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

TL;DR

This study extends turbulent-chemical models to investigate magnetic field tracing in turbulent HI-to-H2 transition regions using the Gradients Technique, demonstrating the superior accuracy of second-order velocity centroid gradients over traditional methods.

Contribution

It introduces the use of higher-order velocity centroid gradients for magnetic field tracing in turbulent PDRs, improving accuracy over existing techniques.

Findings

Second-order velocity centroid gradients outperform traditional methods in magnetic field orientation tracing.

Turbulence disperses H2 and HI distributions, affecting their correlation with magnetic fields.

The energy spectrum of moment maps becomes shallower with higher sonic Mach numbers.

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.