Atomic Chemistry In Turbulent Astrophysical Media I: Effect of Atomic Cooling

TL;DR

This study uses detailed simulations to explore how atomic cooling influences the state of turbulent astrophysical media, revealing the importance of nonequilibrium effects at higher turbulence levels.

Contribution

It provides a comprehensive set of simulation results characterizing atomic cooling effects in turbulent media without ionizing backgrounds, highlighting the limitations of equilibrium models at high turbulence.

Findings

Single-temperature estimates work for low turbulence.

Local equilibrium models fail at high turbulence.

Results are compiled into tables for future research use.

Abstract

We carry out direct numerical simulations of turbulent astrophysical media that explicitly track ionizations, recombinations, and species-by-species radiative cooling. The simulations assume solar composition and follows the evolution of hydrogen, helium, carbon, oxygen, sodium, and magnesium, but they do not include the presence of an ionizing background. In this case, the medium reaches a global steady state that is purely a function of the one-dimensional turbulent velocity dispersion, $\sigma_{\rm 1D},$ and the product of the mean density and the driving scale of turbulence, $n L.$ Our simulations span a grid of models with $\sigma_{\rm 1D}$ ranging from 6 to 58 km s$^{-1}$ and $n L$ ranging from 10$^{16}$ to 10$^{20}$ cm$^{-2},$ which correspond to turbulent Mach numbers from $M=0.2$ to 10.6. The species abundances are well described by single-temperature estimates whenever $M$ is small, but local equilibrium models can not accurately predict the global equilibrium abundances when $M \gtrsim 1.$ To allow future studies to account for nonequilibrium effects in turbulent media, we gather our results into a series of tables, which we will extend in the future to encompass a wider range of elements, compositions, and ionizing processes.