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Timetable (DDPW02)

Planetesimal Formation

Monday 28th September 2009 to Wednesday 30th September 2009

Monday 28th September 2009
09:00 to 09:55 Registration
09:55 to 10:00 Welcome - David Wallace
10:00 to 11:00 J Papaloizou ([Cambridge])
Dust, vortices and density waves in protoplanetary disks
Some recent work on the generation of vortices and their stability is discussd as well as density wave production by turbulence. Consequences for the accumulation of solids are considered.
11:00 to 11:30 Coffee
11:30 to 12:30 C Dullemond ([MPI for Astronomy])
Observation and modeling of proto-planetary disks
I will give an overview of what has been learned about the structure and evolution of protoplanetary disks since the first direct images of such disks were obtained by the HST in the mid nineties. This overview will cover both the various advances made in observations of these disks, as well as the theoretical modeling that was/is aimed to understand these observations. The ultimate aim of these studies is to get a better understanding of the environments in which planets form, and of what the solar nebula might have looked like 4.5 billion years ago. I will show that, while we have now a much better understanding of these issues than before the mid nineties, a great number of new fundamental questions have arisen that are still unsolved. I will end with some thoughts about which kind of observations and modeling need to be done in the next decade to finally be able to answer, at least partially, the question in which kind of environment planets are formed.
12:30 to 13:30 Lunch at Wolfson Court
14:00 to 15:00 M Gounelle ([MNHN])
What meteorites have to say
I will try to present the constraints brought by meteorites on the chronology of and the physical mechanisms within the solar protoplanetary disk.
15:00 to 15:30 Tea
15:30 to 16:30 C Terquem ([Pierre & Marie Curie-Paris VI])
Models of partially ionized discs
17:00 to 18:00 D Lin ([UC, Santa Cruz])
Origin and destiny of planets
Recent discoveries of abundant planetary macrocosms, with diverse masses, sizes, and orbits, post challenges to traditional speculations on the origin and rarity of potentially habitable worlds beyond the visible wanderers in the night sky. The most profound paradigm shift is the mobility of building blocks which promotes the assemblage as well as the destruction of protoplanets and reconstitutes the order of planetary systems. New theories must confront not only relics gathered within the solar system but also signatures of protostellar disks where planetary birth is an ongoing event. Long viewed as a process of stately procession to a foregone conclusion, planetary formation turns out to be startlingly chaotic. The worlds that emerged are the hurly-burlies of competing physical mechanisms. Restless gravitational agitations between individuals lead to unceasing transformation of mature planetary system configurations throughout the lifespan of their host stars. The emergence of habitable worlds and possibility of life elsewhere in the Universe appears to be inevitable. Their eventual discoveries will have fundamental implications on the planetary statistical mechanics, anthropic principle, theory of astrobiology, and the Fermi paradox.
18:00 to 18:30 Welcome Wine Reception
18:45 to 19:30 Dinner at Wolfson Court (Residents Only)
Tuesday 29th September 2009
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.
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.
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.
12:00 to 12:30 H Rein ([Cambridge])
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.
12:30 to 13:30 Lunch at Wolfson Court
14:00 to 15:00 J Cuzzi (NASA Ames Research Center)
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.

15:00 to 16:00 Tea and Poster session
16:00 to 17:00 G Lodato ([Milan])
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.
17:00 to 18:00 Discussion
19:30 to 22:00 Conference Dinner at Emmanuel College
Wednesday 30th September 2009
09:00 to 09:30 MJ Wardle ([Macquarie])
Magnetic activity in protoplanetary discs
Magnetic fields play a key role in the dynamics and evolution of protoplanetary disks. Significant magnetic flux is brought in during the collapse of a molecular cloud core to form a star and disk. Subsequently, the maagnetic field may efficiently transport angular momentum radially outwards by MHD turbulence or remove it vertically through ordered magnetocentrifugal acceleration of outflows from the disk surface. In addition, magnetically-driven transport and mixing impacts disk chemistry, the evolution of the grain population, and planetary migration. I shall outline some of the considerable uncertainties in understanding and quantifying these processes.
09:30 to 10:30 S Fromang (CEA/Saclay)
How reliable are MRI simulations? The role of the magnetic Prandtl number
Thanks to high resolution local simulations, the last few years have seen significant progresses in our understanding of MHD turbulence driven by the magnetorotational instability. Small scale explicit dissipation coefficients (viscosity and resistivity), traditionnally excluded from such simulations in the past, have been shown to be extremely important to set the saturated amplitude of the turbulence. In this talk I will review these recent results and discuss their consequences for protoplanetary disks structure and planet formation theories.
10:30 to 11:00 PY Longaretti ([Joseph Fourier Grenoble])
Prandtl number dependence of MRI-driven transport : new results and future prospects
11:00 to 11:30 Coffee
11:30 to 12:30 NJ Turner ([CALTECH])
Magnetic activity and the separation of dust from gas
From the starlight scattered and reprocessed by protostellar disks we know that micron-sized dust grains remain suspended in the disk atmospheres for several million years. At the same time, measurements at millimeter wavelengths indicate that some particles inside the disks have grown to the size of pebbles. Surely planet formation is underway. Yet according to coagulation models, if the disk gas were laminar, the dust would quickly grow and settle out, leaving the atmosphere optically thin. On the other hand, turbulence at the levels suggested by disk evolutionary timescales would lead to collisional disruption, halting the growth of larger bodies. I will discuss how these difficulties can be resolved if the disk has a magnetically-active, turbulent atmosphere combined with a midplane dead zone.
12:30 to 13:30 Lunch at Wolfson Court
14:00 to 15:00 A Youdin ([Toronto])
Triggering gravitational collapse into planetesimals: The streaming instability and other mechanisms
I will describe recent progress on the formation of planetesimals, focusing on dynamics. Self-gravity is a promising mechanism to collect a sea of small solids into a gravitationally bound planetesimal, but stirring by turbulent gas is a formidable obstacle. However several processes can concentrate particles in gas disks --- both despite and because of turbulence. Perhaps the most powerful is the streaming instability, mediated by two-way drag forces between solids and gas. Sufficiently dense particle clumps trigger collapse into planetesimals. While these mechanisms are promising, many outstanding questions remain. How large must particles grow by coagulation until these dynamical processes take over? How does the final collapse proceed, and what is the initial mass function of planetesimals?
15:00 to 15:30 Tea
15:30 to 16:30 H Klahr ([MPI for Astronomy])
Garvoturbulent planetesimal formation - state and prospects of the field
Planetesimals may well form from the gravitational collapse of a particle cloud, the so called, pre-planetesimals, once they got concentrated by turbulent flow features, possibly enhanced by a streaming instability. This talk shall give an overview of the state of the field and summarize what we know and what we still have to learn about this mechanism. I will discuss the formation and redistribution of the pre-planetesimals. I will also address the role of resolution in our numerical simulations and in how far this relates to the possible initial planetesimal masses. I show that the mass of the planetesimals is actually determined by the rate at which pre-planetesimals drift through the disk. The talk shall further discuss a list of open questions concerning gravoturbulent planetesimal formation and how this model may fit to the observational findings of our solar system.
16:30 to 17:30 Discussion and Conclusions
18:45 to 19:30 Dinner at Wolfson Court (Residents Only)
University of Cambridge Research Councils UK
    Clay Mathematics Institute London Mathematical Society NM Rothschild and Sons