Streaming instability with multiple dust species - I. Favourable conditions for the linear growth

A recent study suggests that the streaming instability, one of the leading mechanisms for driving the formation of planetesimals, may not be as efficient as previously thought. Under some disc conditions, the growth time-scale of the instability can be longer than the disc lifetime when multiple dust species are considered. To further explore this finding, we use both linear analysis and direct numerical simulations with gas fluid and dust particles to mutually validate and study the unstable modes of the instability in more detail. We extend the previously studied parameter space by one order of magnitude in both the range of the dust-size distribution [T-s,T- min, T-s,T- max] and the total solid-to-gas mass ratio epsilon and introduce a third dimension with the slope q of the size distribution. We find that the fast-growth regime and the slow-growth regime are distinctly separated in the epsilon-T-s,T- max space, while this boundary is not appreciably sensitive to q or T-s,T- min. With a wide range of dust sizes present in the disc (e.g. T-s,T- min less than or similar to 10(-3)), the growth rate in the slow-growth regime decreases as more dust species are considered. With a narrow range of dust sizes (e.g. T-s,T- max/T-s,T- min = 5), on the other hand, the growth rate in most of the epsilon-T-s,T- max space is converged with increasing dust species, but the fast and the slow growth regimes remain clearly separated. Moreover, it is not necessary that the largest dust species dominate the growth of the unstable modes, and the smaller dust species can affect the growth rate in a complicated way. In any case, we find that the fast-growth regime is bounded by epsilon greater than or similar to 1 or T-s,T- max greater than or similar to 1, which may represent the favourable conditions for planetesimal formation.

Automated summary

A recent study suggests that the streaming instability may not be as efficient as previously thought in driving planetesimal formation. Through linear analysis and numerical simulations, the research explores the effects of different dust species and parameters on the growth rate and patterns of the instability. The study finds that varying dust sizes and the total solid-to-gas mass ratio can impact the boundaries between fast and slow growth regimes.