Inside-out planet formation – VII. Astrochemical models of protoplanetary discs and implications for planetary compositions

Inside-out planet formation (IOPF) proposes that the abundant systems of close-in Super-Earths and Mini-Neptunes form in situ at the pressure maximum associated with the dead zone inner boundary (DZIB). We present a model of physical and chemical evolution of protoplanetary disc midplanes that follows gas advection, radial drift of pebbles, and gas-grain chemistry to predict abundances from ∼300 au down to the DZIB near 0.2 au. We consider typical disc properties relevant for IOPF, i.e. accretion rates 10¯⁹<m˙/(M⊙yr¯¹)<10¯⁸ and viscosity parameter α = 10¯⁴, and evolve for fiducial duration of 10⁵ yr. For outer, cool disc regions, we find that C and up to 90 per cent of O nuclei start locked in CO and O₂ ice, which keeps abundances of CO₂ and H₂O one order of magnitude lower. Radial drift of icy pebbles is influential, with gas-phase abundances of volatiles enhanced up to two orders of magnitude at icelines, while the outer disc becomes depleted of dust. Discs with decreasing accretion rates gradually cool, which draws in icelines closer to the star. At ≲ 1 au, advective models yield water-rich gas with C/O ratios ≲ 0.1, which may be inherited by atmospheres of planets forming here via IOPF. For planetary interiors built by pebble accretion, IOPF predicts volatile-poor compositions. However, advectively enhanced volatile mass fractions of ∼10 per cent can occur at the water iceline.