Modeling the Atomic-to-Molecular Transition and Chemical Distributions of Turbulent Star-Forming Clouds

TL;DR

This study employs 3D-PDR to model the atomic-to-molecular transition in turbulent star-forming clouds, highlighting the importance of three-dimensional treatment for accurate chemical distribution predictions in complex geometries.

Contribution

It introduces a 3D astrochemistry modeling approach for turbulent clouds, demonstrating its advantages over 1D models in non-symmetric environments.

Findings

Chemical distributions are insensitive to simulation resolution.

3D treatment is essential for spatially varying external radiation fields.

Good agreement with previous in situ chemical evolution models for C and CO abundances.

Abstract

We use 3D-PDR, a three-dimensional astrochemistry code for modeling photodissociation regions (PDRs), to post-process hydrodynamic simulations of turbulent, star-forming clouds. We focus on the transition from atomic to molecular gas, with specific attention to the formation and distribution of H, C+, C, H2 and CO. First, we demonstrate that the details of the cloud chemistry and our conclusions are insensitive to the simulation spatial resolution, to the resolution at the cloud edge, and to the ray angular resolution. We then investigate the effect of geometry and simulation parameters on chemical abundances and find weak dependence on cloud morphology as dictated by gravity and turbulent Mach number. For a uniform external radiation field, we find similar distributions to those derived using a one-dimensional PDR code. However, we demonstrate that a three-dimensional treatment is necessary for a spatially varying external field, and we caution against using one-dimensional treatments for non-symmetric problems. We compare our results with the work of Glover et al. (2010), who self-consistently followed the time evolution of molecule formation in hydrodynamic simulations using a reduced chemical network. In general, we find good agreement with this in situ approach for C and CO abundances. However, the temperature and H2 abundances are discrepant in the boundary regions (Av < 5), which is due to the different number of rays used by the two approaches.