Planetesimal formation around the snow line. II. Dust or pebbles?

Abstract

Around the snow line, icy pebbles and silicate dust may locally pile-up and form icy and rocky planetesimals via streaming instability and/or gravitational instability. We perform 1D diffusion-advection simulations that include the back-reaction to radial drift and diffusion of icy pebbles and silicate dust, ice sublimation, release of silicate dust, and their recycling through recondensation and sticking onto pebbles outside the snow line. We use a realistic description of the scale height of silicate dust obtained from Ida et al. 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 ($\alpha_{\rm Dr}$ and $\alpha_{\rm Dz}$) and for the gas accretion to the star ($\alpha_{\rm acc}$) as well as that on the pebble-to-gas mass flux ($F_{\rm p/g}$). 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 $F_{\rm p/g}-\alpha_{\rm acc}-\alpha_{\rm Dz}(=\alpha_{\rm Dr})$ space) where pebbles no longer drift inward to reach the snow line due to the back-reaction that slows down radial velocity of pebbles. We show that the pile-up of solids around the snow line occurs in a broader range of parameters for $\alpha_{\rm acc}=10^{-3}$ than for $\alpha_{\rm acc}=10^{-2}$. Above a critical $F_{\rm p/g}$ value, the runaway pile-up of silicate dust inside the snow line is favored for $\alpha_{\rm Dr}/\alpha_{\rm acc} \ll 1$, while that of pebbles outside the snow line is favored for $\alpha_{\rm Dr}/\alpha_{\rm acc} \sim 1$. Our results imply that a distinct evolutionary path could produce a diversity of outcomes in terms of planetesimal formation around the snow line.