Thermodynamics of Giant Molecular Clouds: The Effects of Dust Grain Size

TL;DR

This study investigates how variations in dust grain size distribution affect the thermochemistry, radiation penetration, and star formation efficiency in giant molecular clouds using magnetohydrodynamic simulations.

Contribution

It demonstrates that dust grain size significantly influences GMC thermochemistry and star formation, highlighting the importance of grain size dynamics in astrophysical models.

Findings

Larger grains lead to lower dust opacities and deeper radiation penetration.

Star formation efficiency decreases by an order of magnitude with increased grain size range.

Warmer gas suppresses low-mass star formation and increases diffuse ionized gas.

Abstract

The dust grain size distribution (GSD) likely varies significantly across star-forming environments in the Universe, but its impact on star formation remains unclear. This ambiguity arises because the GSD interacts non-linearly with processes like heating, cooling, radiation, and chemistry, which have competing effects and varying environmental dependencies. Processes such as grain coagulation, expected to be efficient in dense star-forming regions, reduce the abundance of small grains and increase that of larger grains. Motivated by this, we investigate the effects of similar GSD variations on the thermochemistry and evolution of giant molecular clouds (GMCs) using magnetohydrodynamic simulations spanning a range of cloud masses and grain sizes, which explicitly incorporate the dynamics of dust grains within the full-physics framework of the \SF project. We find that grain size variations significantly alter GMC thermochemistry: with the leading-order effect is that larger grains, under fixed dust mass, GSD dynamic range, and dust-to-gas ratio, result in lower dust opacities. This reduced opacity permits ISRF and internal radiation photons to penetrate more deeply. This leads to rapid gas heating and inhibited star formation. Star formation efficiency is highly sensitive to grain size, with an order of magnitude reduction when grain size dynamic range increases from $10^{-3}$-0.1 $\rm\mu m$ to 0.1-10 $\rm\mu m$. Additionally, warmer gas suppresses low-mass star formation, and decreased opacities result in a greater proportion of gas in diffuse ionized structures.