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Project Progress

Postdoc Graeme Lufkin joined the node in September 2004. He has worked on two areas of planetary dynamics.

First, he completed simulations from his thesis work on the mechanism of giant planet migration in a gaseous circumstellar disk. His results agree partially with previous simulations using a different numerical technique, suggesting that giant planets should migrate quickly toward their parent star, possibly creating a gap in the gas disk. In addition, the results of his non-linear simulations disagree significantly with the analytical treatment of migration. This is strong evidence that non-linear interaction is a dominant component of giant planet migration, necessitating the use of computer simulations.

Second, Dr. Lufkin, in collaboration with Drs. Richardson and Mundy, has started investigating the effect of giant planet migration on disks of planetesimals. Observations of extrasolar planets suggest that giant planets migrate inward through the traditional habitable zone early in the formation of solar systems. What is the fate of the building blocks of terrestrial planets during such an interaction? To answer this, we have modified our gravitational simulation code to handle a prescribed migration of a giant planet. We simulate disks of planetesimals under the influence of the central star and migrating planet, and observe how the distribution of orbital elements is affected. We are running many simulations, varying both the mass and migration rate of the giant planet, to quantify this dependence. Our initial results suggest that giant planet migration is not a catastrophic event for disks of planetesimals. The eccentricity and inclination of the planetesimals are greatly increased by the giant planet, but less than twenty percent actually get ejected from the system. Further growth of the planetesimals will be postponed until dynamical friction and gas drag can cool their orbital motion.

All the work described above has been performed on the computing cluster of the University of Maryland Astronomy Department. During this year the cluster has been upgraded by Richardson and Lufkin with new nodes and disk space, doubling its speed and storage.

This work was presented at the June 2005 Gordon Research Conference: Origins of Solar Systems, and will be presented at the Protostars and Planets V meeting in October 2005.

Our simulations have thus far been simple, including only the gravitational interaction of the giant planet on the planetesimals. We intend to incorporate additional physics such as gas drag and mutual gravitational interaction. These will require significantly more computing time, but appear feasible.

Dr. Richardson, in addition to working with Graeme Lufkin on the simulations described above, continued work on planet formation simulations with former graduate student Zoë Leinhardt (now a postdoctoral fellow at Harvard University). This work, title “Planetesimals to protoplanets. I. Effect of fragmentation on terrestrial planet formation” and published in ApJ 625:427 (2005), quantified the effect of fragmentation (based on a rubble pile model) during the early and middle stages of planetary growth (it was also presented by Leinhardt at the NAI meeting in April 2005). They found that, for the parameters tested, fragmentation did not play a dominant role compared to simulations that assumed perfect accretion. In addition, debris created in these simulations did not significantly affect the dynamics of the larger bodies. Leinhardt will be visiting Maryland in late October to continue this project (extending to a larger parameter space and implementing new physics) and to plan strategy for their remaining supercomputer time at the Pittsburgh Supercomputer Center. Finally, Richardson designed an algorithm for quantifying and tracking the radial movement of planetesimals in a planet-forming disk that takes into account incorporation of planetesimals into larger bodies. This algorithm will be implemented and tested in the next funding cycle.

Dr. Mundy has continued complementary work at millimeter, submillimeter and infrared wavelengths which focuses on the observational aspects of dust evolution in disks around young and forming stars. In the paper titled “Large Dust Particles in Disks around T Tauri Stars” (accepted to Astronomy & Astrophysics, Rodmann et al Sept 2005), we present 7-mm wavelength continuum data for 14 low-mass young stars in Taurus and argue that dust grains have grown to millimeter size in some systems. Such grain growth is an important first step toward planetesimal formation and is relevant to the simulations discussed above. Dr. Mundy also continues his work on Spitzer projects of relevance to the early evolution of planet-forming disks. Of particular interest are the studies of cold outer disks systems which are being found through their emission in the IRAC 8 micron and MIPS 24 micron bands. The nature and evolutionary state of these systems is unclear at this point; they could be post classical T Tauri systems which have not yet lost their outer disks; they could be more evolved systems with outer debris disks created by planetesimal collisions. Continued work with the large dataset associated with the Cores to Disks Spitzer Legacy project is likely to provide answers.

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