Demographics of Planetesimals Formed by the Streaming Instability

The streaming instability (SI) is a mechanism to aerodynamically concentrate solids in protoplanetary disks and facilitate the formation of planetesimals. Recent numerical modeling efforts have demonstrated the increasing complexity of the initial mass distribution of planetesimals. To better constrain this distribution, we conduct SI simulations including the self-gravity with hitherto the highest resolution. To subsequently identify all of the self-bound clumps, we develop a new clump-finding tool, PLanetesimal ANalyzer (\texttt{PLAN}). We then apply a maximum likelihood estimator to fit a suite of parameterized models with different levels of complexity to the simulated mass distribution. To determine which models are best-fitting and statistically robust, we apply three model selection criteria with different complexity penalties. We find that the initial mass distribution of clumps is not universal regarding both the functional forms and parameter values. Our model selection criteria prefer models different from those previously considered in the literature. Fits to multi-segment power law models break to a steeper distribution above masses close to 100 km collapsed planetesimals, similar to observed Kuiper Belt size distributions. We find evidence for a turnover in the low mass end of the planetesimal mass distribution in our high resolution run. Such a turnover is expected for gravitational collapse, but had not previously been reported.