Arturo Cevallos Soto

Title of Master thesis: Evolving protoplanetary disk composition: coupling full chemical network with transport dynamics in a 1-D model

Context. The Inside-Out Planet Formation (IOPF) theory proposes that close-in super-Earth planets
form in situ at the pressure maximum associated with the Dead Zone Inner Boundary (DZIB). The
chemical composition of pebbles and gas reaching this location would in
uence that of such planets:
both the planetary cores and any primordial atmosphere.
Aims. Our goal is to develop a combined model of physical and chemical evolution of protoplanetary
disk midplanes that follows gas advection, radial drift of pebbles and gas-grain chemistry to predict
the abundances of species from outer disk scales of up to 300 AU down to the small scales of the DZIB
near 0.1 AU.
Methods. We adopt a steady, thin, accretion disk structure that yields midplane properties (tem-
perature, density, etc.) for di erent accretion rates m_ in the range 10􀀀8 to 10􀀀9 M yr􀀀1 and a
viscosity parameter = 10􀀀4. A full chemical network including gas-phase, gas-grain/pebble inter-
actions and grain-surface chemistry evolves the composition of each radial location, which is coupled
with continuous gas and pebble transport for a duration of t = 105 years. Initial abundances are set
by assuming some prior chemical evolution at temperatures < 20 K. The e ect of di erent grain sizes
is also investigated.
Results. We find pebble drift has a large in
uence on the overall solid to gas ratio in the disk
midplane. This is most significant for models with large pebble sizes. It was found that the choice
of abundances for initial and boundary conditions can have a strong impact on the final composition
delivered to the inner region. We find that C and up to 90 % of O nuclei start locked in CO and O2 ice,
which keeps abundances of other species like CO2 and H2O one order of magnitude lower. Volatiles'
gas phases have their abundances enhanced by up to an order of magnitude at their respective iceline
locations due to the radial drift of icy pebbles. Gas advection then brings these species to the hot inner
disk. These region, with fastest gas advection velocities, show the largest di erences in abundances
compared to static models. CO2, which does not form close to the DZIB, is rather transported there
by this process. Lower accretion rates yield lower disk temperatures, which a ects icelines' locations
by shifting them closer to the star. While transport does modify the C/O ratio for gases and solids, we
find that other model assumptions related to initial/boundary conditions and pebble size distribution
have an even larger influence.
Conclusions. We have demonstrated the importance of the combined physical and chemical evolution
to understand the composition of pebbles and gas for planet formation. In particular, we have explored
the sensitivity of key metrics, such as solid and gas phase element ratios to the choices of initial and
boundary conditions of such models.

Supervisor: Jonathan Tan
Examiner: Jonathan Tan
Opponent: Ana-Mari Petrova
Category Student project presentation
Location: Online via Zoom
Starts: 20 September, 2021, 15:00
Ends: 20 September, 2021, 16:00

Page manager Published: Thu 16 Sep 2021.