Evading the dust fragmentation barrier with the streaming instability in protoplanetary disks

TL;DR

The paper investigates whether dust growth coupled to the streaming instability (SI) and mass loading can overcome dust fragmentation barriers in protoplanetary disks. Using high-resolution 2D simulations with a monodisperse dust growth model, the authors show that mass loading reduces turbulent collision velocities, shifting the fragmentation threshold to higher effective Stokes numbers ($\tilde{\mathrm{St}}_{\mathrm{frag}}$) and enabling stronger dust clumping. A two-way synergy emerges: SI-driven concentration promotes growth, and growth-induced mass loading further enhances clumping, provided $\mathrm{St}_{\mathrm{frag}}<1$; when $\mathrm{St}_{\mathrm{frag}}\geq1$, decoupling weakens the feedback and limits clumping gains. These results suggest pathways for forming dense dust filaments that can reach Roche densities and trigger gravitational instability, thereby aiding planetesimal formation in protoplanetary disks.

Abstract

Context: The streaming instability (SI) is a leading candidate for reaching solid densities sufficient to trigger the gravitational collapse needed for the formation of planetesimals. However, dust growth barriers appear to impede the ability to assemble sufficiently large dust particles to trigger strong clumping, providing a serious impediment to planetesimal formation. Aims: We aim to address the possibility to enhance dust clumping with dust growth in SI-produced structures, and to estimate the impact of the shift of the dust fragmentation threshold in regions where the SI has enhanced the dust density. Methods: We perform two-dimensional numerical simulations of the SI with a monodisperse description of dust growth, accounting for the impact of mass loading of the dust on the sound speed of the gas and dust mixture when computing dust collisional velocities. Results: Dust mass loading reduces collision velocities in high density regions, allowing dust particles to survive to larger sizes before shattering. In turn, dust clumping is boosted as particles grow in size, as long as they remain sufficiently coupled to the gas. Conclusions: This two-way synergy between dust growth and clumping, which depends on the initial dust-to-gas ratio and dust elastic properties, allows denser dust clumps to form and thus facilitates the onset of planetesimal formation.

Figures (9)

• Figure 1: Evolution of the maximum dust-to-gas ratio (top row) and maximum St (bottom row) with time for the different dust growth scenarios NGr, GrNML, and GrML. The velocity fragmentation threshold is given above each column. In all cases, the initial dust-to-gas ratio $\epsilon_0 = 0.3$.
• Figure 2: Dust density maps taken at saturation. Each column corresponds to the given dust growth scenario, while each row has the given fragmentation velocity.
• Figure 3: Dust collision velocity driven by turbulence with respect to the Stokes number St of the larger grain, as predicted by the analytical model of Ormel2007, without mass loading. The colored circles refer to the fragmentation velocity values explored in this work. The maximum St to which the dust distribution is expected to grow can be inferred. With mass loading, this limit can be increased.
• Figure 4: Maximum dust density divided by initial dust-to-gas ratio as a function of time for different spatial resolutions for the AB model defined in Johansen2007: fixed $\mathrm{St}=0.1$ and $\epsilon_0 = 1$.
• Figure 5: Same as \ref{['fig: AB test']} but for the BA model defined in Johansen2007: fixed $\mathrm{St}=1$ and $\epsilon_0 = 0.2$.