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

Abstract

The Earth and other rocky bodies in the inner Solar System are significantly depleted in carbon, compared to the Sun and the interstellar medium (ISM) dust. Observations suggest that more than half of the carbon material in the ISM and comets are in a highly refractory form, such as amorphous hydrocarbons and (less refractory) complex organics, which can make up the building blocks of rocky bodies. While amorphous hydrocarbons can be destroyed by photolysis and oxidation, previous studies have suggested that the radial transport of solid particles suppresses carbon depletion. The only exception is the case of strictly complex organics as the refractory carbons, which are considerably less refractory than amorphous hydrocarbons. We aim to reveal the conditions for the severe carbon depletion in the inner Solar System, by adding potentially more realistic settings: different levels of stickiness between icy and silicate particles and high-temperature regions in the upper optically thin layer of the disk, which were not included in the previous works. We performed a 3D Monte Carlo simulation of radial drift and turbulent diffusion of solid particles in a steady accretion disk with the above additional settings as well as ice evaporation and recondensation. We considered the photolysis and oxidation of hydrocarbons in the upper layer as well as the pyrolysis of complex organics to evaluate the radial distribution of carbon fraction in the disk by locally averaging individual particles. The carbon fraction drops off inside the snow line by two orders of magnitude compared to the solar value, under the following conditions: i) when silicate particles are much less sticky than icy particles and ii) when there are high-temperature regions in the disk upper layer. The former leads to fast decay of the icy pebble flux, while the silicate particles are still piling up inside the snow line. The latter contributes to the efficient turbulent stirring up of silicate particles to the upper UV-exposed layer. We have identified simulation settings to reproduce a carbon depletion pattern that is consistent with the observed one in the inner Solar System. The conditions are not too restricted and allow for a diverse carbon fraction of rocky bodies. These effects could be responsible for the observed large diversity of metals on photospheres of white dwarfs and may suggest diverse surface environments for rocky planets in habitable zones.