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Project Progress

Marc Kuchner and his graduate student Erika Nesvold are working on a new tool for modeling the collisional evolution and 3-D distribution of planetesimals in planetary systems and debris disks. We plan to use this tool for interpreting images of planetary systems: modeling images and other data on circumstellar disks. We expect to be able to use this approach to locate hidden exoplanets via their dynamical influence on the shapes of the disks. This technique may be the only way we can find and measure Neptune-like exoplanets: planets that are too faint to image directly, and too slowly orbiting to detect via their effects on their host stars.

We also expect to use our new models to understand the evolution of planetesimals in the solar system. For example, a popular model for the dynamics of the Kuiper Belt and other small bodies in the solar system is the “Nice” model, which posits that the giant planets migrated through a configuration where Jupiter and Saturn were in a 2:1 resonance. This model has ben explored in detail with n-body models, but never with models that incorporate collisions. Our new technique should allow us to constrain the Nice model to a variety of data that it hasn’t been properly compared to before: KBO size distributions, for example. This step will be important for understanding the delivery of water to terrestrial planets by the planetesimals.

Though many research groups have studied the collisional evolution of debris and many have studied the 3-D distribution of planetesimals, it has not yet been possible to model
both the time evolution of a disk due to collisions and the 3-D structure imprinted by a planet on the disk in one a single model. But images of debris disks show a variety of structures, like warps and eccentric rings that can only be properly modeled in this manner. Our model will combine a 3-D n-body integrator (FIGURE 1) with a new algorithm for handling collisions, statistically (FIGURE 2), allowing it to model, for the first time, systems where the
secular time or resonance time is comparable to that of the collision time.

This kind of problem has long been considered difficult because as planetesimals collide, their numbers grow exponentially, while n-body algorithms tend to break down for n>10⁷ or so. Our model works by tracking swarms of particles rather than individual particles. Each swarm is assumed to have the same orbital parameters, but a range of planetesimal sizes, from millimeters to kilometers. When two swarms “collide” we calculate the size-velocity distribution of the resulting spray of daughter particles, and use a Monte-Carlo scheme to choose a manageable number of new swarms to track that represent the post-collision distributions. Additionally, we use a dynamic allocation scheme for the swarms to make best use of computer resources.

Kuchner continues to serve as a disk modeler for several observing teams: the Keck Interferometer Nuller key science program, the NICI key science team, a coronagraphic survey of disks using the STIS instrument on Hubble Space Telescope, some smaller ground-based disk-imaging projects with Keck and VLT/NaCo.