Polydisperse Streaming Instability III. Dust evolution encourages fast instability

Abstract

Planet formation via core accretion requires the production of km-sized planetesimals from cosmic dust. This process must overcome barriers to simple collisional growth, for which the Streaming Instability (SI) is often invoked. Dust evolution is still required to create particles large enough to undergo vigorous instability. The SI has been studied primarily with single size dust, and the role of the full evolved dust distribution is largely unexplored. We survey the Polydispserse Streaming Instability (PSI) with physical parameters corresponding to plausible conditions in protoplanetary discs. We consider a full range of particle stopping times, generalized dust size distributions, and the effect of turbulence. We find that, while the PSI grows in many cases more slowly with a interstellar power-law dust distribution than with a single size, reasonable collisional dust evolution, producing an enhancement of the largest dust sizes, produces instability behaviour similar to the monodisperse case. Considering turbulent diffusion the trend is similar. We conclude that if fast linear growth of PSI is required for planet formation, then dust evolution producing a distribution with peak stopping times on the order of 0.1 orbits and an enhancement of the largest dust significantly above the single power-law distribution produced by a fragmentation cascade is sufficient, along with local enhancement of the dust to gas volume mass density ratio to order unity.

Streaming instability of multiple particle species II -- Numerical convergence with increasing particle number

“…Another result of particle sedimentation is universally high-density conditions in the midplane. These are conditions which linear multi-species streaming instability models also find short growth timescales (Krapp et al 2019;Paardekooper et al 2020Paardekooper et al , 2021McNally et al 2021). As a consequence, the multi-species streaming instability could indeed be successful in forming solid clumps.…”

“…Krapp et al (2019) and Paardekooper et al (2020) find that the linear phase of the instability has longer growth timescales than seen in mono-disperse models. However, McNally et al (2021), using the methodology described in Paardekooper et al (2021), expand the parameter space from their nominal set described in Paardekooper et al (2020) to a larger range of particle sizes and size distributions, which are more in line with those predicted by dust coagulation/fragmentation models (Birnstiel et al 2011(Birnstiel et al , 2015. They see that a fast growth regime is achieved given that the particle size distribution has sufficient fraction of large particles, peaks at a friction time of 0.1Ω −1 and has a local dust-togas ratio above unity.…”