Yoram Lithwick (with Yanqin Wu) worked on the formation of Pluto's newly discovered moons, Nix and Hydra. They showed that the standard model, in which Nix and Hydra are formed close to Pluto and circularized there, cannot work. Instead, Nix and Hydra were likely at one time embedded in a disk of debris, which was accreted from the solar nebula.
Andrew Youdin and Yoram Lithwick (2007, Icarus) completed a project on the dynamics of solid particles in turbulent gas disks. They provided an analytic description of the turbulent stirring combined with Keplerian epicyclic motion. They provided a correction to previous results for the diffusion of larger (bigger than a meter) boulders, and provided detailed expressions which are useful for calibrating numerical simulations of planet formation.
In collaboration with Anders Johansen of MPIA-Heidelberg (now a postdoc with CITA-zen Yuri Levin at Leiden), Andrew Youdin conducted numerical simulations of the coupled dynamics of particles and gas. They found that the feedback of particle inertia on the gas flow promotes the concentration of particles into dense clumps (Youdin & Johansen 2007, Johansen & Youdin 2007 ApJ). This led to a Nature publication (Johansen et al 2007, including Youdin) which showed that these dense particle concentrations can collapse gravitationally to form asteroid-sized planetesimals.
In the core accretion model, gas giant formation is a race between growth and migration; for a core to become a Jovian planet, it must accrete its envelope before it spirals into the host star. Ed Thommes, Leif Nilsson (Physics, UToronto) and Norm Murray used a multizone numerical model to extend their previous investigation of the ``window of opportunity'' for gas giant formation within a disk. When the collision cross section enhancement due to core atmospheres is taken into account, they find that a broad range of protoplanetary disks possess such a window.
Rafikov has investigated the impact of convective cooling in gravitationally unstable disks on the ability of these disks to fragment into self-gravitating objects. This is an important problem for understanding the possibility of giant planet formation via the gravitational instability in protoplanetary disks. Rafikov demonstrated that thermodynamical conditions required to make disks both unstable and fragmenting are virtually incompatible with the properties of the protoplanetary disks, which significantly limits the possibility of giant planet formation by gravitational instability.
Rafikov has studied the irradiation of growing protostars by their own accretion disks from which they gain mass. He demonstrated that at high mass accretion rate typical for the mass buildup stage of the protostars the irradiation flux originating in the inner parts of the accretion disk must have far exceeded the intrinsic stellar luminosity. This intense external illumination dramatically affects the structure of these normally fully convective objects, and suppresses their internal cooling, thus affecting the rate at which such objects contract.
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