# Is the dust-to-gas ratio constant in molecular clouds?

## TL;DR

This study uses numerical simulations to investigate whether the dust-to-gas ratio remains constant in molecular clouds, revealing that it generally varies by 10-20% for small grains and can be significantly higher for larger grains, with implications for cloud observations.

## Contribution

The paper demonstrates through simulations that the dust-to-gas ratio is not constant, challenging previous assumptions and highlighting size-dependent dust behavior in turbulent molecular clouds.

## Key findings

- Dust-to-gas ratio deviates by 10-20% for small grains
- Larger grains can have locally increased dust-to-gas ratios by up to an order of magnitude
- Size-sorting of grains occurs, influencing cloud observations

## Abstract

We perform numerical simulations of dusty, supersonic turbulence in molecular clouds. We model 0.1, 1 and 10 {\mu}m sized dust grains at an initial dust-to-gas mass ratio of 1:100, solving the equations of combined gas and dust dynamics where the dust is coupled to the gas through a drag term. We show that, for 0.1 and 1 {\mu}m grains, the dust-to-gas ratio deviates by typically 10-20% from the mean, since the stopping time of the dust due to gas drag is short compared to the dynamical time. Contrary to previous findings, we find no evidence for orders of magnitude fluctuation in the dust-to-gas ratio for 0.1 {\mu}m grains. Larger, 10 {\mu}m dust grains may have dust-to-gas ratios increased by up to an order of magnitude locally. Both small (0.1 {\mu}m) and large ($\gtrsim$ 1 {\mu}m) grains trace the large-scale morphology of the gas, however we find evidence for 'size-sorting' of grains, where turbulence preferentially concentrates larger grains into dense regions. Size-sorting may help to explain observations of 'coreshine' from dark clouds, and why extinction laws differ along lines of sight through molecular clouds in the Milky Way compared to the diffuse interstellar medium.