Grain Growth During Protostellar Disk Formation

TL;DR

This study models grain growth in protostellar disks using 2D simulations, revealing that significant growth from micron to millimeter sizes during early star formation requires enhanced collision speeds, possibly due to turbulence.

Contribution

First to model grain growth in protostellar environments with 2D radiation hydrodynamics, highlighting the need for collision speed enhancement for substantial growth.

Findings

Grain growth via size-dependent terminal velocity is too slow during Class 0 phase.

Enhanced collision speeds by a factor of 4 promote significant grain growth.

Vertical dust settling increases local dust-to-gas ratio, accelerating growth.

Abstract

Recent observations indicate that mm/cm-sized grains may exist in the embedded protostellar disks. How such large grains grow from the micron size (or less) in the earliest phase of star formation remains relatively unexplored. In this study we take a first step to model the grain growth in the protostellar environment, using two-dimensional (2D axisymmetric) radiation hydrodynamic and grain growth simulations. We show that the grain growth calculations can be greatly simplified by the "terminal velocity approximation", where the dust drift velocity relative to the gas is proportional to its stopping time, which is proportional to the grain size. We find that the grain-grain collision from size-dependent terminal velocity alone is too slow to convert a significant fraction of the initially micron-sized grains into mm/cm sizes during the deeply embedded Class 0 phase. Substantial grain growth is achieved when the grain-grain collision speed is enhanced by a factor of 4. The dust growth above and below the disk midplane enables the grains to settle faster towards the midplane, which increases the local dust-to-gas ratio, which, in turn, speeds up further growth there. How this needed enhancement can be achieved is unclear, although turbulence is a strong possibility that deserves further exploration.