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Research Interests

My research interests are centered around the theoretical study of gravitational dynamics and hydrodynamics in star and planet formation. I have worked on a broad range of problems, spanning scales from planets embedded in their natal disk to the dynamics of massive, dense star clusters over relaxation timescales. The common thread running through most of my work is an interest in the transitional period between the gas-dominated origin of stars and planets and the later n-body regime.

Recent examples of work in each topic:

Planets and Disks

As planets form in a protoplanetary disk, any orbital eccentricities that may develop will be efficiently damped by the disk. Once the gas has cleared, gravitational instabilities are free to develop and the planets may develop crossing orbits, leading to ejection, collisions, and a distribution of eccentricities that closely matches the observed one. Most studies of this process focus on the pure n-body regime, with gas-free initial conditions.

In collaboration with Phil Armitage and Sean Raymond, I have investigated the effect of a remnant gas disk on the dynamical outcomes of planet scattering. Our initial investigation in 2008 hinted at possibly important effects, but was limited by the computational expense of our 3D simulations. In our 2011 followup using the 2D hydro code FARGO, we presented the first statistically significant exploration of this problem with a gas disk simulated in more than 1D. Some movies from that study can be found here.

Three planets interacting in a gas disk. The box size is approximately 10 au. Soon the inner two planets will merge, and the remaining two open eccentric gaps and. Data from Moeckel & Armitage 2011.

Small-n dynamics of young, massive stars

My thesis work at the University of Colorado (supervised by John Bally) dealt with binary formation and disk destruction during encounters between stars and massive disks.

My most recent study in this area has looked at the impact of binary–single encounters on a circumbinary disk, with the BN/KL region in Orion as the motivation. Here a recent scattering event is hinted at by high proper motions of two massive YSOs; complicating the picture, one of them apparently retains a disk. When a binary and a single scatter their gravitational interaction can be extremely complex, and in this case any disk material is likely to be violently disrupted. Sometimes a simple encounter occurs that can leave the disk partially intact. Our first-order study of this process, treating only gravity, is the first scattering study to include material other than the stars. We confirmed that a portion of a disk can survive a velocity-boosting stellar encounter, helping to resolve some of the puzzling observational aspects of the BN/KL region, and set the stage for future studies that will include more computationally expensive physics.

These outcomes are illustrated at right: movies of these two examples can be found here.

Top: stellar paths through a complex scattering event that destroys the disk. The binary begins at the left surrounded by a disk. Gray points show the final disk particles. Bottom: a simple event that preserves some of the disk. Data from Moeckel & Goddi 2011.

Young star clusters

When studying the n-body dynamics of young star clusters, the detailed initial conditions are more important than in some other cluster simulations. For instance, when simulating a post-core collapse globular cluster over a Hubble time, the relaxation of the initial binary population into its steady state may not matter, nor the use of a standard spherical density structure. With very young clusters, where the cluster may be only a few crossing times old, these inputs are crucial. My work has investigated the importance of these initial conditions (e.g. the initial density structure’s effect on mass segregation, with I. Bonnell), and extended n-body methods to be more suitable for young clusters (in my study of collisions in clusters undergoing both accretion and dynamical relaxation in the embedded phase, with C. Clarke).