What specific size range creates the effective Radial Drift or "meter-sized barrier"?
Meter-sized or even millimeter-sized particles.
Even if particles overcome the fragmentation barrier, they face the challenge of radial drift, often termed the 'meter-sized barrier.' This hurdle stems from aerodynamic coupling: particles are bound to the surrounding gas, causing them to drift rapidly inward towards the central star. Particles that are roughly meter-sized or even just millimeters in size are particularly susceptible, especially if their stopping time relative to the orbital frequency (Stokes number $ au_S$) is around 1. Over relatively short timescales, like a few thousand years, these intermediate particles can traverse significant radial distances across the disk. This rapid inward movement means these solids are quickly either destroyed by falling into the star or become too concentrated to efficiently build larger planetesimals through straightforward accretion, creating a strong bottleneck for further growth.
