# The HI-to-H2 Transition in a Turbulent Medium

## TL;DR

This study investigates how turbulence-induced density fluctuations affect the atomic-to-molecular hydrogen transition in photodissociation regions, using analytical models and numerical simulations to interpret HI observations.

## Contribution

It introduces an analytic model for the HI PDF in turbulent media and demonstrates its application to observational data, linking turbulence properties to HI structure.

## Key findings

- Mean HI column density remains consistent with uniform-density models.
- HI PDF width depends on radiation, Mach number, and turbulence scale.
- Application to Perseus cloud suggests small-scale turbulence influences HI structure.

## Abstract

We study the effect of density fluctuations induced by turbulence on the HI/H$_2$ structure in photodissociation regions (PDRs) both analytically and numerically. We perform magnetohydrodynamic numerical simulations for both subsonic and supersonic turbulent gas, and chemical HI/H$_2$ balance calculations. We derive atomic-to-molecular density profiles and the HI column density probability density function (PDF) assuming chemical equilibrium. We find that while the HI/H$_2$ density profiles are strongly perturbed in turbulent gas, the mean HI column density is well approximated by the uniform-density analytic formula of Sternberg et al. (2014). The PDF width depends on (a) the radiation intensity to mean density ratio, (b) the sonic Mach number and (c) the turbulence decorrelation scale, or driving scale. We derive an analytic model for the HI PDF and demonstrate how our model, combined with 21 cm observations, can be used to constrain the Mach number and driving scale of turbulent gas. As an example, we apply our model to observations of HI in the Perseus molecular cloud. We show that a narrow observed HI PDF may imply small scale decorrelation, pointing to the potential importance of subcloud-scale turbulence driving.