Planet-forming material in a protoplanetary disc: the interplay between chemical evolution and pebble drift

Abstract

The composition of gas and solids in protoplanetary discs sets the composition of planets that form out of them. Recent chemical models have shown that the composition of gas and dust in discs evolves on Myr time-scales, with volatile species disappearing from the gas phase. However, discs evolve due to gas accretion and radial drift of dust on time-scales similar to these chemical time-scales. Here we present the first model coupling the chemical evolution in the disc mid-planes with the evolution of discs due to accretion and radial drift of dust. Our models show that transport will always overcome the depletion of CO$_2$ from the gas phase, and can also overcome the depletion of CO and CH$_4$ unless both transport is slow (viscous $\alpha \lesssim 10^{-3}$) and the ionization rate is high ($\zeta \approx 10^{-17}$). Including radial drift further enhances the abundances of volatile species because they are carried in on the surface of grains before evaporating left at their ice lines. Due to large differences in the abundances within 10 au for models with and without efficient radial drift, we argue that composition can be used to constrain models of planet formation via pebble accretion.