  A notable challenge of planet formation is to find a path to directly form
planetesimals from small particles. We aim to understand how drifting pebbles
pile up in a protoplanetary disk with a non-uniform turbulence structure. We
consider a disk structure in which the midplane turbulence viscosity is
increasing with radius in protoplanetary disks as in the outer region of a dead
zone. We perform 1D diffusion-advection simulations of pebbles that include
back-reaction (the inertia) to radial drift and vertical/radial diffusion of
pebbles for a given pebble-to-gas mass flux. We report a new mechanism, the
"no-drift" runaway pile-up, leading to a runaway accumulation of pebbles in
disks, thus favoring the formation of planetesimals by streaming and/or
gravitational instabilities. This occurs when pebbles drifting in from the
outer disk and entering a dead zone experience a decrease in vertical
turbulence. The scale height of the pebble subdisk then decreases, and for
small enough values of the turbulence in the dead zone and high values of the
pebble to gas flux ratio, the back-reaction of pebbles on gas leads to a
significant decrease in their drift velocity and thus their progressive
accumulation. This occurs when the ratio of the flux of pebbles to that of the
gas is large enough so that the effect dominates over any Kelvin-Helmholtz
shear instability. This process is independent of the existence of a pressure
bump.
