Simulating Protoplanetary Disk Dynamics and Investigating Their Exoplanet Outcomes

Abstract

Protoplanetary disks are the formation sites of planets and represent an exciting challenge for any theorist. We have undertaken two studies of protoplanetary disks and two studies of unusual exoplanets to better understand extrasolar planetary systems.

First, we examine the effects of accretion on protoplanetary disk chemical evolution. In many chemical models, the physical conditions of a gas parcel are fixed throughout the simulation. Instead, we develop a simple surface density model to compute the evolution of the disk's surface and volume densities as functions of space and time. This information can be fed into a chemical model, simulating the change in local conditions as the gas parcel accretes towards its central star. We find that cosmic rays play a particularly interesting role in the chemical evolution, since cosmic-ray driven chemistry in the outer disk can then drive different chemistry in the inner disk, compared to a parcel that maintains a fixed location.

Next, we move to a different aspect of disk evolution: grain drift. Small grains in a protoplanetary disk are well-entrained with the gas, but larger ones tend to drift inward towards the central star. Unlike the previous model, in which any accretion track is independent of all others, a model that includes drift allows ``communication'' between intersecting gas and solid trajectories. This effect makes the model more computationally expensive to run, so we limit the model to two chemical species, CO and H2O, in two phases, ice and gas. We find that a region of enhanced CO/H2O is easily and robustly formed in our model disk, which may help explain observations of CO-enhanced comets like 2I/Borisov and C/2016 R2 (PanSTARRS).

Finally, we address extrasolar planets in extreme environments, with a particular focus on KOI 1843.03. The so-called ultra-short period (USP) planets are planets on particularly close-in orbits, and their formation mechanism is a matter of ongoing research. We seek to better understand USP planets by modeling their interiors self-consistently, which allows us to place constraints on their probable iron content and core mass fraction if they are near or at the Roche limit. For KOI 1843.03, we are even able to favor a core comprised of pure Fe over one comprised of pure FeS. Ongoing work involves determining the shapes of transit light curves for non-spherical, tidally-distorted USP planets.