1000 posterior draws from the best-fit planetesimal population. The green lines indicate the evolved planetesimal population, where the planetesimals have been collisionally processed and follow a power law consistent with a collisional cascade. The gray dashed line indicates the point where the collisional cascade transitions from particles whose cohesion is mainly from material strength to cohesion from self-gravity (e.g., M. Pan & R. Sari 2005). The model jointly fits the strength and gravity power-law indices in α₁₂. The black lines indicate the best-fit primordial planetesimal population, which is made up of planetesimals that have individual collision timescales that are longer than the age of the Fomalhaut system. The planetesimal radius where the evolved population transitions to the primordial population is s_break.