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

Abstract

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 disk 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 disk properties relevant for IOPF, i.e. accretion rates $10^{-9}< \dot{m}/ (M_\odot \:{\rm {yr}}^{-1} )<10^{-8}$ and viscosity parameter α = 10−4, and evolve for fiducial duration of 105 yrs. For outer, cool disk regions, we find that C and up to $90{{\%}}$ of O nuclei start locked in CO and $\rm O_2$ ice, which keeps abundances of $\rm CO_2$ and $\rm H_2O$ 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 ice-lines, while the outer disk becomes depleted of dust. Disks 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 $\sim 10{{\%}}$ can occur at the water ice line.

Dust traps in protoplanetary disks efficiently form organic macromolecular matter

“…Various models of protoplanetary disks demonstrate that this is a ux that is readily achieved in the surface layers of disks, due to the irradiation of the central star 31 . The mean opacity is set to κ = 10 cm 2 g −1 , which is in the range of values that are commonly used for protoplanetary disks 32 .…”

A Chemical Modeling Roadmap Linking Protoplanetary Disks and Exoplanet Atmospheres

“…The Walsh code was adopted by Cevallos Soto et al They accounted for pebbles drifting through new material (gas and dust), as they moved through the disk. For their models with stellar accretion rates of ṁ = 10 − 8 nobreak0em0.25em⁣ normalM ⊙ /yr, they find that only pebbles drifting from radii <10 AU make it to their inner disk region within 100kyr.…”