The Streaming Instability Cannot Form Planetesimals from mm-size Grains in Pressure Bumps

I present evidence that it is unlikely that the streaming instability (SI) can form planetesimals from mm-size grains inside pressure bumps. We spent 7 million CPU hours conducting the largest simulation of the SI thus far. We selected the largest pressure bump that is just below the size needed to completely trap dust and modelled a large slice of the disk with mm-size grains and a solar-like dust-to-gas ratio (Z = 0.01). We used a high resolution of 1000/H to resolve as many unstable modes as possible. Our simulation formed a long-lived particle over-density with Z > 0.03 and headwind Π = Δv / c_s = 0.028. That lies well within the region where previous studies predict strong clumping; yet, we observed none. The likely reason is that the time it takes particles to cross the high-Z/Π region (t_cross) is shorter than the growth timescale of the SI (t_grow). We propose an added criterion for planetesimal formation by the SI — that t_cross > t_grow. We ran another experiment: a low-resolution run with a slightly larger bump. We verified that that run formed planetesimals, but exclusively through gravitational instability (GI). Our result significantly restricts the pathways to planet formation: Either protoplanetary disks regularly form grains larger than 1mm, or planetesimals do not form in pressure bumps via the SI. Since bumps strong enough to induce the GI are likely Rossby-wave unstable, the SI may not be responsible for planetesimal formation when the particle size is > 1mm.