We study the linear growth and non-linear saturation of the ‘acoustic Resonant Drag Instability’ (RDI) when the dust grains, which drive the instability, have a wide, continuous spectrum of different sizes. This physics is generally applicable to dusty winds driven by radiation pressure, such as occurs around red-giant stars, star-forming regions, or active galactic nuclei. Depending on the physical size of the grains compared to the wavelength of the radiation field that drives the wind, two qualitatively different regimes emerge. In the case of grains that are larger than the radiation’s wavelength – termed the constant-drift regime – the grain’s equilibrium drift velocity through the gas is approximately independent of grain size, leading to strong correlations between differently sized grains that persist well into the saturated non-linear turbulence. For grains that are smaller than the radiation’s wavelength – termed the non-constant-drift regime – the linear instability grows more slowly than the single-grain-size RDI and only the larger grains exhibit RDI-like behaviour in the saturated state. A detailed study of grain clumping and grain–grain collisions shows that outflows in the constant-drift regime may be effective sites for grain growth through collisions, with large collision rates but low collision velocities.
Recent observations indicate that mm/cm-sized grains may exist in the embedded protostellar discs. 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 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 disc 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 more »
- Award ID(s):
- 1716259
- Publication Date:
- NSF-PAR ID:
- 10369854
- Journal Name:
- Monthly Notices of the Royal Astronomical Society
- Volume:
- 515
- Issue:
- 4
- Page Range or eLocation-ID:
- p. 4780-4796
- ISSN:
- 0035-8711
- Publisher:
- Oxford University Press
- Sponsoring Org:
- National Science Foundation
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