Multifrequency radiation hydrodynamics simulations of H2 line emission in primordial, star-forming clouds

TL;DR

This study uses advanced 3D radiation hydrodynamics simulations with multifrequency H2 line transfer to better understand primordial gas collapse, revealing the importance of detailed line treatment and limitations of simplified escape fraction models.

Contribution

Implemented a multiline, multifrequency ray-tracing scheme in Arepo for accurate H2 line transfer in primordial collapse simulations, highlighting the significance of detailed spectral treatment.

Findings

Multifrequency treatment increases energy escape in high optical depth conditions.

Doppler shifts have a minor effect on energy escape.

Simple density-dependent escape fraction models are insufficient.

Abstract

We investigate the collapse of primordial gas in a minihalo with three-dimensional radiation hydrodynamics simulations that accurately model the transfer of H2 line emission. For this purpose, we have implemented a multiline, multifrequency ray-tracing scheme in the moving-mesh code Arepo that is capable of adaptively refining rays based on the Healpix algorithm, as well as a hybrid equilibrium/non-equilibrium primordial chemistry solver. We find that a multifrequency treatment of the individual H2 lines is essential, since for high optical depths the smaller cross-section in the wings of the lines greatly increases the amount of energy that can escape. The influence of Doppler shifts due to bulk velocities is comparatively small, since systematic velocity differences in the cloud are typically smaller than the sound speed. During the initial collapse phase, the radially averaged escape fraction agrees relatively well with the fit of Ripamonti & Abel. However, in general it is not advisable to use a simple density-dependent fitting function, since the escape fraction depends on many factors and does not capture the suppression of density perturbations due to the diffusion of radiation. The Sobolev method overestimates the escape fraction by more than an order of magnitude, since the properties of the gas change on scales smaller than the Sobolev length.