The Mass and Size Distribution of Planetesimals Formed by the Streaming Instability. I. The Role of Self-Gravity (1512.00009v2)

Abstract: We study the formation of planetesimals in protoplanetary disks from the gravitational collapse of solid over-densities generated via the streaming instability. To carry out these studies, we implement and test a particle-mesh self-gravity module for the Athena code that enables the simulation of aerodynamically coupled systems of gas and collisionless self-gravitating solid particles. Upon employment of our algorithm to planetesimal formation simulations, we find that (when a direct comparison is possible) the Athena simulations yield predicted planetesimal properties that agree well with those found in prior work using different numerical techniques. In particular, the gravitational collapse of streaming-initiated clumps leads to an initial planetesimal mass function that is well-represented by a power-law, dN/dM ~ M^{-p}, with p = 1.6 +/- 0.1, which equates to a differential size distribution dN/dR ~ R^{-q}, with q = 2.8 +/- 0.1. We find no significant trends with resolution from a convergence study of up to 512^3 grid zones and Npar = 1.5x10^8 particles. Likewise, the power-law slope appears indifferent to changes in the relative strength of self-gravity and tidal shear, and to the time when (for reasons of numerical economy) self-gravity is turned on, though the strength of these claims is limited by small number statistics. For a typically assumed radial distribution of minimum mass solar nebula solids (assumed here to have dimensionless stopping time \tau = 0.3), our results support the hypothesis that bodies on the scale of large asteroids or Kuiper Belt Objects could have formed as the high-mass tail of a primordial planetesimal population.

The Mass and Size Distribution of Planetesimals Formed by the Streaming Instability: The Role of Self-Gravity

The paper discusses the formation of planetesimals within protoplanetary disks through gravitational instability initiated by streaming instability. Specifically, the authors explore the role of self-gravity in determining the mass and size distribution of the resultant planetesimals. Using the Athena code, the researchers implemented a particle-mesh self-gravity module to simulate systems of gas and collisionless self-gravitating solid particles. This approach allows for a detailed examination of planar systems where particles interact aerodynamically and gravitationally.

The findings indicate that streaming-induced clumps undergo gravitational collapse, resulting in an initial planetesimal mass function characterized by a power-law $dN/dM \propto M^{-p}$ with $p \approx 1.6 \pm 0.1$. This corresponds to a differential size distribution $dN/dR \propto R^{-q}$ with $q \approx 2.8 \pm 0.1$, which aligns with predictions from previous studies utilizing different numerical methodologies. The paper systematically analyzes the influence of various factors, including resolution, self-gravity strength, tidal shear dynamics, and the timing of self-gravity activation, on the resulting planetesimal properties.

The simulations provide robust evidence supporting the hypothesis that large asteroids and Kuiper Belt Objects may emerge as the high-mass tail of a primordial planetesimal population. The authors present insights into potential variations in size distribution under different physical disk conditions, such as varying metallicity and radial pressure gradients across the disk. Their results demonstrate that as simulation resolution increases, more planetesimals are formed, particularly at lower masses, while the largest planetesimal mass remains approximately constant.

Despite small numerical inaccuracies inherent to any simulation constrained by resolution limitations, the paper contributes to the understanding of planetesimal formation theory. The simulations enhance confidence in the relevance of the streaming instability model for generating planetesimal populations consistent with observations from the main asteroid belt and Kuiper belt in the Solar System.

Several implications arise from this research. Practically, the paper provides quantitative constraints for the sizes of planetesimals that could be pivotal when modeling subsequent planet formation processes, such as accretion into planetary cores. Theoretically, the work invites further investigation into the streaming instability mechanism to understand its universality across diverse disk environments. Future advancements in AI coupled with enhancements in simulation accuracy and capability may offer new pathways to explore complex interactions in protoplanetary disks, potentially unveiling new insights into planetesimal formation.

This simulation-based paper serves as a foundation for further exploration into particle self-gravity and its role within astrophysical environments. Future research could extend this work by incorporating more granular initial conditions or employing AI techniques to optimize simulation parameters, thus paving the way for broader applications in modeling planetary system formations and dynamics.