Dust size distributions in coagulation/fragmentation equilibrium: Numerical solutions and analytical fits

TL;DR

This paper derives analytical and numerical solutions for dust grain size distributions in circumstellar disks, focusing on growth limited by fragmentation and neglecting radial drift, providing simple recipes for modeling such distributions.

Contribution

It introduces a generalized analytical framework and a fitting formula for dust size distributions in disks, validated by numerical simulations, improving modeling efficiency.

Findings

Good agreement between analytical and numerical solutions for simple kernels

Dust distribution shape mainly influenced by gas surface density, turbulence, and temperature

Distribution does not follow the classical MRN distribution

Abstract

Context. Grains in circumstellar disks are believed to grow by mutual collisions and subsequent sticking due to surface forces. Results of many fields of research involving circumstellar disks, such as radiative transfer calculations, disk chemistry, magneto-hydrodynamic simulations largely depend on the unknown grain size distribution. Aims. As detailed calculations of grain growth and fragmentation are both numerically challenging and computationally expensive, we aim to find simple recipes and analytical solutions for the grain size distribution in circumstellar disks for a scenario in which grain growth is limited by fragmentation and radial drift can be neglected. Methods. We generalize previous analytical work on self-similar steady-state grain distributions. Numerical simulations are carried out to identify under which conditions the grain size distributions can be understood in terms of a combination of power-law distributions. A physically motivated fitting formula for grain size distributions is derived using our analytical predictions and numerical simulations. Results. We find good agreement between analytical results and numerical solutions of the Smoluchowski equation for simple shapes of the kernel function. The results for more complicated and realistic cases can be fitted with a physically motivated "black box" recipe presented in this paper. Our results show that the shape of the dust distribution is mostly dominated by the gas surface density (not the dust-to-gas ratio), the turbulence strength and the temperature and does not obey an MRN type distribution.