Planetesimal formation in an evolving protoplanetary disk with a dead zone

Sébastien Charnoz,Frédéric Moynier,Francesco C Pignatale,Ryuki Hyodo,Brandon Mahan,Julien Siebert,Marc Chaussidon

doi:10.1051/0004-6361/201833216

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.

Highlights

The current formation paradigm for planetesimals asserts that they form through streaming instability (Youdin & Goodman 2005; Johansen et al 2007; Bai & Stone 2010)
– Where do planetesimals form in a disk with a dead zone? – What are the key physical parameters? – What is the composition of planetesimals after their formation? – What is the pebble-to-planetesimal mass ratio as a function of space and time? We focus on an already formed protoplanetary disk, i.e., an isolated disk whose initial structure is arbitrarily imposed at the beginning of the simulation, in the spirit of the popular minimum mass solar nebula
The 1D disk model we present here is not really new; it is an extension of the Hueso & Guillot (2005) disk model, with some additional physical modules that take into account multispecies, planetesimal formation, dust growth, dust sublimation, and layering due to the dead zones (DZs)

Summary

Introduction

The current formation paradigm for planetesimals asserts that they form through streaming instability (Youdin & Goodman 2005; Johansen et al 2007; Bai & Stone 2010). Drazkowska et al (2016), Drazkowska & Alibert (2017), and Drazkowska & Dullemond (2018) have studied this problem using a 1D alpha disk model of an active disk in order to simultaneously track the dust growth and planetesimal formation using a parametric prescription physically motivated by local simulations of streaming instability (Johansen et al 2007; Bai & Stone 2010). Schoonenberg & Ormel (2017) found a similar result using a simple and detailed local model As this model is local it does not consider large-scale transport of the dust, and both studies focused on fully active disks, i.e., where the turbulence intensity (quantified by α) is constant everywhere

Methods

Results

Conclusion