Abstracts

Pawel Artymowicz

University of Toronto

"Strange theories"

Unconventional theories play an important role in pushing the boundaries of our knowledge in astrophysics. The range such theories in which Doug Lin was involved far exceeds the scope of my talk. I will therefore sketch just a few of them, where Doug either enlisted my help or inspired me to create my own crazy theories. Quasar accretion disks may rejuvinate normal dwarf stars as the most massive stars, contributing to their surprisingly high metallicities. A variant of this strange theory now provides an insight into the even stranger, observed, world of the central parsec of our own galaxy. Type III migration of bodies gave rise to controversy but turns out to be no less strange than type II migration. Finally, central forces of radiation and gravity may conspire to break the axisymmetry of disks we thought are stable, in a process somewhat akin to the gravitational instability.

Clément Baruteau

University of California, Santa Cruz

Protoplanetary migration in turbulent, isothermal disks

A planet embedded in a protoplanetary disk experiences a torque that generally leads to
its orbital decay. This process, known as migration, plays a major role in the evolution of planetary systems. To reproduce the statistical properties of the observed exoplanets, population synthesis models have shown that the migration of low-mass planets should be significantly slowed down, and that processes stalling migration should be at work. Much current efforts have thus been dedicated to slow down, halt or even reverse migration. Most of these studies rely on the corotation torque, whose long term evolution is intimately related to the disk viscosity. I will show here how these studies are modified by a more realistic treatment of the disk turbulence.

Chas Beichman

Caltech

"Youth is hot and bold, age is weak and cold:" searching for hot young planets

As Shakespeare might have said about Doug Lin, I am going to discuss a search for “Hot and Bold Planets.” Despite the spectacular increase in our knowledge of planets from a variety of techniques, we still know very little about young planets. In particular, we would like to know where and how gas or icy giants form and how their properties evolve with time. I will discuss recent results in ground based imaging as well as prospects for space-based imaging and astrometry to discover and characterize young planets.

Peter Bodenheimer

University of California, Santa Cruz

Protoplanetary accretion disk models and the formation of giant planets

The general properties of the known extrasolar giant planets has led to renewed interest on the question of their formation mechanism. Do they form by core accretion, or gravitational instability, or some by each mechanism? Already in 1980 Doug Lin argued, on the basis of early protoplanetary nebula models, that they form by core accretion, and he has maintained this position consistently ever since. Does this conclusion still hold up in view of recent observations? This contribution will look at the historical development of models of the solar nebula, and the connection with core accretion models of planet formation, and the role Doug played in making progress on these key topics. To answer the formation question observationally will require substantial improvement over current capabilities.

Isabelle Boisse, Francois Bouchy, Guillaume Hebrard, Xavier Bonfils

Institut d'Astrophysique de Paris, Observatoire de Haute-Provence, Laboratoire d'Astrophysique de Grenoble

Stellar limitation in the search and characterization of exoplanets

Most active stars have been rejected from major radial-velocity surveys due to the noise induced by photospheric luminosity variations. Now, they cannot continue to avoid these targets because instruments like CoRoT that need radial-velocity follow-up found planets around active stars. We monitored two active planetary host stars: HD189733 with the SOPHIE spectrograph and IotaHor with HARPS. We used all the parameters available from high-precision spectroscopic measurements to provide a means of disentangling RV variations due to Doppler motion from the noise induced by stellar activity.

Geoff Bryden

JPL/Caltech

Constraints on planet formation from debris disk observations

The Spitzer Space Telescope has greatly expanded the number of nearby solar-type stars observed with debris disks. Combined with advances in planet detection, there are now 15 systems known to have both orbiting planets and dust. I will discuss the properties of these systems and look for any connections between the planets and disks.

Vincenzo Costa (1), Valerio Pirronello (1), Gaetano Belvedere (1), Antonino Del Popolo (1), Giuseppe Lanzafame (2)

(1) Università di Catania, (2) INAF - Osservatorio Astrofisico di Catania

2D SPH simulations of sub-Keplerian accretion discs with two embedded protoplanets: effects of mutual interactions

The mainly proposed models for the formation of planetary systems require migration of protoplanets in an accretion disc of a forming star. Attention is focused here on the mutual interactions between two protoplanets, both embedded in the accretion disc, as a function of the protoplanets masses, their relative positions, the dynamic properties of the accretion disc particles. The study is performed through a 2D SPH code and preliminary results show an oscillation of the distance between the two protoplanets, together with a slow migration of the two planets towards the central star when two Jupiter-like planets are considered. Less correlated behaviour is observed when at least one of the two protoplanets has an Earth-like mass. The role played by the disc particles initial angular momentum is discussed, in sub-Keplerian conditions is discussed.

Vincenzo Costa (1), Valerio Pirronello (1), Gaetano Belvedere (1), Antonino Del Popolo (1) and Giuseppe Lanzafame (2)

(1) Università di Catania, (2) INAF - Osservatorio Astrofisico di Catania

3D SH simulations of a protoplanetary accretion disc: orbital evolution of an asteroid type body and a protoplanet

Interactions of protoplanetary objects with an accretion disc of a forming star are believed to be crucial for the birth of planetary systems. Attention is here focused on the evolution of the orbit of an asteroid type body with an initially tilted orbit, under the influence of the gravitational interaction with the accretion disc and with a more massive protoplanet (both Jupiter-like and Earth-like protoplanets are considered). A SPH based three-dimensional model is used to analyse this physical scenario. Both “internal fragment – external protoplanet” and “external fragment – internal protoplanet” configurations are considered.

Avishai Dekel

The Hebrew University of Jerusalem

What I learned from Doug on galaxy formation: cold streams, clumpy disks and compact spheroids

Observations reveal that most of the stars formed in massive galactic disks at high redshifts. These disks have many common features with other astrophysical disks, such as planetary and accretion disks. A simple theoretical analysis reveals that their evolution is governed by the interplay between smooth and clumpy cold cosmological streams, disk instability and bulge formation. Intense, relatively smooth streams maintain an unstable dense gas-rich disk. Instability with high turbulence and giant clumps is self-regulated by gravitational clump interactions. Clump migration into a bulge is induced by encounters and dynamical friction in a couple of orbital periods. The cosmological streams replenish the draining disk and prolong the clumpy phase to several Gyrs in a steady state, with comparable masses of disk, bulge and dark matter within the disk radius. The overall SFR in the clumps follows the accretion rate, ~100 M_solar/yr. While the clumps coalesce dissipatively into a compact bulge, the disk appears extended because of the associated angular momentum transport outward, and because the streams feeding the outer disk preferentially induce instability there. Passive spheroid-dominated galaxies form when the streams are more clumpy. The external clumps merge into a massive bulge and stir up disk turbulence, which stabilize the disk and suppress in-situ clump and star formation. This scenario predicts a bimodality in galaxy type by z~3, with giant-clump star-forming disks alongside with spheroids of suppressed SFR. High-resolution cosmological hydro simulations reveal clumpy disks consistent with our analysis.

Ian Dobbs-Dixon

McGill University

Close-in gas giant planets are now familiar members of the growing family of extra-solar planets. Their short period orbits and proclivity for transiting has made them the target of numerous observational campaigns, and our knowledge of their structure and composition has increased dramatically over the past few years. However, despite their prevalence and important role in constraining a wide range of planetary models, fundamental questions about the dynamical behavior of their atmospheres remain, crucial for interpreting observations. I will discuss three-dimensional radiative hydrodynamical simulations of atmospheric flows on a wide variety of such objects, ranging from the well-known HD209458b to the more exotic rapidly rotating or highly eccentric objects. Such objects exhibit a range of unusual behavior including supersonic winds, shocks and instabilities, and time dependent behavior. I will review the results from models we have developed to study these processes with the goal of both explaining individual objects and the observed diversity among this class of planets.

Sandra M. Faber

University of California, Santa Cruz

From Darkness into Light: Illuminating the Role of Dark Matter in Galaxy Formation

In the heady days of the early 1980's, when we were just beginning to identify the core concepts of galaxy formation, Doug Lin and I achieved a first. Together we made the first identification of
a galaxy (the Ursa Minor dwarf spheroidal) that was NEARLY ALL dark matter. We interpreted it as a severe case of "arrested development" in which gas was either swept out, driven out, or
never accreted. A quarter century later, we now see that Ursa Minor and its twins are at one extreme end of a long continuum that demonstrates the key role that dark matter plays in galaxy evolution. I will argue that this continuum is primarily a mass sequence set by the mass of the dark halo. To first order, the visible properties of galaxies are set by the properties of the invisible halos that we cannot see.

Paulina Assmann & Michael Fellhauer

Departamento de Astronomia, Universidad de Concepcion, Chile

The Formation of Dwarf Spheroidal (dSph) Galaxies

In this work we study numerically the evolution of star clusters. Intense star formation bursts in gas-rich galaxies typically produce a few to a few hundred young massive star clusters, within a region of just a few hundred pc. The dynamical evolution of these star clusters may explain the
formation of dwarf galaxies, faint fuzzy star clusters or off-center nuclei of dwarf galaxies. Here we perform a numerical experiment to show that the evolution of star cluster complexes in dark matter haloes can explain the formation of the luminous components of dSph galaxies. The dwarf galaxy population of the Milky Way, consisting mainly of dSph galaxies, provides a unique template for the investigation of the process of galaxy formation and evolution on its smallest scales and also tests the nature of dark matter. The proximity of these dwarfs makes it possible to study their dynamical state at a level of detail unprecedented by any external system. The questions addressed with this project are: How do dwarf galaxies form out of their smallest star forming constituents? How is the dark matter distributed within dwarf galaxies and are all dwarfs dark matter dominated?

Debra Fischer

San Francisco State University

The planet-stellar metallicity connection

The correlation between formation of giant planets and stellar metallicity was predicted by Doug Lin as "the Last of the Mohicans" fell into their host stars and polluted the stellar atmospheres with refractory elements. Inspired by this prediction, Valenti & Fischer carried out spectral synthesis modeling for more than 800 stars with uniform velocity sets in 2005. We now present an update of the Fischer & Valenti (2005) planet-metallicity correlation, incorporating an additional 5 years of radial velocity measurements and an extended sample of stars to examine the stellar metallicity for hosts of hot Jupiters and longer period planets.

Hideaki Fujiwara (1), Takuya Yamashita (2), Daisuke Ishihara (3), Takashi Onaka (1), Hirokazu Kataza (3), Takafumi Ootsubo (3), Misato Fukagawa (4), Jonathan P. Marshall (5), Hiroshi Murakami (3), Takao Nakagawa (3), Takanori Hirao (3), Keigo Enya (3) and Glenn J. White (5,6)

(1) University of Tokyo, (2) National Astronomical Observatory of Japan, (3) Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, (4) Osaka University, (5) The Open University, (6) Space Science & Technology Department, The Rutherford Appleton Laboratory

Hot debris dust around HD 106797

Photometry of the A0 V main-sequence star HD 106797 with AKARI and Gemini/T-ReCS is used to detect excess emission over the expected stellar photospheric emission between 10 and 20 micron, which is best attributed to hot circumstellar debris dust surrounding the star. The temperature of the debris dust is derived as Td ~ 190 K by assuming that the excess emission is approximated by a single temperature blackbody. The derived temperature suggests that the inner radius of the debris disk is ~ 14 AU. The fractional luminosity of the debris disk is 1000 times brighter than that of our own zodiacal cloud. The existence of such a large amount of hot dust around HD 106797 cannot be accounted for by a simple model of the steady state evolution of a debris disk due to collisions, and it is likely that transient events play a significant role. Our data also show a narrow spectral feature between 11 and 12 micron attributable to crystalline silicates, suggesting that dust heating has occurred during the formation and evolution of the debris disk of HD 106797.

P. Garaud, A. Traxler, S. Stellmach

University of California, Santa Cruz

Whatever happened to the other Mohicans

Most theories of planetary migration through early proto-stellar disks imply that the vast majority of proto-planets and planets fall into the central star, eventually leaving behind as the disk clears a few stragglers. Among those, tidal interactions between the star and the planet, and/or dynamical rearrangements between the planets are likely to lead to late infall onto the star. A significant but highly stochastic fraction of high-metallicity material is deposited on the stellar surface, while the finally remaining planets are the very "Last of the Mohicans".

Late pollution has been proposed as one of two possible explanation to the observed trend between stellar metallicity and planet-bearing probability. However, it is difficult to reconcile this idea with the *lack* of trend between stellar metallicity and stellar type. Indeed, a planetary body infalling into a star with deep convection zone leads to a much lower apparent increase in metallicity than the same body falling into a star with a shallow convection zone. On a similar argument, the incredibly small metallicity dispersion seen in stars among the same cluster puts a strict upper limit on the allowable late infall.

Or does it? As proposed by Vauclair, double-diffusive instabilities cause an increase in the turbulent diffusion of metals into stably stratified regions. This additional mixing could possibly reconcile the late infall theories with observations. Unfortunately, all currently available theories and experimental results on double-diffusion are only applicable to the high-Prandtl number regime of the Earth's oceans. Here, we present the first numerical simulations and theories of double diffusion which quantify this phenomenon for the low-Prandtl number of stellar interiors, and discuss how they can apply to constrain models of planet formation.

Douglas Gough

IoA and DAMTP, University of Cambridge

Asteroseismology after planet consumption

It is presumed that if a star consumes its planets it is enriched somewhat with heavy elements. Could such a process be detectable? Others at this conference will address this matter from classical astronomical perspectives. I have been charged to consider whether direct evidence for surface enrichment is likely to be found asteroseismologically. Accreted material must surely be mixed rapidly at least through the outermost convective portion of the stellar envelope. If the star is massive and near the main sequence, that zone contains only little mass, so the contamination might be substantial. In that case there is the chance of a detectable change in the depth of the convection zone, which could perhaps be inferred if only one could be sure of what the depth would have been had the star not been contaminated. More interestingly, one might think, is that if the contaminating material were confined strictly to the convection zone there would also be a direct seismic signature: a consequent small discontinuity in mean molecular mass at the base of the zone could in principle be detected via an oscillatory contribution to the spacing in the power spectrum of low-degree acoustic oscillations. But is material confinement plausible? Fingering is bound to occur into the radiative interior, smoothing the putative discontinuity and thereby reducing the amplitude of the seismic signature. On what timescale does that occur? Surely any dynamically driven macroscopic fluid motion must occur on a timescale brief compared with stellar evolution timescales. It behoves us to address whether that statement is actually correct? Direct measurements of timescales on Earth are difficult to extrapolate to stellar conditions because the mixing process is not well understood, despite its apparent importance in ocean dynamics; and by the same token the reliability of astrophysical recipes must surely be questioned in the absence of experimental or observational verification. How do we proceed?

Pin-Gao Gu (1), Takeru K. Suzuki (2), Evgenya Shkolnik (3)

(1) ASIAA, (2) Univ. of Tokyo, (3) Carnegie Institute

Planet-Induced Radiation from a Solar-like Atmosphere

We investigate the thermal response of the atmosphere of a solar-type star to an electron beam injected from a hot Jupiter by performing a 1-D MHD simulation with non-linear wave dissipation, radiative cooling, and thermal conduction. In our work, the stellar atmosphere is non-rotating and is modelled as a 1-D open flux tube expanding super-radially from the stellar photosphere in the coronal hole to the planet. An electron beam is assumed to be generated from the reconnection site of the planet's magnetosphere. The effects of the electron beam are then implemented in our simulation as dissipation of the beam momentum and energy at the base of the corona where the Coulomb collisions become effective. When the average stellar field at the photosphere is about a few to 10 Gauss, a warm region forms in the chromosphere driven by the beam dissipation. The planet-induced radiation is estimated and compared to the observed value.

Javiera Guedes, Eugenio Rivera, Gregory Laughlin, Debra Fischer

UCSC, UCSC, UCSC, SFSU

The Earths of Alpha Centauri B

Alpha Centauri B belongs to a triple star system comprised of a central A/B binary and a third companion, Proxima Centauri. The system, our closest stellar neighbor, is of great interest for planet hunters because it may harbor Earth-mass planets possibly in the habitable zone of the stars of the A/B binary. To assess whether terrestrial planets can indeed form around these stars, we run a series of N-body accretionary simulations of a 1/r disk populated with 400-900 lunar-mass oligarchs is followed for 200 Myr. Most simulations lead to the formation of multiple-planet systems with at least one planet in the 1-2 M_Earth mass range at 0.5-1.5 AU. We examine the detectability of our simulated planetary systems by generating synthetic radial velocity observations including noise. Using these synthetic observations, we find that we can reliably detect a 1.8 M_Earth planet in the habitable zone of Alpha Centauri B after only three years of high cadence observations. The observations will be carried out at the CTIO site in northern Chile on a 1.5-m underutilized telescope. We expect to report the detection (or non-detection) of Earth-like planets around Alpha Centauri B in 3 to 5 years.

Tristan Guillot

Observatoire de la Cote d'Azur

Evolution and compositions of extrasolar planets

With a steadily growing number of known transiting planets and the realisation that no less than 10,000,000 planets transit stars in our Galaxy alone, a lot is to be learned from the present discoveries and much more is to be expected for the future. The vast majority of transiting planets known today are gaseous giant planets. For them, a proper knowledge of their evolution is crucial because how they contract directly affects what we can infer on their composition. However, uncertainties abound: on the equations of state to be used, on the opacities in little known pressure-temperature regimes, and on physical processes themselves, in particular heat dissipation due to stellar tides. Taken individually, these uncertainties generally prevent from infering the planet's global composition. For example, some planets are found to be larger than possible for a hydrogen-helium planet of that age, mass and irradiation level when calculated with standard evolution models. However, by using the same hypotheses for all planets, it is then possible to infer model-dependent global planetary compositions and relate it to other observables. It is thus found that the "metallicity" of stars and that planets are correlated, giant planets orbiting close to very metal-rich stars being found to possess up to ~100 Earth masses in heavy elements. I will discuss how further insight can be obtained from statistical models of the star and planet populations and their comparison to transiting surveys. I will finally present a few intringuing planets, in connection to the CoRoT mission.

Misha Haywood

Gepi, Paris Observatory

On the correlation between metallicity and the presence of giant planets

The correlation between stellar metallicity and the presence of giant planets is well established. It has been tentatively explained by the possible increase of planet formation probability in stellar disks with enhanced amount of metals. However, there are two caveats to this explanation. First, giant stars with planets do not show a metallicity distribution skewed towards metal-rich objects, as found for dwarfs. Second, the correlation with metallicity is not valid at intermediate metallicities, for which it can be shown that giant planets are preferentially found orbiting thick disk stars.

None of these two peculiarities is explained by the proposed scenarios of giant planet formation. We contend that they are galactic in nature, and probably not linked to the formation process of giant planets. It is suggested that the same dynamical effect, namely the migration of stars in the galactic disk, is at the origin of both features, with the important consequence that most metal-rich stars hosting giant planets originate from the inner disk, a property that has been largely neglected until now. We illustrate that a planet-metallicity correlation similar to the observed one is easily obtained if stars from the inner disk have a higher percentage of giant planets than stars born at the solar radius, with no specific dependence on metallicity. We propose that the density of H2 in the inner galactic disk (the molecular ring) could play a role in setting the high percentage of giant planets that originate from this
region.

Shigeru Ida

Tokyo Institute of Technology

Planetesimal dynamics and theoretical modeling for formation of extrasolar planets

Planetesimal dynamics is one of fundamental processes in planet formation. I will summarize my works on planetesimal dynamics/accretion and diversity of planetary systems, which have been competition and collaboration with Doug Lin.

The mass distribution of planetesimals evolves due to collisional accretion among them. While the collision cross section is determined by relative velocity between planetesimals, the relative velocity is regulated by the mass distribution. This complicated structure leads to non-linear evolution of planetesimal growth. Early growth of planetesimals is "runaway growth," in which initially slightly larger planetesimals predominantly grow. The runaway growth eventually slows down and "oligarchic growth" starts, in which similar-sized protoplanets are formed with similar orbital separations. Since the other planetesimals remain small, the mass distribution becomes bimodal. The growth of the protoplanets is stalled, when all the planetesimals in their feeding zones are accreted, The asymptotic mass is called "isolation mass." In a nominal disk model, the isolation mass is larger in outer regions, in particular, in the regions beyond the ice line in which icy grains condense.

Growth beyond the isolation mass is only possible after disk gas becomes so severely depleted that the eccentricity damping due to tidal interaction with disk gas no more inhibits secular eccentricity growth due to distant perturbations among the isolated bodies. Gas accretion onto a planet is possible after the planet mass becomes larger than a critical value that is a few to ten earth masses and before disk gas is significantly depleted. The preferred locations of formation of gas giants are beyond the ice line. While the planet mass is limited by the isolation mass in inner regions, slow growth limits the mass that can be attained before depletion of disk gas. The regions in which gas giants can be formed are also dependent of disk surface density. Monte Carlo simulations of the sequential planet formation model (Ida & Lin 2004a, b, 2005, 2008a, b), which incorporates these planetesimal dynamics/accretion processes as well as orbital migrations and gas accretion onto protoplanets explain many features in distributions of extrasolar planetary systems that observations have revealed.

Gareth F. Kennedy (1) and Rosemary A. Mardling (2)

(1) ICCUB, IEEC Barcelona; (2) Monash University, Australia

Signatures of resonant terrestrial planets in long-period systems

The majority of extrasolar planets discovered to date have significantly eccentric orbits, some if not all of which may have been produced through planetary migration. During this process, any planets interior to such an orbit would therefore have been susceptible to resonance capture (as well as ejection and collision). While no energy is exchanged between planets in non-resonant systems so that their orbital periods remain constant, significant energy can be exchanged in resonant systems resulting in substantial long-term period modulation of the observed planet, even when the mass of the companion planet is small. Here we examine the possibility of detecting low-mass planets which have suffered resonance capture, particularly into the strong 2:1 resonance. Using simulated data we show that it is possible to identify the existence of a low-mass companion in the internal 2:1 resonance by calculating the orbital period using piecewise sections of radial velocity data. This works as long as the amplitude of modulation of the orbital period is greater than its uncertainty, which in practice means that the system should not be too close to exact resonance. We provide simple expressions for the libration period and the change in the observed orbital period, these being valid for arbitrary eccentricities and planet masses. They in turn allow one to constrain the mass and eccentricity of a companion planet if the orbital period is sufficiently modulated.

Hubert Klahr

Max-Planck-Institute for Astronomy

Turbulence in protoplanedougy disks and planetesimal formation

Following pioneering work by Doug we investigate the mechanism and the role of turbulence for the formation of planets. In this presentation we focus on the early stages of transforming dust into kilometer sized planetary building bricks (Planetesimals) in the early solar system.

A very attractive way to do so is the “Gravoturbulent Fragmentation” of a cloud of relatively small icy and dusty objects. A pure hit and stick scenario (Coagulation) for planetesimals fails as either the large radial drift velocity leads to a quick decay of meter sized boulders into the young star or collisional velocities are large enough to grind boulders to small debris, as was shown theoretically and experimentally. Hence, we came up with a hybrid scenario in which turbulent concentration of centimeter to meter sized icy and dusty material leads to sufficiently large densities in which self gravity dominates over gas shear and tidal forces of the star, thus the heaps of material collapse spontaneously under their own weight into many kilometer sized planetesimals (see nature: Johansen et al. 2007). Therefore, we simulate the motion of millions of particles in magneto-hydro-dynamically and particle driven turbulence and include particle feedback on the turbulence and the gravity among gas and particles, all in one huge simulation.

Willy Kley

University of Tuebingen

Forming resonant planetary systems

Among the observed extrasolar multi-planet systems a considerable fraction has a configuration where the planets are engaged in a low-order mean motion resonance. Among the observed resonant systems the majority lies in a 2:1 resonance. The most famous example being GJ 876 which is in a state of apsidal corotation, where both resonant angles are aligned with only small amplitude libration. In other cases (eg. HD 73526 and HD 128311) only one angle is librating while the second one is circulating. Recently, new systems in 3:2 (HD45364) and 3:1 (HD60532) resonant configurations have been discovered.

These special orbital arrangements seem to be unlikely to be obtained by pure chance. Even though direct formation through scattering has been suggested, more likely they are a consequence of differential and convergent migration of a pair of planets. Standard migration scenarios lead to configurations with apsidal corotation, and several new mechanism have been suggested to break this state. In the talk I will present a short overview of the main physical processes that might have operated in shaping the present configurations of observed resonant planetary systems.

Eiichiro Kokubo (1), Shigeru Ida (2)

(1) National Astronomical Observatory of Japan, (2) Tokyo Institute of Tecnology

Formation of Terrestrial Planets from Protoplanets: Effect of Initial System Radial Range

The final stage of terrestrial planet formation is known as the giant impact stage where protoplanets collide with one another to form planets. We have been investigating this final assemblage of terrestrial planets from protoplanets using N-body simulations. So far we have systematically changed the surface density, surface density profile, and orbital separation of the initial protoplanet system, and the bulk density of protoplanets, while the initial system radial range has been fixed as 0.5-1.5AU. For the standard disk model, typically two Earth-sized planets form in the terrestrial planet region. In the present paper, we systematically change the initial system radial range. We find that as the radial range increases, the number of Earth-sized planets increases, while the number of planets per radial width barely changes. The mass of the largest planet increases almost linearly with the total mass of protoplanets and its semimajor axis converges to around 1 AU. Based on these results, we discuss optimal protoplanet systems to reproduce the terrestrial planets in the solar system.

Katherine Kretke, D.N.C. Lin

University of California, Santa Cruz
Kavli Institute of Astronomy and Astrophysics, Beijing

Stalling Super-Earths and Protoplanetary Cores

Type-I migration, migration due to tidal torques on an embedded planet, both poses a challenge to Jovian planet formation and a provides source for short-period low-mass planets. As such, many studies have looked in detail at the mechanisms which determine the type-I migration rate and
have demonstrated that type-I migration is sensitive to many aspects of the disk structure, including the radial gradients in temperature and surface density. Therefore, in order to calculate the migration of a planet in a realistic disk we have create a detailed 1+1D model of a partially MRI active disk (a disk in which turbulence due to the magneto-rotational instability provides the effective viscosity for disk evolution, but where the ionization fraction is too small to sustain MRI
turbulence at the mid-plane at many regions in the disk). We self-consistently calculate the density and thermal structure of such a disk and compare the migration of planets in these disk to more standard power-law models. Additionally we discuss the likely stopping locations for low mass planets which do migrate towards the inner regions of the protoplanetary disk.

Greg Laughlin

University of California, Santa Cruz

Probing the physical properties of Extrasolar Planets

The science of extrasolar planets is progressing rapidly through its golden age of discovery, and we are entering a golden age of planetary characterization. In this talk, I will give an overview of how follow-up efforts from ground and space are yielding solid information about the physical properties and atmospheric conditions of a variety of extrasolar planets. Surprisingly detailed portraits of many planets can be constructed by drawing on a full range of observational clues.

Zoe Leinhardt

DAMTP Cambridge

Forming Protoplanets from Planetesimals

Over the past decade hundreds of Jupiter-sized extra-solar planets have been discovered. Innovations in space and ground based techniques have resulted in detections of smaller Neptune and super-Earth-sized planets with the promise of increasing the planet inventory by orders of magnitude in the next decade. In addition, observations strongly suggest that our own solar system is only one of a diverse population of solar system configurations. In this presentation I will present a brief discussion of planetesimal formation. I will then focus on the growth of protoplanets from planetesimals (middle stage of the core accretion model for planet formation) and present results on how the initial conditions of the protoplanetary disk can effect this growth process.

Karen M. Lewis

Monash University

The Effect of Realistic Photometric Noise on the Detectability of Moons of Transiting Extra-Solar Planets Using the Photometric Transit Timing Technique

The photometric transit timing technique was proposed by Szabó et.al. (2006) as a method for discovering moons of transiting extra-solar planets. In the preliminary analysis of this technique, it was assumed that the noise in the transit lightcurve was well described by uncorrelated white noise. Unfortunately, real stellar lightcurves contain an excess of lower frequency components due to processes such as rotational modulation of active regions and granulation. To determine the effect of using more realistic lightcurves, timing uncertainties were calculated using additive white noise, solar photometric noise and filtered solar photometric noise. It was found that moons of planets with long transit durations, and moons which were distant from their host planet, were less easily detected when realistic or filtered stellar noise was used than when white noise of the same power was used.

Donald Lynden-Bell

IoA Cambridge

Dynamics without inertial frames

I shall first describe a classical dynamics without Inertial Frames and then discuss Einstein's and others' efforts to remove them from General Relativity.

George Mamatsashvili

Institute for Astronomy, University of Edinburgh

Coupling of vortices and density waves in astrophysical discs

Analytical and numerical study of flow non-normality/shear/differential rotation induced generation of density waves by vortices in thin astrophysical discs is presented. First in the shearing sheet approximation we demonstrate physics and properties of linear coupling of vortices and density waves in terms of individual spatial Fourier harmonics using non modal approach. In particular, we show that generation of density waves by vortices occurs in the so-called non-adiabatic region in the wavenumber space when time-dependent (due to Keplerian shear) radial wavenumber crosses zero point, or equivalently when the timescales of vortex and wave modes become comparable. Then after an analytical non-modal description we follow the process of wave generation by considering the evolution of an initially imposed small amplitude coherent localized circular vortex structure by means of direct global numerical analysis. This allows us to see the spatial aspect of wave generation phenomenon.

Rosemary Mardling

Monash University

Chaotic scattering in massive eccentric systems from first principles

Chaotic scattering seems to play a fundamental role at every stage of planet formation, from bringing planetesimals together to form planetary cores, to rearranging planet orbits as the disk potential diminishes, to scattering planetesimals out of the system or onto highly eccentric orbits during migration. These processes are further complicated by the presence of stellar companions, including the hyperbolic flyby of a nascent cluster sibling. For realistic mass ratios and eccentricities, modelling the scattering process has necessarily relied on numerical integrations, while analytical insight has remained at the level of the coplanar circular restricted three-body problem.

In this talk I will present the elements of a new parameter-free formulation of the general three-body problem which allows one to determine the stability characteristics of any triple system, including arbitrary inclinations, eccentricities and mass ratios. A simple algorithm will also be presented.

Michel Mayor

Observatoire de Geneve

The mass-semimajor axis distribution of planetary systems from super-earths to gaseous giant planets

M.Mayor, C.Lovis ,S.Udry

In the last six years, at La Silla observatory, we have carried out a systematic survey to search for planetary systems. The extreme sensitivity of the HARPS spectrograph has led to the detection of an impressive population of low-mass planets on close-in orbits. A first comparison will be done between the observed (m2sini -a) distribution and predictions from models of planetary formation.

Stefano Meschiari

University of California, Santa Cruz

The Systemic Console Package

During the past decade, the characterization of extrasolar planets has become a major branch of Astronomy. The field is energized by a variety of ground and space-based programs that are successfully detecting planets using a variety of techniques. The Systemic Console is a software package for the analysis and combined multiparameter fitting of these fast-growing Doppler radial velocity (RV) and transit timing observations data sets. It integrates an extensible array of tools that facilitate error estimation, Monte-Carlo simulations and long-term stability analysis, providing a common suite of routines to the planet hunters community. We illustrate the capabilities of the package by analyzing an updated Keck radial velocity data set for the HD128311 planetary system. HD128311 harbors a dynamically interacting pair of planets that appear to be participating in a 2:1 mean motion resonance. We show that the dynamical configuration cannot be fully determined from the RV dataset alone. We find that if a planetary system like HD128311 is found to undergo transits, then self-consistent Newtonian fits to combined radial velocity data and a small number of timing measurements of transit midpoints can provide an immediate and vastly improved characterization of the planet’s dynamical state. (Download at http://www.oklo.org)

Cezary Migaszewski & Krzysztof Gozdziewski

Torun Centre for Astronomy, N. Copernicus University, Poland

Secular, relativistic dynamics of circumbinary planets

In this work, we construct a secular theory of hierarchical triple system of gravitating point masses. This model takes into account the general relativity PPN corrections. Our approach relies on the expansion of the perturbing secular Hamiltonian in the semi-major axes ratio $\alpha$, which is considered as a small parameter. This secular theory derived up to the order of $10$ has no direct restrictions on eccentricities. In fact, in typical cases, already the analytic, octupole expansion leads to qualitatively the same results as the basically exact theory derived with quasi-analytic averaging. Here, we apply the secular theory to study the long-term dynamics of a compact stellar (or sub-stellar) binary and a low mass companion (a planet or a brown dwarf) in inclined orbits. We found that the low-massive object on a wide orbit may excite large eccentricity of the binary when the mutual inclination of the orbits is larger than $\sim 60^{\circ}$. This unusual behavior may be attributed to the so called outer Lidov-Kozai resonance (LKR). In that case, the secular motions may became strongly chaotic (in the sense of the Lyapunov exponent). Our study shows that the restricted model does not properly describe the long-term dynamics of the system involved in the outer LKR. We also analyse the influence of the PPN corrections on the parametric structure of a few families of stationary solutions existing in this problem, when the initial mutual inclination remains in the $[0^{\circ},90^{\circ}]$-range. We found that the dynamics of hierarchical systems with small $\alpha$ may be qualitatively different in the realm of the Newtonian (classic) and relativistic models. It holds true even for relatively large masses of the secondaries. The PPN corrections may influence the character of motion of the secular system, regarding significant changes of the eccentricity and the secular apsidal angle.

Alessandro Morbidelli

Observatoire de la Cote d'Azur

Origin of the orbital architecture of the planets of the solar system

I will discuss how the giant planets of our solar system could avoid Type II migration towards the Sun. The dynamics, ruled by the interaction of Jupiter and Saturn with the gas disk would have left the four giant planets on fully resonant orbits with very small eccentricities and inclinations. After the disappearance of the gas disk, the interaction of the planets with the planetesimals extracted the former from their original quadruple resonance and led to a late but short phase of dynamical instability in the planetary motion. The current orbital configuration of the giant planets could then be achieved from the gravitational interaction between the planets and the disk of planetesimals. In particular, we will discuss how the amplitudes of the secular modes that characterize the current secular motion of the planets could be achieved. In the scenario where the dynamical instability of the giant planets occurred late, which is tempting to explain the origin of the so-called Late Heavy Bombardment, the orbital stability of the terrestrial planets is at risk. We will discuss how the terrestrial planets could have survived the sweeping of powerful secular resonances through their region and eventually acquire their current orbits. Constraints from the orbital distribution in the asteroid belt will also be discussed.

Makiko Nagasawa

Tokyo Institute of Technology

Sweeping secular resonance and its role in forming terrestrial planets from protoplanets

The secular resonance occurs when the precession rate of a body coincides with one of the eigenfrequencies of the system that are normally close to the precession rate of perturber. Near the location of the secular resonance, the eccentricity or inclination of the body is excited. The location of the secular resonance is determined by the gravitational potential in the system. A long term variation of the gravitational potential causes the change of the location of the secular resonances. This phenomenon is known as the sweeping secular resonances.

An outstanding problem concerning the standard scenario of final terrestrial planet formation is that the formed planets remain the modest eccentricities they got during orbital instability phase. The damping mechanisms are proposed to overcome this issue, but since the damping mechanism tends to keep isolation of protoplanet, some kind of instability process is required for further evolution. The sweeping secular resonance caused by the gas disk depletion is a good candidate, because it can happen together with gravitational drag.

We investigate the effect of disk depletion in final assemblage of the terrestrial planets. We find that a combination process of sweeping secular resonance and gravitational drag can form the analog of the terrestrial region of the solar system. I will talk the role of the sweeping secular resonance in forming the orbital properties of terrestrial region.

Gordon Ogilvie

DAMTP, University of Cambridge

Tidal interactions of planets and stars

Extrasolar planets can experience strong thermal, gravitational and magnetic interactions with their host stars. Tidal forcing excites low-frequency internal waves (inertial or inertia-gravity waves) in both the planet and the star. I will discuss the role of tides in determining the observed properties of short-period planets.

Chris Ormel

Max-Planck-Institute for Astronomy

Runaway growth in protoplanetary disks

The runaway growth stage of planet formation occurs when bodies --also referred to as planetesimals -- have reached a size such that their escape velocity becomes larger than the relative velocity between them. Gravitational focusing then operates to enhance the collisional cross section in such a way that the more massive bodies outpace their competitors in terms of growth. Eventually, one body will dominate the growth in its neighborhood.

Because runaway growth (RG) is the stage preceding to the later oligarchy and giant planet formation stages, it is paramount to understand and subsequently model this critical phase. I will argue that, due to the discrete nature of RG, particle based models are more natural to apply. Furthermore, in a particle-oriented approach relevant particle properties like their random velocity, excitation stage, etc, can be included at relatively little expense. I will present the first preliminary results of this new approach towards runaway growth in the planetesimal accretion stage, focusing on the runaway/oligarchy transition and the timescales involved.

John Papaloizou

DAMTP Cambridge

Disk planet Interaction and orbital migration

The popular theory that disk planet interactions that occur during the later stages of planet formation lead to orbital migration, and close orbiting giant planets, developed from related work on close binary sytems carried out by Doug Lin over thirty years ago. We here present a review of the early developments as well as recent work in this area.

Miguel Preto (1) & Prasenjit Saha (2)

(1) ARI, Heidelberg, (2) ITP, Zurich

Adaptive, symplectic integrators for galactic nuclei and planetary systems

The dynamics of galactic nuclei and of planetary systems are both dominated by a central mass -- e.g. a massive black hole or a star. We formulate the post-Newtonian effects on the motion of Galactic-center stars as weak, non-separable perturbations to the Kepler Hamiltonian. In this framework, we propose an adaptive, symplectic integrator for fast and accurate numerical calculation of highly eccentric, near-Keplerian orbits. The fully symplectic time-stepping scheme thus introduced can also be applied when the Hamiltonian is written in terms of "democratic heliocentric coordinates"; as a result, it may substantially alleviate the difficulties faced by popular symplectic codes, e.g. SYMBA or MERCURY, when dealing with eccentric orbits. Furthermore, we propose to adopt an exact Kepler solver that is fully implemented in cartesian coordinates (no need for solving Kepler's equation or use Gauss' f,g functions). In summary, these developments

may substantially improve N-body integrations of nearly Keplerian systems by allowing: (i) the adoption of individual time steps; (ii) the efficient integration of highly eccentric orbits; (iii) the speed up of Kepler drifts; (iv) the introduction of non-separable perturbations (e.g post-Newtonian or Poynting-Robertson drag) without breaking the symplecticity of the overall scheme.

Derek Richardson

University of Maryland

The dynamics of small body satellite formation

Since the first small body satellite was discovered in 1993 (tiny Dactyl orbiting main belt asteroid Ida), over 100 examples of binaries and even multiples among the small body population of our solar system have been found. In fact, 15% of all near-Earth asteroids are believed to be binary. Formation scenarios for small body satellites include 3-body capture, collisional ejecta retention, and rotational disruption, with the latter either caused by tidal encounters with planets, or the thermal YORP effect. A recent numerical model (Walsh et al. 2008) has successfully matched the main characteristics of the well-studied near-Earth asteroid binary 1999 KW4 by invoking YORP-induced spin-up of a rubble-pile asteroid. I will discuss this model, and others, and outline some of the future directions in this field of study. I will even explain how all this (loosely) relates to the man of the hour, the venerable Douglas N. C. Lin.

Harvey Richer

University of British Columbia

Kicked White Dwarfs and the Dynamical Evolution of Globular Star Clusters

It appears likely that white dwarfs are given a small kick when they are born. I will present evidence to support this idea as well as some ideas on where in the evolution of a star the kick occurs. If correct, the white dwarf kick is capable of heating a cluster, fluffing up its core and reducing the need for intermediate mass black holes in at least some of them.

Jerry Sellwood

Rutgers University

New developments in spiral structure theory

Progress in the theory of spiral structure has stalled for many years largely because no observational data existed that could discriminate between the various mechanisms proposed to account for the phenomenon. This situation changed dramatically with the publication of the Geneva-Copenhagen Survey, which provides full phase-space information for a large, and unbiased, sample of nearby F & G stars. I will review the current state of spiral structure theory before showing how data from the GCS impinge on the generating mechanism for spirals.

V. V. Salmin

Siberian Federal University, Russia

3D Hydrodynamic model of Solar System formation

Ideal Hydrodynamics and MHD are widely used in astrophysical models due to not only low viscosity but because of large scale of the systems. According to general conception, Solar System was formed from primordial nebula. Our basic idea is to describe nebular structures as precursors of planetary systems by steady Euler equation. According to Arnold's structural theorem 3D steady Euler equation of ideal incompressible fluid flow has solution with toroidal topology described by divergent free vector field. It is easy to show that structural theorem is also applicable to description of steady ideal incompressible MHD and steady isentropic or isothermal flow of ideal compressible fluid. Divergent free vector fields in Euclidian space may be settled by stereographic projection of vector fields with closed flow lines on 3D sphere. The minimal energy among all the fields with closed flow lines on 3D sphere has a structure called Hopf field. It was recently proved that Hopf field on 3D sphere is stable. Also, stereographic projection of Hopf field into Euclidian space induces toroidal coordinates, and has image as divergent free vector field with toroidal topology where flow lines are Villarceau circles lying on tori corresponding to the levels of Bernoulli function. Using calculation of stress tensor in toroidal coordinates we showed that optimal level corresponding to "optimal torus" exists where relative surface free energy is minimal. Beat of oscillations with wave numbers corresponding to structural radii of "optimal torus" lying in accretion plane leads to scaling of "optimal tori". "Optimal tori" are considered as precursors of planetary orbits. Suggested model allows describing the distribution of semi-major axes planets of Solar System and all regular satellites. We found that scaling factors of Solar System and HR 8799 system are similar and precisely correspond to an "optimal torus". We have shown that scaling factors of satellite systems had deviations depending on central body axial tilt and local ratio of semi-major axes of neighbor planets that explained by coincidence of flow lines involved in toroidal flow around protoplanets and around proto-Sun. Also the model gives good description of volumes of regular satellites of giant planets. Mechanism of accretion in retrograde direction in satellite systems based on toroidal symmetry breaking is discussed. Arguments for Triton's origin are presented.

Frank Shu

University of California, San Diego

Magnetized star formation and accretion disks around young stellar objects

We review the theory and observational evidence governing the formation of low-mass stars from the magnetized cores of molecular clouds. We show that the ubiquitous presence of accretion disks around forming stars requires non-ideal effects to be present in the magnetohydrodynamics of the gravitational collapse, and we argue that such circumstances lead naturally to a theory of accretion disks where magneto-turbulence plays a critical role in the disk viscosity and resistivity, as anticipated by Doug Lin's supervisor, Donald Lynden-Bell, forty years ago. Such disks automatically find it difficult to launch powerful, significantly mass-loaded, magneto-centrifugal winds. We argue that observed protostellar jets are likely to originate instead from the interaction of the inner edges of such disks with the stellar magnetospheres, and we review recent observational evidence and numerical simulations that bear on these points.

Rainer Spurzem

ARI Heidelberg

Dynamics of planetary systems in star clusters

At least 10-15% of nearby sunlike stars have known Jupiter-mass planets. In contrast, very few planets are found in mature open and globular clusters such as the Hyades and 47 Tuc. We explore here the possibility that this dichotomy is due to the post-formation disruption of planetary systems associated with the stellar encounters in long-lived clusters. One supporting piece of evidence for this scenario is the discovery of freely floating low-mass objects in star forming regions. We use two independent numerical approaches, a hybrid Monte Carlo and a direct N-body method, to simulate the impact of the encounters. It is found that moderately close stellar encounters, which are likely to occur in dense clusters, can excite planets' orbital eccentricity as well as repeated hyperbolic adiabatic encounters which accumulate small-amplitude changes. In our model, which only includes single planet systems, disruption of planetary systems occurs primarily through occasional nearly parabolic, nonadiabatic encounters. The detached planets are generally retained by the potential of their host clusters as free floaters in young stellar clusters such as Sigma Orionis. We compute effective cross sections for the dissolution of planetary systems and show that, for all initial eccentricities, dissolution occurs on timescales which are longer than the dispersion of small stellar associations, but shorter than the age of typical open and globular clusters. In future work we plan to extend the study to multi-planetary systems, where encounter induced eccentricities can lead to instabilities and, for short-period planets, subsequent tidal dissipation may lead to orbital decay, tidal inflation, and even disruption of the close-in planets. Computational perspectives for the next years using special hardware are discussed, including a planned supercomputing facility in Beijing at the National Astronomical Observatory of the Chinese Academy, in close cooperation with Doug Lin and the Kavli Institute for Astronomy and Astrophysics of Peking University (The Silk Road Project).

A.H.M.J. Triaud (1), D. Queloz (1), D. Pollaco (2), C. Hellier (3), A. Collier Cameron (4) and the SuperWasp Consortium (2,3,4,5,6,7,8,9)

(1) Observatoire de Geneve, (2) Queen's University Belfast, (3) University of Keele, (4) University of St Andrews, (5) Open University, (6) University of Leicester, (7) University of Cambridge, (8) Newton Group of Telescopes, (9) Instituto de Astrofisica de Canarias.

Searching and characterising transiting gas giants

Using the SuperWasp North and South arrays, a list of transiting planet candidates is established. These are followed-up spectroscopically and photometrically in order to extract the true planetary bodies from false positives, binaries and giants. Once confirmed, work does not stop and characterisation by high precision photometry, transit timing and Rossiter McLaughlin observations as well long term radial velocity variations take place bringing in a wealth of information. Here we present the current state of the SuperWasp planets.

Eduard Vorobyov

The Institute for Computational Astrophysics, St Mary’s University, Halifax

Disk masses and mass accretion rates in T Tauri stars and brown dwarfs

Using numerical hydrodynamic simulations of disk formation and long-term evolution, we study mass accretion rates and disk masses in T Tauri stars and brown dwarfs. Our modelling suggests that the mean disk masses in T Tauri stars are considerably larger than those inferred from observations. For instance, we predict the mean disk masses of 0.1-0.11 Msun in the Class 0/I phases, while Andrews & Williams (2005) report almost a factor of four smaller values. The largest contrast is found in mean masses of the Class II disks, for which we obtain 0.06-0.12 Msun but Andrews & Williams report 0.003 Msun. On the other hand, our numerical modelling reproduces well the observed relation between mass accretion rates and stellar masses. The mass accretion rates depend (either directly or indirectly) on the disk mass, and we would not have succeeded in reproducing the observed mass accretion rates if our model disk masses had been much in error. This allows us to believe that the disk masses inferred from observations may have been considerably underestimated by conventional observational methods.

Wei Wang, Steve Boudreault, Coryn A.L. Bailer-Jones, Bertrand Goldman, Thomas Henning, Jose Caballero

MPIA, UCM

The substellar population in the old open cluster Praesepe

The measurement of cluster mass function is a fundamental tool for assessing models of star formation, especially the case in the substellar regime, where competing models make different predictions about the relative formation efficiency at different masses. The brown dwarf evolution can be traced by the comparison of mass functions at different ages. We have carried out a deep, multi-band (r,i, z & Y) imaging survey in the central part of an old open cluster, namely Praesepe (age~650 Myr) with the LBT/LBC red and blue cameras. Our proposed 5sigma detection limit at i-SLOAN band is about 26, corresponding to mass limit about 45 M_Jup. However, due to the imperfect status of the instrument during our observation, the actual i band magnitude limit is about 1 mag shallower, but still reaching the substellar regime. We will report the preliminary result in this poster.

A. Wolszczan (1,2), A. Niedzielski (3), M. Adamów (3), G. Nowak (3), P. Zielinski (3), S. Gettel (1,2)

(1) Department of Astronomy & Astrophysics, Penn State, (2) Center for Exoplanets and Habitable Worlds, Penn State, (3) Torun Centre for Astronomy, Nicolaus Copernicus University

A search for planets around intermediate-mass stars with the Hobby-Eberly telescope

We describe the status of our long-term radial velocity (RV) survey of a sample of ~1000 GK-giants with the 9.2-m Hobby-Eberly Telescope and its High Resolution Spectrograph, in search for planetary-mass companions. The survey uses the I2 cell calibration method to achieve a ~4-6 m s-1 precision of RV measurements, which is quite sufficient, given the observed ≥20 m s-1 intrinsic RV variability of these stars. The substellar-mass companions discovered so far include planets around HD 17092, HD 102272, and HD 240210, and two bodies orbiting BD +20 2457, which are likely to be brown dwarfs. There is also a growing number of other plausible candidates currently undergoing the standard verification process. We present a representative sample of RV curves of these stars and describe the line bisector and photometry analyses applied to the data, in order to separate the RV variations generated by orbiting planets from those resulting from stellar phenomena. We also discuss the preliminary characteristics of the GK-giant planets population, which begin to emerge, as its size continues to grow.

Ji-lin Zhou

Nanjing University

Migration and final location of hot super-Earths in the presence of gas giants

Hot super-Earths are exoplanets with masses less than 10 Earth mass and orbital periods less then 20 days. Around 8 hot super-Earths have been discovered around stars in the neighborhood of solar system. In this lecture, we review the mechanisms for the formation of hot super-Earths, dynamical effects that play important roles in sculpting the architecture of the multiple planetary systems, with emphases on the cooperation works with DNC Lin. Several example systems are presented to show the formation and evolution of hot super-Earths or Neptunes.

Andras Zsom (1), Carsten Guettler (2), Chris Ormel (1), Juergen Blum (2) & Cornelis Dullemond (1)

(1) Max-Planck-Institute for Astronomy
(2) Institut für Geophysik und extraterrestrische Physik, TU Braunschweig, Germany

Circumstellar disk and dust aggregates: the biggest billiard game with the smallest balls

A conclusive bottom-up model on the formation of pre-planetesimal bodies has so far only been established for the very first fractal growth regime. The major reason for this drawback is that, after the restructuring and compaction of the dust aggregates, laboratory experiments show diverse results in the collisional outcomes (e.g. cratering, erosion, bouncing, etc.) which proved to be difficult to incorporate into dust collision models.

In this study we introduce a collision model which, due to its Monte Carlo nature, can handle the full complexity of aggregate collisions. We categorize dust aggregate collisions into eight regimes based on the porosity of the collision partners and their relative masses. In these regimes, aggregate masses, porosities and collision velocities determine the outcome. From experimental observations we distinguish nine classes of outcomes: 4 types of sticking, 2 types of bouncing and 3 types of fragmentation. Although experiments are not available for all possible collisions, we are able to cover the whole parameter space by plausible extrapolations.

Based on this recipe we run a local growth simulation at 1 AU in the midplane of two disk models: in the MMSN disk, and one disk model introduced by Brauer et al. (2008) which has lower density at 1 AU than the MMSN model. In general we find that evolution of dust does not follow the previously assumed growth-fragmentation cycles. Actually, catastrophic fragmentation plays an insignificant role. Furthermore we see long lived, quasi-steady states in the mass distribution function of the aggregates (due to bouncing).