09:00 to 10:00 S Weidenschilling (Planetary Science Institute)Planetesimal formation: numerical modeling of particle growth, settling, and collective gas-grain interactions In a relatively quiescent solar nebula, solid particles settle to form a dense layer in the midplane. The density of this layer is set by a balance between settling and diffusion caused by shear-generated turbulence. I present results of a numerical model for the equilibrium structure of a layer of particles of arbitrary size or a mixture of sizes. Radial drift rates and relative velocities are computed. Another model includes coagulation of particles throughout the thickness of the nebula; it is used to determine timescales for growth of aggregates and their settling, and the range of impact strength necessary for the production of macroscopic bodies by collisional sticking. Finally, a model for gravitational coagulation of bodies in Keplerian orbits is used to infer consequences of initial planetesimal sizes for accretion of planetary embryos. INI 1 10:00 to 11:00 J Blum ([TU Braunschweig])The growth of macroscopic bodies in protoplanetary disks: experimental evidences I will review the various laboratory and microgravity experiments on low-velocity dust-aggregate collisions and will present a systematic description of the possible outcomes of such collisions. Depending on the aggregate masses, mass ratios, porosities, and collision velocities, we can distinguish between four types of sticking, two types of bouncing, and three types of fragmentation. Careful interpolation between and extrapolation of the experimental results into yet uncovered parameter space allows us to describe the protoplanetary dust growth from initially micrometer-sized grains in a self-consistent way, using a recently-developed Monte-Carlo method. The results of the first simulations show that bouncing is the dominant process after a rapid initial growth stage, which limits the maximum aggregate sizes achievable in protoplanetary disks to 100 ?m - 1 cm, depending on the disk model used. The simulations also reveal the growth path of the dust aggregates through the multi-parameter space, which is being used to define new laboratory experiments. With this bi-directional feeback between experiments and modeling, we are able to refine the results of protoplanetary dust growth modeling and to achieve solid data for the maximum size, the size distribution, and the porosities of the aggregates. INI 1 11:00 to 11:30 Coffee 11:30 to 12:00 A Morbidelli (Observatoire de Nice)Asteroids formed big How big were the first planetesimals? We attempt to answer this question by conducting coagulation simulations in which the planetesimals grow by mutual collisions and form larger bodies and planetary embryos. The size frequency distribution (SFD) of the initial planetesimals is considered a free parameter in these simulations, and we search for the one that produces at the end objects with a SFD that is consistent with asteroid belt constraints. We find that, if the initial planetesimals were small (e.g. km-sized), the final SFD fails to fulfill these constraints. In particular, reproducing the bump observed at diameter D~100km in the current SFD of the asteroids requires that the minimal size of the initial planetesimals was also ~100km. This supports the idea that planetesimals formed big, namely that the size of solids in the proto-planetary disk jumped'' from sub-meter scale to multi-kilometer scale, without passing through intermediate values. Moreover, we find evidence that the initial planetesimals had to have sizes ranging from 100 to several 100km, probably even 1,000km, and that their SFD had to have a slope over this interval that was similar to the one characterizing the current asteroids in the same size-range. This result sets a new constraint on planetesimal formation models and opens new perspectives for the investigation of the collisional evolution in the asteroid and Kuiper belts as well as of the accretion of the cores of the giant planets. INI 1 12:00 to 12:30 On the validity of the super-particle approximation of planetesimals in simulations of gravitational collapse The formation mechanism of planetesimals in protoplanetary discs is hotly debated. Currently, the favoured model involves the accumulation of meter-sized objects within a turbulent disc, followed by a phase of gravitational instability. At best one can simulate a few million particles numerically as opposed to the several trillion meter-sized particles expected in a real protoplanetary disc. Therefore, single particles are often used as super-particles to represent a distribution of many smaller particles. However, the super- particle approximation is not always valid when applied to planetesimal formation because the system can be marginally collisional (of order one collision per particle per orbit). The super-particle approximation is valid only when the system is collisionless. In many recent numerical simulations this is not the case and the approach leads to spurious results and enhanched clumping. We present new results from numerical simulations of planetesimal formation through gravitational instability. A scaled system is studied that does not require the use of super-particles. We find that the scaled particles can indeed be used to model the initial phases of clumping if the porperties of the scaled particles are chosen such that all important timescale in the system are equivalent to what is expected in a real protoplanetary disc. This method is explained in detail in this paper and we give constraints on the number of particles that one has to use in order to achieve numerical convergence. In order to illustrate this we simplify the system: the evolution of particles is studied in a local shearing box; the particle- particle interactions such as gravity, physical collisions, and gas drag are solved directly but a constant background shear flow without any feedback from the particles is assumed. We compare this new method to the standard super-particle approach and find significant discrepancies in both the require- ment for gravitational collapse and the resulting clump statistics. Our study shows that the formation of planetesimals in a trubulent disk is much harder than previously reported. INI 1 12:30 to 13:30 Lunch at Wolfson Court 14:00 to 15:00 Primary accretion of large planetesimals from chondrule size particles Primary accretion is the process by which the first large objects formed from freely floating nebula particles. Several clues as to the nature of this process are to be found in primitive meteorites and asteroids. The most primitive chondritic meteorites display a characteristic texture: predominance of mm-sized, once-molten chondrules, metal grains, and refractory oxide particles, each surrounded by fine-grained dust rims and all embedded in a granular matrix. The size distribution of the chondrules in all classes of chondrite is quite narrow and nearly universal in shape, but with a mean size distinctive of each class. At least two entire chondrite classes are each thought to derive from only one or two planetesimals, roughly 100 km in size and originally composed largely of chondrules with very similar properties. This ubiquitous and unusual texture is surely telling us something important about primary accretion, but there is no explanation for it at present. Moreover, the extended duration of meteorite parent body formation as revealed in isotopic age-dating, and the scarcity of melted asteroids, suggest that primary accretion went on for a long time. We have shown how well-sorted, chondrule-sized mineral particles can be concentrated, by orders of magnitude, into dense zones in weak nebula turbulence. This turbulent concentration explains the characteristic size and size distribution of chondrules in a natural way. We developed a cascade model of the statistics of dense zones and their correlation with gas vorticity, which incorporates the effects of particle mass loading on the gas and predicts the fractional volume of particle-rich zones which can evolve directly into objects with some physical cohesiveness. We have derived threshold conditions (combinations of particle density, clump lengthscale, gas density, and local vorticity) which allow dense clumps to proceed to become actual planetesimals. Combination of these thresholds with our cascade models recently led us to a method for predicting the relative abundance of primary planetesimals as a function of mass - their birth function - and even their production rate. The predictions can be extended easily from the asteroid belt to the Kuiper belt; similar size populations are found to arise. A number of challenges remain in validating and solidifying this scenario. The key elements of the cascade model must be validated (or modified) using deeper inertial ranges, further from the dissipation scale. The settling of dense clumps in the vertical component of solar gravity increases the local density of chondrule-size components in regions near the midplane, and must be modeled. Finally, the self-gravity of dense particle clumps in turbulence must be modeled to assess their stability and mutual interactions. INI 1 15:00 to 16:00 Tea and Poster session 16:00 to 17:00 Planetesimal formation in self-gravitating protostellar discs In this talk, I will review the main features of gravitational instabilities in gaseous discs. I will discuss the conditions under which protostellar discs are expected to be unstable and describe their evolution. I will discuss the role of gravitationally induced structures in the formation of planetesimals, also in the light of recent developments on the structure of self-regulated protostellar discs. I will show that planetesimal formation through gravitational instabilities in the gas disc can only occur at large radii, beyond a few tens astronomical units and discuss its implications. INI 1 17:00 to 18:00 Discussion 19:30 to 22:00 Conference Dinner at Emmanuel College