Modeling H2 formation in the turbulent ISM: Solenoidal versus compressive turbulent forcing

TL;DR

This study uses high-resolution 3D simulations to compare how solenoidal and compressive turbulence affect molecular hydrogen formation in the interstellar medium, revealing that compressive turbulence accelerates H2 formation.

Contribution

It provides the first detailed comparison of solenoidal versus compressive turbulence effects on H2 formation using high-resolution simulations, and tests a simple modeling prescription.

Findings

Compressive turbulence leads to faster H2 formation due to higher peak densities.

The difference in H2 formation rate is up to an order of magnitude early on.

The Gnedin et al. (2009) prescription works well at low H2 fractions but fails at high densities.

Abstract

We present results from high-resolution three-dimensional simulations of the turbulent interstellar medium that study the influence of the nature of the turbulence on the formation of molecular hydrogen. We have examined both solenoidal (divergence-free) and compressive (curl-free) turbulent driving, and show that compressive driving leads to faster H2 formation, owing to the higher peak densities produced in the gas. The difference in the H2 formation rate can be as much as an order of magnitude at early times, but declines at later times as the highest density regions become fully molecular and stop contributing to the total H2 formation rate. We have also used our results to test a simple prescription suggested by Gnedin et al. (2009) for modeling the influence of unresolved density fluctuations on the H2 formation rate in large-scale simulations of the ISM. We find that this approach works well when the H2 fraction is small, but breaks down once the highest density gas becomes fully molecular.