Planet formation in a dusty disk: effects of collisional dust growth and dynamics
Date
2019
Authors
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Publisher
University of Delaware
Abstract
Despite making a small contribution to total protoplanetary disk mass, dust affects the disk temperature by controlling the absorption of starlight. As grains grow from their initial ISM-like size distribution, settling depletes the disk’s upper layers of dust and decreases the optical depth, cooling the interior. In this dissertation, we will discuss the effects of collisional growth of dust grains and their dynamics on the thermal and optical profiles of the disk, the vertical distribution of dust grains, and the possibility that cooling induced by grain growth and settling could lead to gravitational instability in an otherwise marginally stable disk. We also discuss how the critical gap-opening mass of a growing planet changes with the growth, settling and inward radial drift of solids in the course of a disk's evolution. ☐ First, we present a new fast and numerically inexpensive Monte Carlo method with a weighting technique, which models collisional growth of dust, along with vertical settling, turbulent diffusion and radial drift. We present a comprehensive description of the structure of the massively parallel code we have developed. Next, as the first application of our dust model, we explore three disk models, the Minimum Mass Solar Nebula (MMSN), the Minimum-Mass Extra-solar Nebula (MMEN), and a “heavy” disk with higher surface density than the MMEN, and perform simulations for both constant and spatially variable profiles of the turbulence efficiency, $\alpha$. The variable α profile is computed from the ionization fraction determined by an ionization-recombination chemical network. We then calculate wavelength-dependent opacities for the evolving disks and perform radiative transfer to calculate the temperature profile. We find that the growth of large particles in the mid-plane can make a massive disk optically thick at millimeter wavelengths, making it difficult to determine the surface density of dust available for planet formation in the inner disk. Finally, we calculate the Toomre Q parameter, a measure of the disk's stability to gravitational perturbations, for each disk model after it reaches a steady state dust-size distribution, and show that for an initially massive disk, grain growth and settling can reduce the Toomre Q parameter, making the disk unstable under its self-gravity and possibly triggering spiral instabilities. ☐ In the second application, we apply our dust model to calculate the new hydrostatic equilibrium for vertical gas columns and show that the local gas scale heights become significantly less compared to the canonical value of $h(R)/R \sim 0.05$ for isothermal disk models, and can become as low as $0.005$ in a disk with weak turbulence. We also find that the gap opening criteria is not sensitive to the mass of the disk, but basically depends on the turbulence strength. We discuss this result in the context of the minimum mass for a planet to open a gap in a settled disk, and its possible implications for planet migration.