Global Modeling of Nebulae With Particle Growth, Drift, and Evaporation Fronts. III. Redistribution of Refractories and Volatiles

TL;DR

This paper models how particle growth, drift, and evaporation fronts influence the redistribution of refractories and volatiles in protoplanetary disks, affecting planetesimal composition and formation zones.

Contribution

It introduces a comprehensive model of disk composition evolution considering turbulence, particle drift, and evaporation fronts, revealing new insights into volatile distribution and planetesimal formation.

Findings

Radial drift causes significant inward migration of larger particles.

Volatile-rich belts form in outer nebula, potentially explaining observed pebble bands.

Long-lived volatile bands influence planetesimal composition and planetary formation zones.

Abstract

Formation of the first planetesimals remains an unsolved problem. Growth by sticking must initiate the process, but multiple studies have revealed a series of barriers that can slow or stall growth, most of them due to nebula turbulence. In a companion paper, we study the influence of these barriers on models of fractal aggregate and solid, compact particle growth in a viscously evolving solar-like nebula for a range of turbulent intensities $\alpha_{\rm{t}} = 10^{-5}-10^{-2}$. Here, we examine how disk composition in these same models changes with time. We find that advection and diffusion of small grains and vapor, and radial inward drift for larger compact particles and fractal aggregates, naturally lead to diverse outcomes for planetesimal composition. Larger particles can undergo substantial inward radial migration due to gas drag before being collisionally fragmented or partially evaporating at various temperatures. This leads to enhancement of the associated volatile in both vapor inside, and solids outside, their respective evaporation fronts, or ``snowlines''. In cases of lower $\alpha_{\rm{t}}$, we see narrow belts of volatile or supervolatile material develop in the outer nebula, which could be connected to the bands of ``pebbles" seen by ALMA. Volatile bands, which migrate inwards as the disk cools, can persist over long timescales as their gas phase continues to advect or diffuse outward across its evaporation front. These belts could be sites where supervolatile-rich planetesimals form, such as the rare CO-rich and water-poor comets; giant planets formed just outside the H$_2$O snowline may be enhanced in water.