Effects from different grades of stickiness between icy and silicate particles on carbon depletion in protoplanetary disks

TL;DR

This study models how variations in particle stickiness and high-temperature zones in protoplanetary disks influence carbon depletion, revealing conditions that lead to significant carbon reduction in forming rocky bodies.

Contribution

It introduces a realistic 3D Monte Carlo simulation incorporating particle stickiness differences and high-temperature effects, advancing understanding of carbon depletion mechanisms.

Findings

Carbon drops by two orders of magnitude inside the snow line.

Less sticky silicate particles promote rapid carbon depletion.

High-temperature regions stir silicates into UV-exposed areas.

Abstract

Earth and other rocky bodies in the inner Solar System are significantly depleted in carbon compared to the Sun and interstellar medium (ISM) dust. Observations indicate that over half of carbon in the ISM and comets is in refractory forms, like amorphous hydrocarbons and complex organics, which can be building blocks of rocky bodies. While amorphous hydrocarbons are destroyed by photolysis and oxidation, radial transport of solid particles can limit carbon depletion, except when complex organics, which are less refractory, are the main carbon source. We aim to identify conditions for severe carbon depletion in the inner Solar System by introducing more realistic factors: differences in stickiness between icy and silicate particles, and high-temperature regions in the disk's upper optically-thin layer, which were not considered in previous studies. We perform a 3D Monte Carlo simulation of radial drift and turbulent diffusion in a steady accretion disk, incorporating ice evaporation/re-condensation, photolysis/oxidation of hydrocarbons in the upper layer, and pyrolysis of complex organics. Our results show that the carbon fraction drops by two orders of magnitude inside the snow line under two conditions: i) silicate particles are much less sticky than icy particles, leading to a rapid decline in icy pebble flux while silicates accumulate inside the snow line, and ii) high-temperature regions in the disk's upper layer stir silicate particles into UV-exposed areas. These conditions reproduce carbon depletion patterns consistent with observations and allow for diverse carbon fractions in rocky bodies. This diversity may explain the wide variation of metals in white dwarf photospheres and suggest different surface environments for rocky planets in habitable zones.