperse the distribution of H2 and HI fraction. We find the energy spectrum of moment maps gets shallower when sonic Mach number Ms is increasing. 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 (VC2Gs) are more accurate than VCGs and IGs in terms of probing the magnetic field orientation.

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