# Workshop Programme

## for period 17 - 21 August 2009

### Dynamics of Discs and Planets

17 - 21 August 2009

Timetable

 Monday 17 August 08:30-09:50 Registration 09:50-10:00 Welcome - Ben Mestel Session: 1 - Observations of Protoplanetary Discs 10:00-11:00 Hartmann, L (Michigan) Review - protostellar disk observations CMS MR2 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. 11:30-11:50 Wilner, D (Harvard Smithsonian Center) Submm observations of protoplanetary disks CMS MR2 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. 11:50-12:10 Bouwman, J (Max-Planck-Institute, Heidelberg) Observational evidence for grain growth CMS MR2 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. 12:10-12:40 Discussion (session chair: Jim Pringle) 11:00-11:30 Coffee and posters 12:40-13:30 Lunch at Wolfson Court and posters Session: 2 - Protoplanetary Disc Modelling 14:00-14:40 Dominik, C (Amsterdam) Review - models of protoplanetary disks CMS MR2 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. 14:40-15:00 Calvet, N (Michigan) Recent results on the interpretation of observations of protoplanetary disks CMS MR2 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. 15:30-15:50 Lesur, G (Cambridge) Turbulent convection in accretion discs CMS MR2 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. 15:50-16:10 Sano, T (Osaka) Dead zones in protoplanetary disks CMS MR2 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. 16:10-16:30 Gammie, C (Illinois) Self-gravitating disc evolution CMS MR2 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. 16:30-17:00 Clarke, C (Cambridge) The role of photoevaporation in disc dispersal CMS MR2 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. 17:00-17:30 Discussion (session chair: Steve Balbus) 15:00-15:30 Tea and posters 17:30-18:30 Welcome Wine Reception 18:45-19:30 Dinner at Wolfson Court (Residents Only)
 Wednesday 19 August Session: 5 - Disc-planet Interactions and Migration 09:40-10:00 Paardekooper, S-J (Cambridge) Corotation torques and type I planetary migration CMS MR2 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. 10:00-10:20 Crida, A (Cambridge) Migration in resonance CMS MR2 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. 10:20-10:40 Adams, F (Michigan) Type I planetary migration with stochastic fluctuations CMS MR2 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 $0.01 < p_S < 0.1$. 10:40-11:00 Burns, J (Cornell) The real thing: Saturn's ring CMS MR2 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. 11:30-11:50 Matsumura, S (Northwestern) Evolution of planetary systems emerging out of gas disks CMS MR2 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. 11:50-12:30 Discussion (session chair: John Papaloizou) 11:00-11:30 Coffee and posters 12:30-13:30 Lunch at Wolfson Court and posters 20:00-23:00 Conference Dinner at Corpus Christi College (Dining Hall)