Planetesimal formation around the snow line

II. Dust or pebbles?

¹ ISAS/JAXA, Sagamihara, Kanagawa, Japan
e-mail: hyodo@elsi.jp
² Université Côte d’Azur, Laboratoire J.-L. Lagrange, CNRS, Observatoire de la Côte d’Azur, 06304 Nice, France
³ Earth-Life Science Institute, Tokyo Institute of Technology, Meguro-ku, Tokyo 152-8550, Japan
⁴ Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Meguro-ku, Tokyo 152-8551, Japan
⁵ Steward Observatory/The Lunar and Planetary Laboratory, University of Arizona, Tucson, Arizona 85721, USA

Received: 12 November 2020
Accepted: 11 December 2020

Abstract

Context. Forming planetesimals is a major challenge in our current understanding of planet formation. Around the snow line, icy pebbles and silicate dust may locally pile up and form icy and rocky planetesimals via a streaming instability and/or gravitational instability. The scale heights of both pebbles and silicate dust released from sublimating pebbles are critical parameters that regulate the midplane concentrations of solids.

Aims. Here, using a realistic description of the scale height of silicate dust and that of pebbles, we wish to understand disk conditions for which a local runaway pile-up of solids (silicate dust or icy pebbles) occurs inside or outside the snow line.

Methods. We performed 1D diffusion-advection simulations that include the back-reaction (the inertia) to radial drift and diffusion of icy pebbles and silicate dust, ice sublimation, the release of silicate dust, and their recycling through the recondensation and sticking onto pebbles outside the snow line. We used a realistic description of the scale height of silicate dust obtained from a companion paper and that of pebbles including the effects of a Kelvin-Helmholtz instability. We study the dependence of solid pile-up on distinct effective viscous parameters for turbulent diffusions in the radial and vertical directions (α_Dr and α_Dz) and for the gas accretion to the star (α_acc) as well as that on the pebble-to-gas mass flux (F_p/g).

Results. Using both analytical and numerical approaches, we derive the sublimation width of drifting icy pebbles which is a critical parameter to characterize the pile-up of silicate dust and pebbles around the snow line. We identify a parameter space (in the F_p/g − α_acc − α_Dz(= α_Dr) space) where pebbles no longer drift inward to reach the snow line due to the back-reaction that slows down the radial velocity of pebbles (we call this the “no-drift” region). We show that the pile-up of solids around the snow line occurs in a broader range of parameters for α_acc = 10⁻³ than for α_acc = 10⁻². Above a critical F_p/g value, the runaway pile-up of silicate dust inside the snow line is favored for α_Dr∕α_acc ≪ 1, while that of pebbles outside the snow line is favored for α_Dr∕α_acc ~ 1. Our results imply that a distinct evolutionary path in the α_acc − α_Dr − α_Dz − F_p/g space could produce a diversity of outcomes in terms of planetesimal formation around the snow line.