Simulating Dispersal of Protoplanetary Disks and Planetary Atmospheres with Consistent Radiation and Thermochemistry

Abstract

This thesis focuses on combining (magneto-)hydrodynamic (HD/MHD) simulations with consistent radiation and thermochemistry. We implement a simulation code, an extensions of the Godunov HD/MHD solver Athena++, that co-evolves radiation and thermochemistry with HD/MHD (Chapter 2). A stoichiometry-compatible scheme for advection of chemical species is proposed that has lower computation costs than conventional schemes. Radiative transfer and thermochemistry solvers are optimized for graphics processing units (GPUs), showing considerable acceleration.

Chapter 3 simulates unmagnetized photoevaporation in protoplanetary disks (PPDs) with this code. Most models have a three-layer structure: a cold mid-plane, warm intermediate layer, and hot wind, the last having typical speeds ~40 km/s and mass-loss rates ~1e-9 SolarMass/yr when driven primarily by ionizing UV radiation. H2O molecules re-form in the intermediate layer and survive at relatively high wind temperatures due to reactions being out of equilibrium. Wind mass loss rates are sensitive to the treatment of both the hydrodynamics and the thermochemistry.

We use the code specifically on hydrostatic grids to study the puzzle of transitional disks, which have inner cavities in gas and dust but mostly signs of accretion (Chapter 4). Results indicate that gas near the mid-plane has a dimensionless ambipolar parameter in the right general range for wind solutions to drive transonic accretion in the predominantly neutral mid-plane.

Chapter 5 carries out global MHD simulations of magnetized PPD, thus combining magnetic acceleration with photoevaporation, and linking outflow (winds) to inflow (accretion). When magnetization is sufficient to drive accretion rate ~1e-8 SolarMass/yr as is typically observed, the mass loss rates in the winds are comparable. Models lacking EUV photons feature warm magnetized winds at typical poloidal speeds > 4 km/s . Mass loss rate are subtly affected by ionization due to UV and X-ray radiation near wind bases. EUV photons yields the three-layer structure of photoevaporation, whereas the warm intermediate layer, which remains bound to the star

in the magnetized models of Chapter 3, becomes a magneto-thermal wind.

The code is also used to investigate the photoevaporation of primordial planetary atmospheres (Chapter 6). EUV photons are most important in launching the photoevaporative wind, while the EUV photospheres sensitively depend on the entropy and mass in the convective layer. We then find the photoevaporation model capable to reproduce the observed bimodal radius distribution of sub-Neptune Kepler planets semi-quantitatively.