Depletion of refractory carbon in protoplanetary disks using Monte Carlo models for dust coagluation

Vignesh Vaikundaraman, Joanna Drążkowska, Fabian Binkert, Til Birnstiel, Anna Miotello

When compared to the interstellar medium, the inner solar system is significantly depleted in carbon. The reason for this is still unknown. Delving into why planet-forming dust loses its carbon gives us important insights into understanding the puzzle of planet formation. We used a Monte Carlo dust evolution code that includes vertical settling, turbulent mixing, and dust coagulation to explore how efficiently high-energy stellar radiation depletes carbon. Modeling photolysis required an accurate description of the regions in the disk surface exposed to far-ultraviolet radiation (FUV) and we applied a flux prescription taking into account the scattering by the dust grains. We found that the forward scattering nature of the dust grains and resonant scattering effects help the FUV photons penetrate deeper into the disk. We calculated carbon destruction rates and the timescale of carbon depletion at 1 AU and compared them with the present calculations and observations of carbon abundances in the inner solar system. We arrived at carbon depletion timescales of 100 - 300 Kyr, mostly depending on the strength of the turbulence of the disk. The depletion timescales are fast enough to deplete carbon within the window only if radial drift is not considered. Ideally, a faster process in combination with photolysis is needed to provide a robust mechanism of carbon depletion and solve the carbon deficit puzzle.