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Timetable (DDPW01)

Dynamics of Discs and Planets

Monday 17th August 2009 to Friday 21st August 2009

Monday 17th August 2009
08:30 to 09:50 Registration
09:50 to 10:00 Welcome - Ben Mestel
10:00 to 11:00 L Hartmann ([Michigan])
Review - protostellar disk observations
Rapid progress is being made in developing observational constraints on the structure and evolution of protoplanetary disks, primarily due advances in spectral sensitivity due to the Spitzer Space Telescope and improvements in spatial resolution from mm-wave interferometry. Unfortunately there still are considerable uncertainties in the masses and mass distributions of disks. I will review the observational limits we now have on disk masses, the evidence for dust growth and settling in these disks, and the increasingly common indications of inner disk clearing at early evolutionary times (~ 1 Myr). I will then argue that the time-dependence of disk accretion in the protostellar phase strongly suggests that something like a dead zone or high-surface-density, low-viscosity, region exists in inner disks, at least initially, providing the conditions for more rapid formation of relatively massive bodies.
11:00 to 11:30 Coffee and posters INI 1
11:30 to 11:50 D Wilner ([Harvard Smithsonian Center])
Submm observations of protoplanetary disks
Observations over a wide wavelength range provide diagnostic information on protoplanetary disks, but the submillimeter regime is especially important because (1) optically thin dust emission probes particles through the entire disk, including the cold midplane, (2) these are the longest wavelengths where dust is readily detectable, and therefore the last direct link on the chain of sizes from sub-micron interstellar particles to planetesimals, (3) aligned dust particles can produce polarized emission that traces the magnetic field, and (4) spectral line emission from a variety of species show the detailed disk kinematics and constrain nebular chemistry. I will describe recent results from the Submillimeter Array that take advantage of several of these key features, with implications for disk structure, planet forming potential, and the physics of accretion. In particular, I will discuss a high resolution (0.3 arcsec = 40 AU) 870 micron survey of dust continuum emission from young disks in the Ophiuchus star-forming region, where we have used 2D radiative transfer calculations to fit simultaneously the resolved submillimeter data and the broadband spectral energy distributions with a parametric model in an effort to characterize the viscous properties and the likelihood of future (and perhaps even past) planet formation in these disks.
11:50 to 12:10 J Bouwman ([Max-Planck-Institute, Heidelberg])
Observational evidence for grain growth
The disks in Herbig Ae/Be and T Tauri systems are believed to be the birth sites of planetary systems. These disks are known to dissipate in about 10Myr, after which giant planet formation will be terminated. In this review I will discuss observational evidence for the onset of planet formation:the growth of sub-micron sized dust grains, typical for the ISM, into mm sized dust grains. To correctly interpret these observations, comparisons to experimental and theoretical studies elucidating the processes (growth, evaporation, condensation, crystallisation, and large scale mixing) acting on dust in protoplanetary disks, are required. This review will, therefore, extensively discuss the interplay between observations and experiments/theory.
12:10 to 12:40 Discussion (session chair: Jim Pringle)
12:40 to 13:30 Lunch at Wolfson Court and posters
14:00 to 14:40 C Dominik ([Amsterdam])
Review - models of protoplanetary disks
In my talk I will address a variety of modeling approaches to protoplanetary disks. I will discuss radiative transfer models that are used to derive spectral energy distributions and address the issues related to various geometrical structures in such disks and the pitfall of modeling these with codes not appropriate for complex geometries. I will discuss the latest models of the inner boundary of protoplanetary disks near the dust evaporation zone and show that a detailed treatment of the evaporation physics leads to interesting structure, size and chemical sorting, and possibly to instabilities as a source for observational variability, both in SED features and in interferometric observations. I will also discuss the latest suite of models covering dust settling and coagulation on a global scale in disks. Hydrodynamic and magnetohyrdodynamic models are beyond the scope of this review talk.
14:40 to 15:00 N Calvet ([Michigan])
Recent results on the interpretation of observations of protoplanetary disks
I will show recent results on the interpretation of SEDs and spectra of protoplanetary disks around low mass stars. I will talk about recent Spitzer/IRS observations that indicate that disks are very settled even in extremely young populations. I will then talk about Spitzer/IRAS observations showing that the inner disks get increasingly settled as the population ages in primordial disks. I will show UV observations of molecular H that indicate that the gas in the inner disk disappears when the stars stop accreting, even if some dust and probably gas is left in the outer disks. I will talk about the transitional and pre-transitional disks, that is, disks with inner clearing and gaps, and speculate that they are possible phases for the final clearing of the inner disks.
15:00 to 15:30 Tea and posters INI 1
15:30 to 15:50 G Lesur ([Cambridge])
Turbulent convection in accretion discs
Transport of angular momentum has always been a central problem of accretion disc theory. Since the discovery of the magnetorotational instability in accretion discs by Balbus and Hawley (1991), MRI-driven turbulence is believed to be the best candidate to explain anomalous transport in discs. Despite this result, several other routes to turbulence have been considered over the last two decades, with limited success. A possible alternative to MRI turbulence is turbulent convection, driven by an unstable vertical entropy gradient in the disc. Several studies have shown that convection was actually transporting angular momentum inward, and is therefore not favourable to accretion. In this presentation, I will revisit the problem of turbulent convection in accretion discs, using modern numerical methods. In particular, I will show that this hydrodynamic process could actually drive outward angular momentum transport if certain conditions are met, with an efficiency compatible with protoplanetary discs observations.
15:50 to 16:10 T Sano ([Osaka])
Dead zones in protoplanetary disks
MHD turbulence driven by the magnetoroational instability (MRI) is the most promising mechanism of angular momentum transport in accretion disks. However protoplanetary disks are dense and cold so that the ionization fraction is extremely low. It is known that there must be dead zones in protoplanetary disks where the growth of MRI is suppressed significantly due to non-ideal MHD effects. The size of dead zones are related to the characteristics of dust grains. The gas and dust evolutions, or planet formation, are affected by the existence of the dead zones. The roles of the dead zones in planet formation scenario are summarized in this talk.
16:10 to 16:30 C Gammie ([Illinois])
Self-gravitating disc evolution
I will briefly review recent advances in modeling self-gravitating disc evolution, as well as some unsolved problems, particularly the long-term interaction of density waves with other forms of angular momentum transport such as MHD turbulence.
16:30 to 17:00 C Clarke ([Cambridge])
The role of photoevaporation in disc dispersal
I first recapitulate former work explaining how the interplay between viscous evolution and extreme ultraviolet (EUV) photoevaporation produces a characteristic pattern of disc clearing, which invoves first rapid viscous draining of the inner disc (within a few A.U.) followed by rapid photoevaporation of the outer disc. This behaviour sets in at late times when the accretion rate through the disc is very low ($\sim 10^{10} M_\odot$ yr$^{-1}$). I then describe recent work which demonstrates that, contrary to previous estimates, Xray photoevaporation is in fact likely to be a major disc dispersal agent. The sequence of disc clearing phases is qualitatively similar to that described above but with two key differences: i) the photoevaporation rate is ten times higher and thus this clearing sets in earlier, when the disc accretion rate is $\sim 10^{-9} M_\odot$ yr$^{-1}$ and ii) a combination of the greater penetrating power of Xrays and the somewhat lower temperatures attained by Xray heated gas compared with the EUV case means that the peak wind mass loss occurs at $\sim 20 $ A.U.. The size of the inner hole is thus $\sim 4$ times larger than in EUV photoevaporative models. We discuss the implications of this new result for models of disc clearing and the production of transition discs.
17:00 to 17:30 Discussion (session chair: Steve Balbus)
17:30 to 18:30 Welcome Wine Reception INI 1
18:45 to 19:30 Dinner at Wolfson Court (Residents Only)
Tuesday 18th August 2009
09:00 to 09:40 G Wurm ([Münster])
Review - there and back again: the making and destruction of dusty planetesimals
A dust particle has to take a number of different roads to be incorporated into a planet. On the constructive side, collisions gentle enough might stick the particle to other particles and lead to growth of larger objects. Ultimately, beyond any detail, this is how planetesimals, km-size precursors to planets and terrestrial planets further on form. Collisions are therefore among the most fundamental processes in planet formation. In detail they have their share in shaping the size distribution and morphology of evolving bodies and to set formation time scales of larger objects in protoplanetary disks. Sticking, rebound, fragmentation, and reaccretion (the latter especially by gas drag) are important results of individual collisions. How far we can get along the size scale of growth from dust upward and which collisions / conditions would be needed to get to planetesimals will be covered in this talk. As the evolving bodies are not isolated in their collisions but embedded in a gaseous protoplanetary disk, other processes can support, prevent, or even undo the growth of large bodies. Eventually, these processes might be of similar importance as the collisional growth and put some dependence of the formation processes to the radial distance from the star within the disk. Planetesimal and planet formation does not necessarily proceed the same way all over the disk. I will suggest some thoughts on this.
09:40 to 10:00 A Johansen ([Leiden])
The crucial role of metallicity for planetesimal formation
The probability of finding exoplanets around a main sequence star rises sharply for metallicities around solar or higher. I present computer simulations of particle clumping and planetesimal formation in protoplanetary discs with varying amounts of solid material. The sedimentary mid-plane layer of pebbles is unstable to both Kelvin-Helmholtz and streaming instabilities. For metallicities below the solar value the equilibrium mid-plane layer is thick and displays no clumping. However, already at slightly super-solar metallicities, strong clumping occurs in the mid-plane layer. These particle clumps can locally obtain more than a hundred times the gas density. We interpret the onset of clumping as an effect of a curious trait of the streaming instability: the strength of the turbulence increases with a decreasing solids-to-gas mass ratio. Particles inside dense clumps have collision speeds of a few meters per second, and the clumps readily contract gravitationally into a number of interacting 100-km-size planetesimals. Our results show that the metallicity dependence of exoplanets may have been imprinted already in the early stages of planet formation, or during the dispersal of the gaseous part of the disc.
10:00 to 10:40 E Kokubo ([NAOJ])
Review - formation of terrestrial planets: the basic dynamical model
In the standard scenario for formation of planetary systems, a planetary system forms from a protoplanetary disk that consists of gas and dust. The formation scenario can be divided into three stages: (1) formation of planetesimals from dust, (2) formation of protoplanets from planetesimals, and (3) formation of planets from protoplanets. In stage (1), planetesimals form from dust through gravitational instability of a dust layer or coagulation of dust grains. Planetesimals are small building blocks of solid planets. Planetesimals grow by mutual ollisions to protoplanets or planetary embryos through runaway and oligarchic growth in stage (2). The final stage (3) depends on a type of planets. The final stage of terrestrial planet formation is giant impacts among protoplanets while sweeping residual planetesimals. In the present talk, I review the basic elementary processes of terrestrial planet formation, showing some recent simulations.
10:40 to 11:00 D McNeil ([QMUL])
Oligarchic growth and migration scenarios for short-period neptune and super-earth formation
The discovery of short-period Neptune-mass objects, now including the remarkable system HD69380 with three Neptune analogues, presents challenges to current formation models. Several formation scenarios have been proposed, where most combine the canonical oligarchic picture of core accretion with type I migration and planetary atmosphere physics (e.g. Terquem & Papaloizou 2007; Alibert et al. 2006). These consider only a very small number of progenitors at late times, raising questions about the earlier evolution. Using global N-body simulations, we ask whether the standard model of oligarchic core accretion with embryos experiencing type I migration can generate a population of hot Neptune systems. This problem is investigated using both traditional semianalytic methods for modelling oligarchic growth as well as a new code designed specifically for treating formation problems with large dynamic range (McNeil & Nelson 2009). We consider a wide range of plausible disc parameters, and find that it is difficult for oligarchic migration models to reproduce the observed distribution. By comparison, it is relatively straightforward to form short-period icy super-Earths. We conclude that either the conditions in discs which produce hot Neptunes differ significantly from those of our simple disc models, or we are missing important physics that modifies the migratory behaviour of forming planets.
11:00 to 11:30 Coffee and posters INI 1
11:30 to 12:10 Poster Presentations INI 1
12:10 to 12:30 C Agnor ([QMUL])
Giant impacts and planetary evolution
The planetesimal hypothesis posits that solid rocky or icy planets form via the accumulation of smaller bodies. In this picture, collisions between bodies are the mechanism by which planets acquire mass and a principal process of planetary evolution. Giant collisions between like-sized planets have been invoked to explain several bulk planetary characteristics (e.g. the origin of Earth's Moon and Mercury's large iron core). In this talk, I will discuss how these giant impacts arise in the context of planetary formation and our recent results to explicitly model these collisions. I will discuss the connections between the different stages and regimes of planetary growth, the giant impact outcomes expected, and the implications for the thermal, rotational and compositional evolution of emerging planets.
12:30 to 13:30 Lunch at Wolfson Court and posters
14:00 to 14:20 Z Leinhardt ([Cambridge])
The evolution of collision outcomes in the protoplanetary disk
Although hundreds of extrasolar planets have been detected, the earlier phases of planet formation are much more difficult to observe. As a result, theorists and numericists are still struggling to explain the planet formation process in detail. One of the fundamental problems in explaining the formation of our own solar system is reproducing the low eccentricity and inclination of the terrestrial planets. The dominant growth mechanism of planetesimals in the terrestrial region is collisions. However, the details of the collisions and the evolution of the post-collision remnants are not well understood. Although there has been a significant amount of work incorporating simple fragmentation models into numerical simulations of planet formation, thesesimulations have yet to produce the low eccentricities and inclinations of our own solar system. In this talk I will present numerical simulations that show how the criteria for catastrophically disrupting planetesimals can change by orders of magnitude as theimpact velocity and mechanical properties of the planetesimals are varied. These simulations suggest that the collisional response of planetesimals will change significantly as the protoplanetary disk evolves. The results presented here validate previous work (Benz, 2000), and expand upon their conclusions. The critical impact velocity required to begin collisional erosion of weak aggregate bodies is only a few metres per second. Therefore, the transition from the coagulation phase to collisional erosion for km-sized bodies begins much earlier during planet formation than usually considered. Thus, it seems likely that additional mechanisms (besides collisions) are needed for planetesimals to grow beyond km-sizes in the young protoplanetary disk. Our result that km-scale aggregates are particularly susceptible to disruption is supported by the observed deficit of small bodies in the outer solar system. With these results in mind we strongly suggest the use of a velocity dependent disruption law in N-body simulations of planet formation and evolution.
14:20 to 14:40 E Thommes ([Guelph])
From gas disks to gas giants
The ensemble of now well over 300 discovered planetary systems displays a wide range of masses, orbits and in multiple systems, dynamical interactions. These represent the endpoint of a complex sequence of events, wherein an entire protostellar disk converts itself into a small number of planetary bodies. Here we present self-consistent numerical simulations of this process, which produce results in agreement with some of the key trends observed in the properties of the exoplanets. Though the typical formation history of a planetary system is highly stochastic, there are nevertheless clear correlations between a system's birth disk and the characteristics of the mature planetary system which ultimately grows from it. Analogues to our own Solar System are naturally accounted for in this picture, as the products of disks just slightly above the giant-planet-forming threshold. However, such outcomes are in the minority, and a "typical" planetary system tends to have significant eccentricities and shorter-period orbits, akin to the discovered exoplanets.
14:40 to 15:00 H Genda (Tokyo Institute of Technology)
Formation of heavy-element rich giant planets
More than twenty extrasolar planets are known to transit their star. >From planetary radius observed by the transit method and planetary mass observed by the radial velocity method, one can determine the densities of the extrasolar planets. The density of the planet informs us about the planetary interior. According to calculation of the interior structure of gas giant planet by Guillot et al. (2006), the core mass and core mass ratio in the planets increases with the metallicity of their star ([Fe/H]). This dependency seems to be reasonable, because the star with higher [Fe/H] had the protoplanetary disk with enough solid materials to form more heavy element-rich planet. However, the simple formation theory of gas giant planets cannot fully explain this dependency. We have performed the smoothed particle hydrodynamic (SPH) simulations of collisions between two gas giant planets. Changes in masses of the ice/rock core and the H/He envelope due to the collisions are investigated. The main aim of this study is to constrain the origin and probability of a class of extrasolar hot Jupiters that have much larger cores and/or higher core/envelope mass ratios than those predicted by theories of accretion of gas giant planets. A typical example is HD 149026b. Theoretical models of the interior of HD 149026b (Sato et al. 2005; Fortney et al. 2006; Ikoma et al. 2006) predict that the planet contains a huge core of 50-80 Earth masses relative to the total mass of 110 Earth masses. Our SPH simulations demonstrate that such a gas giant is produced by a collision with an impact velocity of typically more than 2.5 times escape velocity and an impact angle of typically less than 10 degrees, which results in an enormous loss of the envelope gas and complete accretion of both cores.
15:00 to 15:30 Tea and posters INI 1
15:30 to 16:10 D Mordasini ([Max-Planck-Institute, Heidelberg])
Giant planet formation by core accretion
A review of the standard paradigm for giant planet formation, the core accretion theory is presented. First, an overview of the physical concepts used in this model is given, and results of classical core accretion models are shown. Then, various improvements and modifications to the original model are discussed. Such improvements are the inclusion of more realistic envelope opacities and solid accretion rates, of concurrent migration and disk evolution and of results of hydrodynamical simulations. The corresponding consequences are compared with the observational constraints given by the giant planets of our own solar system. Finally it is discussed how recently several groups have started to use core accretion models to understand the statistical properties of the quickly growing population of extrasolar giant planets.
16:10 to 16:50 A Boley ([Zürich])
Review - gravitational instabilities and the formation of gas giant planets
Gas giant planets can be formed directly through disk instability if the Toomre Q parameter is near unity and the local cooling time is comparable to or less than the local dynamical time. Whether these conditions can ever be realistically met during disk evolution has been the focus of heated debate. In this talk, I will review the constraints that analytical work and detailed radiation hydrodynamics simulations have placed on the disk instability model. I will outline where there is general agreement in the field and where controversy remains. In particular, I will summarize recent work by multiple authors that suggests that planet formation by disk instability can operate at large disk radii (~100 AU).
16:50 to 17:10 D Stamatellos ([Cardiff])
The thermodynamics of disc fragmentation and the properties of the objects produced
We suggest that stars like our Sun should sometimes form with massive discs, and we show, by means of radiative hydrodynamic simulations, that the outer parts of such discs are likely to fragment on a dynamical time-scale, forming low-mass stars, brown dwarfs, and planetary- mass objects. The disc thermodynamics play a critical role in this process. We will present the predictions of this model and we will compare these predictions with the observed properties of low-mass stars and brown dwarfs. In particular, we will show that the model of disc fragmentation can explain the binary properties of low-mass stars, the brown dwarf desert, and the existence of free-floating planetary mass objects. We will also discuss predictions of the model that can be tested by future observations.
17:10 to 17:50 Discussion (session chair: Lucio Mayer)
18:45 to 19:30 Dinner at Wolfson Court (Residents Only)
Wednesday 19th August 2009
09:40 to 10:00 S-J Paardekooper ([Cambridge])
Corotation torques and type I planetary migration
In the standard picture of planet migration, Type I migration is due to a linear response of the disc to the presence of a low-mass planet. This mode of migration is driven by torques generated at Lindblad resonances, can be alarmingly fast, and takes all planets up to a few times the mass of the Earth very close to the central star. Corotation torques, in the linear picture generated at corotation resonances, were thought to play only a minor role. Recent work has shown, however, that this simple linear model is not correct. Corotation torques are always non-linear, and can be much larger than the linear estimate, to the extent that they can even dominate over the Lindblad torques, especially when non-barotropic effects are considered. I will give an overview of the current state of affairs concerning the new picture that is emerging for Type I migration, and how this may affect planet formation in general.
10:00 to 10:20 A Crida ([Cambridge])
Migration in resonance
It is well known that a planet embedded in a protoplanetary gaseous disk migrates, generally towards the central star. If two planets are migrating in the same disk at different speeds, they may get caught in a Mean Motion Resonance. It has been shown for instance that Jupiter and Saturn in a same disk should most likely end in the 2:3 MMR. Other configurations are possible, in particular some planets could share the same orbit in 1:1 resonance. The resonance has several effects on the migration of the pair of planets. First, their eccentricities should increase. We have shown that the damping of the eccentricity of the inner planet by the inner disk can explain the eccentricities of observed systems. Second, the migration rate may be completely changed. If the outer planet is lighter than the inner one, and if the two planets in resonance lie inside a common gap, they may migrate outwards (Masset & Snellgrove, 2001). We have shown that this can proceed on the long run, towards up to ~100 AU in flared disks. This could explain the presence of the recently directly detected exo-planets, orbiting at several dozens of AU around HD8799 and Fomalhaut. In addition, under some conditions, the migration rate could be negligible over the life-time of the disk. This should apply to the outer solar system, in the frame of the Nice model (Morbidelli et al., 2007). Consequences of this idea on the Minimum Mass Solar Nebula will be presented.
10:20 to 10:40 F Adams ([Michigan])
Type I planetary migration with stochastic fluctuations
This talk presents a generalized treatment of Type I planetary migration in the presence of stochastic perturbations. In many planet-forming disks, the Type I migration mechanism, driven by asymmetric torques, acts on a short time scale and compromises planet formation. If the disk also supports MHD instabilities, however, the corresponding turbulent fluctuations produce additional stochastic torques that modify the steady inward migration scenario. This work studies the migration of planetary cores in the presence of stochastic fluctuations using complementary methods, including a Fokker-Planck approach and iterative maps. Stochastic torques have two main effects: [1] Through outward diffusion, a small fraction of the planetary cores can survive in the face of Type I inward migration. [2] For a given starting condition, the result of any particular realization of migration is uncertain, so that results must be described in terms of the distributions of outcomes. In addition to exploring different regimes of parameter space, this talk considers the effects of the outer disk boundary condition, varying initial conditions, and time-dependence of the torque parameters. For disks with finite radii, the fraction of surviving planets decreases exponentially with time. We find the survival fractions and decay rates for a range of disk models, and find the expected distribution of locations for surviving planets. For expected disk properties, the survival fraction lies in the range $0.01
10:40 to 11:00 J Burns ([Cornell])
The real thing: Saturn's ring
Of all dense astrophysical discs, only Saturn's rings can be studied in detail. Cassini observations reveal examples of many processes that are likely relevant in the dynamical evolution of debris discs, such as the interactions of the disc's particles with one another, with local masses and with more distant masses via resonances. Material accretion and breakup have been inferred elsewhere, and even non-gravitational forces are found to sculpt some regions. Resonances account for much of the rings's architecture that is understood. Lindblad resonances with Mimas (2:1) and the co-orbital moons (7:6) constrain the exterior perimeters of the A and B rings. Density and bending waves, initiated at resonances with satellites, are abundant in the outer A ring, where they transfer angular momentum between the satellites and the rings. These waves indicate disc's physical properties, which vary smoothly across this region. Gaps may also be opened by resonances with a lumpy planetary gravity field or with non-uniform rings. Structures in dust-laden rings are visible at Lindblad resonances with the planetary spin rate, likely driven by electromagnetic interactions. Satellites with radii ~ 15km and ~ 4km open the Encke and Keeler gaps, generating undulations along the gap edges that are remarkably persistent and surprisingly complex; the A and B peripheries are also complicated. It is unclear whether this morphology alters angular-momentum transfer. “Propellers”, believed to be disturbances generated by unseen embedded moonlets (tens to scores of meters), are concentrated in three bands in the mid-A ring. Some very large propellers (from >100-m objects) are found in the outermost A ring; one's orbit is noticed to evolve, perhaps exhibiting smooth Type-I migration or stochastically scattering off density clumps. Self-gravity wakes develop in the A ring, but the full agglomeration of moonlets is frustrated by Saturn's tides. These clumps form ephemeral elongated structures with height-to-width ratios of ~1x10; regions between wakes are fairly clear. Close-in moons have low densities (~0.5 g/cc) and nearly fill their Hill spheres. Even though the dense B ring is almost opaque (optical depth ƒ ~ 5), concentric holes are occasionally visible; in places, its ƒ jumps repeatedly between two values over radial spans of hundreds of km.
11:00 to 11:30 Coffee and posters INI 1
11:30 to 11:50 S Matsumura ([Northwestern])
Evolution of planetary systems emerging out of gas disks
Previous N-body simulations of multiple planetary systems without a gas disk have successfully reproduced the observed eccentricity distribution by assuming that the planetary systems are dynamically "active" when the gas disk dissipates. The planet-planet interactions alone, however, cannot explain the semi-major axis distribution. We numerically study the evolution of planetary systems as the gas disk dissipates by using a hybrid N-body and 1D gas disk code, and highlight disk's role in shaping the planetary systems.
11:50 to 12:30 Discussion (session chair: John Papaloizou)
12:30 to 13:30 Lunch at Wolfson Court and posters
20:00 to 23:00 Conference Dinner at Corpus Christi College (Dining Hall)
Thursday 20th August 2009
09:00 to 09:40 H Levison (Southwest Research Institute)
Modeling the formation of giant planet cores
One of the most challenging problems we face in our understanding of planet formation is how Jupiter and Saturn could have formed before the the solar nebula dispersed. The most popular model of giant planet formation is the so-called 'core accretion' model. In this model a large planetary embryo formed first, mainly by two-body accretion. This is then followed by a period of inflow of nebular gas directly onto the growing planet. The core accretion model has an Achilles heel, namely the very first step. We have undertaken the most comprehensive study of this process to date. In this study we numerically integrate the orbits of a number of planetary embryos embedded in a swarm of planetesimals. In these experiments we have included a large number of physical processes that might enhance accretion. In particular, we have included: 1) aerodynamic gas drag, 2) collisional damping between planetesimals, 3) enhanced embryo cross-sections due to their atmospheres, 4) planetesimal fragmentation, and 5) planetesimal driven migration. We find that the gravitational interaction between the embryos and the planetesimals lead to the wholesale redistribution of material - regions are cleared of material and gaps open near the embryos. Indeed, in 90% of our simulations without fragmentation, the region near that embryos is cleared of planetesimals before much growth can occur. The remaining 10%, however, the embryos undergo a burst of outward migration that significantly increases growth. On timescales of ~100,000 years, the outer embryo can migrate ~6 AU and grow to roughly 30 Earth-masses. We also find that the inclusion of planetesimal fragmentation tends to inhibit growth.
09:40 to 10:00 P Armitage ([Colorado])
Planetary system architecture from planet-planet and planetesimal scattering
Observational evidence points to a dominant role of small-body scattering in early outer Solar System dynamics, while planet-planet scattering is a plausible explanation for extrasolar planet eccentricity. Both processes are likely to operate during the early evolution of systems of low-mass giant planets at moderate orbital radii (of the order of 10 AU), potentially leading to architectures distinct from either the Solar System or currently known extrasolar planetary systems. I will present results from a very large set of N-body simulations of marginally stable multiple planet systems surrounded by planetesimal disks. The simulations suggest that a surprisingly sharp transition from typically eccentric to typically circular orbits ought to be observed as surveys detect lower mass planets at larger orbital radii, and that resonant configurations may dominate among more massive planets whose dynamics was affected by planetesimal disks.
10:00 to 10:20 E Ford ([Florida])
Planet scattering, eccentricity excitation and the long-term evolution of planetary systems
The discovery of extrasolar planets on eccentric orbits has motivated theoretical investigations of numerous mechanisms for exciting the eccentricities of giant planets during the planet formation process and subsequent orbital evolution. The orbital properties of the growing number of extrasolar planetary systems and multiple planet systems are beginning to provide clues that constrain planet formation models. I will discuss the implications of exoplanet observations for eccentricity evolution from planet-disk interactions in young planetary systems to the chaotic evolution of multiple planet systems. I will discuss the potential role of planet scattering in the formation of planets in wide orbits, such as those recently found around A stars. I will also discuss the implications of planet scattering and secular interactions for the orbits of Neptune or super-Earth-mass planets. I will conclude will speculations as to why our own solar system has settled to a state with nearly circular orbits.
10:20 to 10:40 Y Wu ([Toronto])
Secular instability: organization of planetary systems and the origin of hot Jupiters
In a planetary system where planets are well spaced and interact only through their secular (i.e., non-resonant) perturbations, there exists a nonlinear instability that leads to chaotic behaviour. Some planets (the most unstable ones) may gradually obtain very large values of eccentricities and/or inclinations. We elucidate the condition under which this occurs. When a most unstable planet is removed from the system, either through close encounter with another planet, or through close encounter with the star (where tidal effects might kick in), the remaining planetary system becomes increasingly more stable. This, as opposed to planet-planet scattering, may explain the stable architecture of the observed systems. In the case of the most unstable planet moving in toward close encounter with the star, its pericentre dipping may be tidally stalled around a few stellar radii. When these orbits are tidally circularized, we obtain hot jupiters (or hot neptunes). This scenario for hot jupiter formation explains a variety of observed phenomena, including correlations between planet mass and semi-major axis, coplanarity of planet orbit and stellar spin, as well as the paucity of second planets around hot jupiters.
10:40 to 11:00 M Davies ([Lund])
Turning solar systems into extrasolar planetary systems
Many stars are formed in some form of cluster or association. These environments can have a much higher number density of stars than the field of the galaxy. Such crowded places are hostile environments: a large fraction of initially single stars will undergo close encounters with other stars or exchange into binaries. We describe how such close encounters and exchange encounters will affect the properties of a planetary system around a single star. We define singletons as single stars which have never suffered close encounters with other stars or spent time within a binary system. It may be that planetary systems similar to our own solar system can only survive around singletons. Close encounters or the presence of a stellar companion will perturb the planetary system, leading to strong planet-planet interactions, often leaving planets on tighter and more eccentric orbits. Thus, planetary systems which initially resembled our own solar system may later more closely resemble the observed extrasolar planetary systems.
11:00 to 11:30 Coffee and posters INI 1
11:30 to 12:00 Discussion (session chair: Martin Duncan)
12:00 to 12:40 Poster presentations
12:40 to 13:30 Lunch at Wolfson Court and posters
14:00 to 14:30 J Patience ([Exeter])
Direct imaging of planets
Direct imaging searches for exoplanets are sensitive to wide orbit planets and provide complementary information on the exoplanet population compared to indirect detection techniques. Recent high-contrast imaging has revealed a multiple planet system orbiting the young, dusty A-star HR 8799. The three planets are located projected separations of 24-68 AU and have masses estimated to be 7-10 times the mass of Jupiter. The discovery of massive planets at the orbital radii around HR 8799 system and other recently imaged exoplanets present important test cases for models of planet formation and evolution. Ongoing imaging surveys and upcoming instrument projects will further characterize the population of outer giant planets.
14:30 to 15:00 D Fischer ([Yale])
The search for planets around alpha centauri A and B
We are carrying out an intensive Doppler search for low mass planets in the alpha Centauri system at the 1.5-m CTIO telescope. The binary star system presents a challenging environment for the accretion of planetesimals into protoplanets. However, dynamical simulations show that if they form, terrestrial mass planets within about 2 AU of either star could survive in stable orbits. We present the motivation for our exoplanet search and describe the auxiliary science: the power spectrum of p-mode oscillations, a study of the convective zone depth in A and B, and a measurement of the helium abundances in the stars. This program will also help us to determine the fundamental limits of Doppler planet searches. We will present the status of the project and our Doppler analysis.
15:00 to 15:30 Tea and posters INI 1
15:30 to 15:50 M Fridlund (ESA/ESTeC)
The CoRoT mission - first results, successes and the future
The CoRoT mission was launched into space on the 27:th of December of 2006 and have carried out scientific measurements since mid-February of 2007. Over 45 000 light curves with lengths of between 22 and 155 days have been obtained with a dutycycle of over 95% have been obtained and a large number are already to be found on the public server of the CoRoT mission. So far 12 planets have been discovered and further studied (of which for 7 the first results have been published). Several of these objects have very interesting characteristics. The most spectacular find is designated 7b and is the first proven terrestrial type ('rocky') planet found outside the solar system. This is because we have measured both a very precise radius as well as a well determined mass, proving that its average density is similar to the 'rocky'worlds found in our own system. As such this planet form the beginning of the study of worlds like our own in the Galaxy, as well as being the kind of object that the CoRoT mission was designed to find. Nevertheless, literally hundreds of exo-planetary candidates have been found and new ones are being added to the list for follow-up observations on almost a daily basis. The talk will discuss the confirmed planets, CoRoT-7b in particular and give a hint of things to come
15:50 to 16:10 K Horne ([St Andrews])
From hot Jupiters to cool earths
I will summarise ground-based programmes using small wide-angle camera systems (e.g. SuperWASP) to discover and characterise transiting Hot Jupiters, and complementary surveys to discover and characterise cool planets down to the mass of the Earth by intensive follow-up of Galactic Bulge gravitational microlens lightcurves.
16:10 to 16:30 J Cho ([QMUL])
Spitzer observations of hot Jupiters and their interpretation
A brief review of the existing Spitzer, and other, data on hot Jupiters is given. Some interpretations and implications for theory are presented.
16:30 to 17:10 Discussion (session chair: Tristan Guillot)
18:45 to 19:30 Dinner at Wolfson Court (Residents Only)
Friday 21st August 2009
09:00 to 09:40 M Meyer (ETH Zürich)
To see a world in a grain of sand: observations of debris disks as tests of planet formation theory
We will review recent observations of debris disks with a focus on what they can reveal about the formation and evolution of planetary systems. In this presentation, we define a debris disk as one where the opacity we see is dominated by dust produced in collisions of planetesimals. We will concentrate on observed properties of disks as a function of wavelength (as a proxy for orbital radius) and compare results as a function of stellar mass when possible. We will start by summarizing the observational evidence for the appearance of dust debris and final gas disk dispersal. We then consider the observational signatures of terrestrial planet formation and giant impacts. We will briefly comment on specific physical properties inferred for debris disks based on application of simple models. Finally, we will review the observational connection (or lack thereof) between known exoplanets and debris disks. If most planetary systems are dynamically full, then it may be those systems lacking signatures of debris that represent the richest planetary architectures. Related Links •http://www.exp-astro.phys.ethz.ch/meyer/ - Star and Planet Formation Research Group, Institute for Astronomy, ETH •http://feps.as.arizona.edu/ - Spitzer Legacy Science Program FEPS
09:40 to 10:00 G Rieke ([Arizona])
Spitzer tracers of dynamical activity in debris discs
I will review the patterns of dynamical activity in debris disks as they are revealed by observations with the Spitzer Telescope. The Spitzer data show an overall decline in such activity among all stellar types, but recent detailed analyses reveal significant differences with stellar type. They also suggest that the disks around earlier-type host stars may have a general pattern of relatively complex structure, whereas those around solar-type stars may have simpler structures. On top of these overall patterns, there are a number of classes of rare but extreme systems: 1.) an absence of disks around young stars with very high stellar winds; 2.) huge excesses around stars in the 30 – 120 Myr age range; and 3.) large outflows in A stars. Each of these classes represents interesting phases in the overall evolution of the planetary systems generating the disks.
10:00 to 10:40 M Wyatt ([Cambridge])
Collisional and dynamical evolution of debris discs
The observed properties of debris discs are seen to change with stellar age in a manner indicative of the discs' evolution. The majority of the observed trends agree with what would be expected from the steady state collisional evolution of planetesimal belts formed in the protoplanetary disc. However, there remains uncertainty as to how debris discs are stirred, and about the role of stochastic processes (due to either massive collisions or planetary system instability) in their evolution. More generally the study of debris discs offers a unique opportunity to probe how planetesimals and planetary systems form and evolve. This talk will review our current understanding of the theory of debris disc evolution, due to both collisional processes within the disc, and due to the dynamics of its interaction with a (typically unseen) planetary system.
10:40 to 11:00 A Moro-Martin ([Madrid])
Exchange of debris between planetary systems
The exchange of meteorites among the terrestrial planets of our Solar System is a well established phenomenon. Similarly, could solid material be transferred between planetary systems? We examine a dynamical process that yields very low escape velocities using nearly parabolic trajectories, and the reverse process that allows for low velocity capture. These processes are chaotic and provide a mechanism for minimal energy transfer that yield an increased transfer probability compared to that of previously studied mechanisms that have invoked hyperbolic trajectories. However, they require a small relative velocities and would therefore be applicable when the stars are still embedded in their maternal cluster. We estimate the transfer probability in a stellar cluster as a function of stellar mass and cluster size. We find that significant amounts of solid material could potentially have been transferred from the early Solar System to our nearest neighbor stars in the cluster. Regarding the exchange of km-size debris today, given the high relative velocities between the Sun and its neighbors, any incoming extra-solar debris would be identified as a hyperbolic comet. No hyperbolic comet has been observed so far, but future surveys with Pan-STARRS and LSST will provide wide coverage maps of the sky to a very high sensitivity ideal to detect moving objects. In anticipation of these observations, we estimate the number of extrasolar comets that might be detected taking into account recent results on the frequency of planetesimal and planet formation (from debris disks and planet surveys), the amount of solid material that might be available to form planetesimals (from protoplanetary disks studies), and the size distribution of planetesimals (from the study of the small body population in the Solar System).
11:00 to 11:30 Coffee and posters INI 1
11:30 to 11:50 J-C Augereau ([Joseph Fourier Grenoble])
Observations of exozodiacal disks
The zodiacal cloud has long been suspected to have extrasolar analogs, exozodiacal, debris clouds that remained elusive until very recently. Over the last decade, the presence of exozodiacal dust in the habitable zone around nearby stars, has essentially been discussed as a potential noise source that may compromise the ability of future exo-Earth finding missions to reach their goals. Our pioneering detection of exozodiacal dust around Vega in 2006 by near-IR interferometry shows that exozodis are by themself very interesting astrophysical objects. During this talk, I will review the current observations of exozodiacal dust disks around nearby main sequence stars, and show that, as a rule of thumb, the detected exozodiacal dust disks differ from the zodiacal cloud. I will then discuss possible dynamical scenarios that may give rise to an abundant production of exozodiacal dust. I will show that a promising scenario involves the outward migration of a planet destabilizing a planetesimal belt similar to the Kuiper belt, and responsible for a cometary bombardment.
11:50 to 12:30 S Lubow ([Space Telescope Science Institute])
Review of disc-planet interactions
The interactions of a young planet with its surrounding gaseous disc are likely responsible for the properties of many of the observed extra-solar planets. The masses of these planets are determined by tidal truncation or gas dispersal. Their orbital radii are determined in part by planet migration. Disc-planet interactions may play a role in exciting the orbital eccentricities of planets. A major uncertainty in evaluating these interactions is the physical state of the disc about which we have little direct observational evidence. The nature of disc turbulence and structure of the disc can have an important influence on the outcome of the interactions. Understanding how these interactions affect the planet formation process is a major challenge.
12:30 to 13:30 Lunch at Wolfson Court and posters
14:00 to 15:00 D Lin ([UCSC/KIAA])
Conference summary
15:00 to 15:30 Tea INI 1
18:45 to 19:30 Dinner at Wolfson Court (Residents Only)
University of Cambridge Research Councils UK
    Clay Mathematics Institute The Leverhulme Trust London Mathematical Society Microsoft Research NM Rothschild and Sons