Planetesimal formation in an evolving protoplanetary disk with a dead zone

Abstract

Context. When and where planetesimals form in a protoplanetary disk are highly debated questions. Streaming instability is considered the most promising mechanism, but the conditions for its onset are stringent. Disk studies show that the planet forming region is not turbulent because of the lack of ionization forming possibly dead zones (DZs). Aims. We investigate planetesimal formation in an evolving disk, including the DZ and thermal evolution. Methods. We used a 1D time-evolving stratified disk model with composite chemistry grains, gas and dust transport, and dust growth. Results. Accretion of planetesimals always develops in the DZ around the snow line, due to a combination of water recondensation and creation of dust traps caused by viscosity variations close to the DZ. The width of the planetesimal forming region depends on the disk metallicity. For Z = Z⊙, planetesimals form in a ring of about 1 au width, while for Z > 1.2 Z⊙ planetesimals form from the snow line up to the outer edge of the DZ ≃ 20 au. The efficiency of planetesimal formation in a disk with a DZ is due to the very low effective turbulence in the DZ and to the efficient piling up of material coming from farther away; this material accumulates in region of positive pressure gradients forming a dust trap due to viscosity variations. For Z = Z⊙ the disk is always dominated in terms of mass by pebbles, while for Z > 1.2 Z⊙ planetesimals are always more abundant than pebbles. If it is assumed that silicate dust is sticky and grows up to impact velocities ~10 m s−1, then planetesimals can form down to 0.1 au (close to the inner edge of the DZ). In conclusion the DZ seems to be a sweet spot for the formation of planetesimals: wide scale planetesimal formation is possible for Z > 1.2 Z⊙. If hot silicate dust is as sticky as ice, then it is also possible to form planetesimals well inside the snow line.