Study Turbulence and Probe Magnetic Fields Using the Gradient Technique: Application to H i-to-H 2 Transition Regions

Abstract

The atomic-to-molecular (H i -to-H 2 ) transition in photodissociation regions (PDRs) has been investigated over the past several decades through analytic and numerical modeling. However, classical PDR models typically assume a uniform-density gas, ignoring the turbulent nature of the interstellar medium. Recently, Bialy et al. have presented a theoretical framework for studying the H i -to-H 2 transition in a realistic turbulent medium with a nonhomogeneous density structure. Here we extend these turbulent-chemical models to explore the possibility of tracing the magnetic field direction in turbulent PDRs using the gradient technique. We utilize both subsonic and supersonic magnetohydrodynamic numerical simulations for chemical H i /H 2 balance calculations. We confirm that the density fluctuations induced by turbulence can disperse the distribution of the H 2 and H i fractions. We find that the energy spectrum of moment maps gets shallower when the sonic Mach number M S 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.