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The formation of planets around a single star is a complex process that depends on many parameters. Planetary systems with a secondary star too close to the primary star can significantly complicate the process. In this work, we explored the formation of γ Cephei b, a giant planet located in a close binary system, using a hydrodynamic approach. We tested different parameters for the discs and protoplanet in an attempt to reproduce the observed features. We found that in situ formation can reach the observational mass of the planet. Both isothermal and radiative disc conditions exhibited similar planetary evolution. However, our results demonstrated a high planetary eccentricity. To understand this behaviour, we applied secular theory to the γ Cephei system. We found that the system can reproduce the observational and simulated values of eccentricities. [ABSTRACT FROM AUTHOR]

Context. Recent observations found close-in planets with significant atmospheres of hydrogen and helium in great abundance. These are the so-called super-Earths and mini-Neptunes. Their atmospheric composition suggests that they formed early during the gas-rich phase of the circumstellar disk and were able to avoid becoming hot Jupiters. As a possible explanation, recent studies explored the recycling hypothesis and showed that atmosphere-disk recycling is able to fully compensate for radiative cooling and thereby halt Kelvin-Helmholtz contraction to prevent runaway gas accretion. Aims. To understand the parameters that determine the efficiency of atmospheric recycling, we extend our earlier studies by exploring the effects of the core mass, the effect of circumstellar gas on sub-Keplerian orbits (headwind), and the optical depth of the surrounding gas on the recycling timescale. Additionally, we analyze their effects on the size and mass of the forming atmosphere. Methods. We used three-dimensional (3D) radiation-hydrodynamic simulations to model a local shearing box centered on the planet. Our planet is located at a separation of ap= 0.1 au from its solar-type host star, and we scanned the core mass range from 1 to 10 MEarth. In order to measure and track the recycling of the atmosphere, we employed tracer particles as well as tracer fluids after thermodynamic equilibrium was reached. Results. For the explored parameter space, all simulations eventually reach an equilibrium where heating due to hydrodynamic recycling fully compensates radiative cooling. In this equilibrium, the atmosphere-to-core mass ratio stays well below 10%, preventing the atmosphere from becoming self-gravitating and entering runaway gas accretion. Higher core masses cause the atmosphere to become turbulent, which further enhances recycling. Compared to the core mass, the effect of the headwind on the recycling timescale is negligible. The opacity has no significant effect on the recycling timescale, which demonstrates that the Kelvin-Helmholtz contraction timescale and the atmosphere-disk recycling timescale are independent of each other. Conclusions. Even for our highest core mass of 10 MEarth, atmosphere-disk recycling is efficient enough to fully compensate for radiative cooling and prevent the atmosphere from becoming self-gravitating. Hence, in-situ formation of hot Jupiters is very unlikely, and migration of gas giants is a key process required to explain their existence. Our findings imply that atmosphere-disk recycling is the most natural explanation for the prevalence of close-in super-Earths and mini-Neptunes. [ABSTRACT FROM AUTHOR]

Context. The most accurate method for modelling planetary migration and hence the formation of resonant systems is using hydrodynamical simulations. Usually, the force (torque) acting on a planet is calculated using the forces from the gas disc and the star, while the gas accelerations are computed using the pressure gradient, the star, and the planet's gravity, ignoring its own gravity. For a nonmigrating planet the neglect of the disc gravity results in a consistent torque calculation while for a migrating case it is inconsistent. Aims. We aim to study how much this inconsistent torque calculation can affect the final configuration of a two-planet system. We focus on low-mass planets because most of the multi-planetary systems, discovered by the Kepler survey, have masses around ten Earth masses. Methods. Performing hydrodynamical simulations of planet-disc interaction, we measured the torques on non-migrating and migrating planets for various disc masses as well as density and temperature slopes with and without considering the self-gravity of the disc. Using this data, we found a relation that quantifies the inconsistency, used this relation in an N-body code, and performed an extended parameter study modelling the migration of a planetary system with different planet mass ratios and disc surface densities, to investigate the impact of the torque inconsistency on the architecture of the planetary system. Results. Not considering disc self-gravity produces an artificially larger torque on the migrating planet that can result in tighter planetary systems. The deviation of this torque from the correct value is larger in discs with steeper surface density profiles. Conclusions. In hydrodynamical modelling of multi-planetary systems, it is crucial to account for the torque correction, otherwise the results favour more packed systems. We examine two simple correction methods existing in the literature and show that they properly correct this problem. [ABSTRACT FROM AUTHOR]

The formation of satellite systems in circum-planetary discs is considered to be similar to the formation of rocky planets in a proto-planetary disc, especially super-Earths. Thus, it is possible to use systems with large satellites to test formation theories that are also applicable to extrasolar planets. Furthermore, a better understanding of the origin of satellites might yield important information about the environment near the growing planet during the last stages of planet formation. In this work, we investigate the formation and migration of the Jovian satellites through N-body simulations. We simulated a massive, static, low-viscosity, circumplanetary disc in agreement with the minimum mass sub-nebula model prescriptions for its totalmass. In hydrodynamic simulations,we found no signs of gaps, therefore type II migration is not expected. Hence, we used analytic prescriptions for type I migration, eccentricity and inclination damping, and performed N-body simulations with damping forces added. Detailed parameter studies showed that the number of final satellites is strong influenced by the initial distribution of embryos, the disc temperature, and the initial gas density profile. For steeper initial density profiles, it is possible to form systems with multiple satellites in resonance while a flatter profile favours the formation of satellites close to the region of the Galilean satellites. We show that the formation of massive satellites such as Ganymede and Callisto can be achieved for hotter discs with an aspect ratio of H/r ∼ 0.15 for which the ice line was located around 30RJ. [ABSTRACT FROM AUTHOR]

The surroundings of massive protostars constitute an accretion disc which has numerically been shown to be subject to fragmentation and responsible for luminous accretion-driven outbursts. Moreover, it is suspected to produce close binary companions which will later strongly influence the star's future evolution in the Hertzsprung-Russel diagram. We present three-dimensional gravitation-radiation-hydrodynamic numerical simulations of 100M⊙ prestellar cores. We find that accretion discs of young massive stars violently fragment without preventing the (highly variable) accretion of gaseous clumps on to the protostars. While acquiring the characteristics of a nascent low-mass companion, some disc fragments migrate on to the central massive protostar with dynamical properties showing that its final Keplerian orbit is close enough to constitute a close massive protobinary system, having a young highand a low-mass components. We conclude on the viability of the disc fragmentation channel for the formation of such short-period binaries, and that both processes - close massive binary formation and accretion bursts - may happen at the same time. FU-Orionis-type bursts, such as observed in the young high-mass star S255IR-NIRS3, may not only indicate ongoing disc fragmentation, but also be considered as a tracer for the formation of close massive binaries - progenitors of the subsequent massive spectroscopic binaries - once the high-mass component of the system will enter the main-sequence phase of its evolution. Finally, we investigate the Atacama Large (sub-)Millimeter Array observability of the disc fragments. [ABSTRACT FROM AUTHOR]

Accretion-driven luminosity outbursts are a vivid manifestation of variable mass accretion on to protostars. They are known as the so-called FU Orionis phenomenon in the context of low-mass protostars. More recently, this process has been found in models of primordial star formation. Using numerical radiation hydrodynamics simulations, we stress that present- day forming massive stars also experience variable accretion and show that this process is accompanied by luminous outbursts induced by the episodic accretion of gaseous clumps falling from the circumstellar disc on to the protostar. Consequently, the process of accretion-induced luminous flares is also conceivable in the high-mass regime of star formation and we propose to regard this phenomenon as a general mechanism that can affect protostars regardless of their mass and/or the chemical properties of the parent environment in which they form. In addition to the commonness of accretion-driven outbursts in the star formation machinery, we conjecture that luminous flares from regions hosting forming high-mass stars may be an observational implication of the fragmentation of their accretion discs. [ABSTRACT FROM AUTHOR]

Massive stars that have been ejected from their parent cluster and supersonically sailing away through the interstellar medium (ISM) are classified as exiled. They generate circumstellar bow-shock nebulae that can be observed. We present two-dimensional, axisymmetric hydro-dynamical simulations of a representative sample of stellar wind bow shocks from Galactic OB stars in an ambient medium of densities ranging from nISM = 0.01 up to 10.0 cm-3. Independently of their location in the Galaxy, we confirm that the infrared is the most appropriated waveband to search for bow shocks from massive stars. Their spectral energy distribution is the convenient tool to analyse them since their emission does not depend on the temporary effects which could affect unstable, thin-shelled bow shocks. Our numerical models of Galactic bow shocks generated by high-mass (≈40 Mʘ) runaway stars yield H a fluxes which could be observed by facilities such as the SuperCOSMOS H-Alpha Survey. The brightest bow-shock nebulae are produced in the denser regions of the ISM. We predict that bow shocks in the field observed at H α by means of Rayleigh-sensitive facilities are formed around stars of initial mass larger than about 20 Mʘ. Our models of bow shocks from OB stars have the emission maximum in the wavelength range 3 ≤ λ ≤ 50 µm which can be up to several orders of magnitude brighter than the runaway stars themselves, particularly for stars of initial mass larger than 20 Mʘ. [ABSTRACT FROM AUTHOR]

Aims. Massive planets that open a gap in the accretion disk are believed to migrate with exactly the viscous speed of the disk, a regime termed type II migration. Population synthesis models indicate that standard type II migration is too rapid to be in agreement with the observations. We study the migration of massive planets between 2 × 10-4 and 2 × 10-3 Mʘ, corresponding to 0.2 to 2 Jupiter masses MJ to estimate the migration rate in comparison to type II migration. Methods. We follow the evolution of planets embedded in two-dimensional, locally isothermal disks with non-zero mass accretion, which is explicitly modelled using suitable in- and outflow boundary conditions to ensure a specific accretion rate. After a certain relaxation time we release the planet and measure its migration through the disk and the dependence on parameters, such as viscosity, accretion rate, and planet mass. We study accreting and non-accretion planets. Results. The inferred migration rate of the planet is determined entirely by the disk torques acting on it and is completely independent of the viscous inflow velocity, so there is no classical type II migration regime. Depending on the local disk mass, the migration rate can be faster or slower than type II migration. From the torques and the accretion rate profile in the disk we see that the gap formed by the planet does not separate the inner from the outer disk as necessary for type II migration, rather gas crosses the gap or is accreted onto the planet. [ABSTRACT FROM AUTHOR]

Context. Disk accretion onto weakly magnetized white dwarfs (WDs) in cataclysmic variables (CVs) leads to the formation of a boundary layer (BL) between the accretion disk and the WD, where the accreted matter loses its excess kinetic energy and angular momentum. It is assumed that angular momentum is effectively transported in the BL, but the transport mechanism is still unknown. Aims. Here we compute detailed model spectra of recently published optically thick one-dimensional radial BL models and qualitatively compare them with observed soft X-ray/extreme ultraviolet (EUV) spectra of dwarf novae in outburst. Methods. Every considered BL model with given effective temperature and surface density radial distribution is divided into a number of rings, and for each ring, a structure model along the vertical direction is computed using the stellar-atmosphere method. The ring spectra are then combined into a BL spectrum taking Doppler broadening and limb darkening into account. Results. Two sets of model BL spectra are computed, the first of them consists of BL models with fixed WD mass (1 M☉) and various relative WD angular velocities (0.2, 0.4, 0.6 and 0.8 break-up velocities), while the other deals with a fixed relative angular velocity (0.8 break-up velocity) and various WD masses (0.8, 1, and 1.2 M☉). The model spectra show broad absorption features because of blending of numerous absorption lines, and emission-like features at spectral regions with only a few strong absorption lines. The model spectra are very similar to observed soft X-ray/EUV spectra of SS Cyg and U Gem in outburst. The observed SS Cyg spectrum could be fitted by BL model spectra with WD masses 0.8-1 M☉ and relative angular velocities 0.6-0.8 break up velocities. These BL models also reproduce the observed ratio of BL luminosity and disk luminosity. The difference between the observed and the BL model spectra is similar to a hot optically thin plasma spectrum and could be associated with the spectrum of outflowing plasma with a mass loss rate compatible with the BL mass accretion rate. Conclusions. The suggested method of computing BL spectra seems very promising and can be applied to other BL models for comparison with EUV spectra of dwarf novae in outburst. [ABSTRACT FROM AUTHOR]

Aims. The aim of this work is to gain a deeper insight into how much different aggregate types are afected by erosion. Especially, it is important to study the influence of the velocity of the impacting projectiles. We also want to provide models for dust growth in protoplanetary disks with simple recipes to account for erosion effects. Methods. To study the erosion of dust aggregates we employed a molecular dynamics approach that features a detailed micro-physical model of the interaction of spherical grains. For the first time, the model has been extended by introducing a new visco-elastic damping force, which requires a proper calibration. Afterwards, different sample generation methods were used to cover a wide range of aggregate types. Results. The visco-elastic damping force introduced in this work turns out to be crucial to reproduce results obtained from laboratory experiments. After proper calibration, we find that erosion occurs for impact velocities of 5 ms-1 and above. Though fractal aggregates as formed during the first growth phase are most susceptible to erosion, we observe erosion of aggregates with rather compact surfaces as well. Conclusions. We find that bombarding a larger target aggregate with small projectiles results in erosion for impact velocities as low as a few ms-1. More compact aggregates suffer less from erosion. With increasing projectile size the transition from accretion to erosion is shifted to higher velocities. This allows larger bodies to grow through high velocity collisions with smaller aggregates. [ABSTRACT FROM AUTHOR]

Context. Within the sequential accretion scenario of planet formation, planets are built up through a sequence of sticking collisions. The outcome of collisions between porous dust aggregates is very important for the growth from very small dust particles to planetesimals. In this work we determine the necessary material properties of dust aggregates as a function of porosity. Aims. Continuum models such as SPH that are capable of simulating collisions of macroscopic dust aggregates require a set of material parameters. Some of them, such as the tensile and shear strength, are difficult to obtain from laboratory experiments. The aim of this work is to determine these parameters from ab initio molecular dynamics simulations. Methods. We simulated the behavior of porous dust aggregates using a detailed micro-physical model of the interaction of spherical grains that includes adhesion forces, rolling, twisting, and sliding. Using different methods of preparing the samples, we studied the strength behavior of our samples with varying porosity and coordination number of the material. Results. For the tensile strength, we can reproduce data from laboratory experiments very well. For the shear strength, there are no experimental data available. The results from our simulations differ significantly from previous theoretical models, which indicates that the latter might not be sufficient to describe porous dust aggregates. Conclusions. We have provided the functional behavior of tensile and shear strength of porous dust aggregates as a function of the porosity, which can be directly applied to continuum simulations of these objects in planet formation scenarios. [ABSTRACT FROM AUTHOR]

Context. Bouncing collisions of dust aggregates within the protoplanetary disk may have a significant impact on the growth process of planetesimals. Yet, the conditions that result in bouncing are not very well understood. Existing simulations studying the bouncing behavior used aggregates with an artificial, very regular internal structure. Aims. Here, we study the bouncing behavior of sub-mm dust aggregates that are constructed applying different sample preparation methods. We analyze how the internal structure of the aggregate alters the collisional outcome and we determine the influence of aggregate size, porosity, collision velocity, and impact parameter. Methods. We use molecular dynamics simulations where the individual aggregates are treated as spheres that are made up of several hundred thousand individual monomers. The simulations are run on graphic cards (GPUs). Results. Statistical bulk properties and thus bouncing behavior of sub-mm dust aggregates depend heavily on the preparation method. In particular, there is no unique relation between the average volume filling factor and the coordination number of the aggregate. Realistic aggregates bounce only if their volume filling factor exceeds 0.5 and collision velocities are below 0:1 ms-1. Conclusions. For dust particles in the protoplanetary nebula we suggest that the bouncing barrier may not be such a strong handicap in the growth phase of dust agglomerates, at least in the size range of μ100 μm. [ABSTRACT FROM AUTHOR]

Context. The strength and direction of migration of low mass embedded planets depends on the disk's thermodynamic state. It has been shown that in active disks, where the internal dissipation is balanced by radiative transport, migration can be directed outwards, a process which extends the lifetime of growing embryos. Very important parameters determining the structure of disks, and hence the direction of migration, are the viscosity and the adiabatic index. Aims. In this paper we investigate the influence of different viscosity prescriptions (α-type and constant) and adiabatic indices on disk structures. We then determine how this affects the migration rate of planets embedded in such disks. Methods. We perform three-dimensional numerical simulations of accretion disks with embedded planets.We use the explicit/implicit hydrodynamical code NIRVANA that includes full tensor viscosity and radiation transport in the flux-limited diffusion approximation, as well as a proper equation of state for molecular hydrogen. The migration of embedded 20 MEarth planets is studied. Results. Low-viscosity disks have cooler temperatures and the migration rates of embedded planets tend toward the isothermal limit. Hence, in these disks, planets migrate inwards even in the fully radiative case. The effect of outward migration can only be sustained if the viscosity in the disk is large. Overall, the differences between the treatments for the equation of state seem to play a more important role in disks with higher viscosity. A change in the adiabatic index and in the viscosity changes the zero-torque radius that separates inward from outward migration. Conclusions. For larger viscosities, temperatures in the disk become higher and the zero-torque radius moves to larger radii, allowing outward migration of a 20-MEarth planet to persist over an extended radial range. In combination with large disk masses, this may allow for an extended period of the outward migration of growing protoplanetary cores. [ABSTRACT FROM AUTHOR]

Context. The strength and direction of migration of embedded low mass planets depends on the disc's thermodynamic state. It has been shown that, in discs where the viscous heating is balanced by radiative transport, the migration can be directed outwards, a process which extends the lifetime of growing planetary embryos. Aims. We investigate the influence of opacity and stellar irradiation on the disc thermodynamics. We focus on equilibrium discs, which have no net mass flux. Utilizing the resulting disc structure, we determine the regions of outward migration in the disc. Methods. We performed two-dimensional numerical simulations of equilibrium discs with viscous heating, radiative cooling, and stellar irradiation. We used the explicit/implicit hydrodynamical code NIRVANA that includes a full tensor viscosity and stellar irradiation, as well as a two temperature solver that includes radiation transport in the flux-limited diffusion approximation. The migration of embedded planets was studied by using torque formulae. Results. In the constant opacity case, our code reproduces the analytical results corresponding to a black-body disc: the stellar irradiation dominates in the outer regions - leading to flaring (H/r ∞ r3/7) - while the viscous heating dominates close to the star. In particular, we find that the inner edge of the disc should not be significantly puffed-up by the stellar irradiation. If the opacity depends on the local density and temperature, the structure of the disc is different, and several bumps in the aspect ratio H/r appear, due to transitions between different opacity regimes. The bumps in the disc structure are very important, as they can shield the outer disc from stellar irradiation. Conclusions. Stellar irradiation is an important factor for determining the disc structure and has dramatic consequences for the migration of embedded planets. Compared to discs with only viscous heating and radiative cooling, a stellar irradiated disc features a much smaller region of outward migration for a range of planetary masses. This suggests that the region where the formation of giant planet cores takes place is smaller, which in turn might lead to a shorter growth phase. [ABSTRACT FROM AUTHOR]

As planets form and grow within gaseous protoplanetary disks, the mutual gravitational interaction between the disk and planet leads to the exchange of angular momentum and migration of the planet. We review current understanding of disk-planet interactions, focusing in particular on physical processes that determine the speed and direction of migration. We describe the evolution of low-mass planets embedded in protoplanetary disks and examine the influence of Lindblad and corotation torques as a function of the disk properties. The role of the disk in causing the evolution of eccentricities and inclinations is also discussed. We describe the rapid migration of intermediate-mass planets that may occur as a runaway process and examine the transition to gap formation and slower migration driven by the viscous evolution of the disk for massive planets. The roles and influence of disk self-gravity and magnetohydrodynamic turbulence are discussed in detail, as a function of the planet mass, as is the evolution of multiple planet systems. Finally, we address the question of how well global models of planetary formation that include migration are able to match observations of extrasolar planets. [ABSTRACT FROM AUTHOR]

Context. In 2D-simulations of thin gaseous disks with embedded planets or self-gravity the gravitational potential needs to be smoothed to avoid singularities in the numerical evaluation of the gravitational potential or force. The softening prescription used in 2D needs to be adjusted properly to correctly resemble the realistic case of vertically extended 3D disks. Aims. We analyze the embedded planet and the self-gravity case and provide a method to evaluate the required smoothing in 2D simulations of thin disks. Methods. Starting from the averaged hydrodynamic equations and using a vertically isothermal disk model, we calculated the force to be used in 2D simulations.We compared our results to the often used Plummer form of the potential, which runs as ∝1/(r2 + ϵ2)1/2. For that purpose we computed the required smoothing length ϵ as a function of distance r to the planet or to a disk element within a self-gravitating disk. Results. We find that for longer distances ϵ is determined solely by the vertical disk thickness H. For the planet case we find that outside r ≈ H a value of ϵ = 0.7H describes the averaged force very well, while in the self-gravitating disk the value needs to be higher, ϵ = 1.2H. For shorter distances the smoothing needs to be reduced significantly. Comparing torque densities of 3D and 2D simulations we show that the modification to the vertical density stratification as induced by an embedded planet needs to be taken into account to obtain agreeing results. Conclusions. It is very important to use the correct value of ϵ in 2D simulations to obtain a realistic outcome. In disk fragmentation simulations the choice of ϵ can determine whether a disk will fragment or not. Because a wrong smoothing length can change even the direction of migration, it is very important to include the effect of the planet on the local scale height in 2D planet-disk simulations. We provide an approximate and fast method for this purpose that agrees very well with full 3D simulations. [ABSTRACT FROM AUTHOR]

Context. The early planetesimal growth proceeds through a sequence of sticking collisions of dust agglomerates. Very uncertain is still the relative velocity regime in which growth rather than destruction can take place. The outcome of a collision depends on the bulk properties of the porous dust agglomerates. Aims. Continuum models of dust agglomerates require a set of material parameters that are often difficult to obtain from laboratory experiments. Here, we aim at determining those parameters from ab initio molecular dynamics simulations. Our goal is to improve on the existing model that describe the interaction of individual monomers. Methods. We use a molecular dynamics approach featuring a detailed micro-physical model of the interaction of spherical grains. The model includes normal forces, rolling, twisting and sliding between the dust grains. We present a new treatment of wall-particle interaction that allows us to perform customized simulations that directly correspond to laboratory experiments. Results. We find that the existing interaction model by Dominik & Tielens leads to a too soft compressive strength behavior for uniand omni-directional compression. Upon making the rolling and sliding coefficients stiffer we find excellent agreement in both cases. Additionally, we find that the compressive strength curve depends on the velocity with which the sample is compressed. Conclusions. The modified interaction strengths between two individual dust grains will lead to a different behavior of the whole dust agglomerate. This will influences the sticking probabilities and hence the growth of planetesimals. The new parameter set might possibly lead to an enhanced sticking as more energy can be stored in the system before breakup. [ABSTRACT FROM AUTHOR]

Context. As of today, over 50 planetary systems have been discovered in binary star systems, some of which have binary separations that are smaller than 20AU. In these systems the gravitational forces from the binary have a strong influence on the evolution of the protoplanetary disk and hence the planet formation process. Aims. We study the evolution of viscous and radiative circumstellar disks under the influence of a companion star. We focus on the eccentric γ Cephei and α Centauri system as examples and compare disk quantities such as disk eccentricity and precession rate to previous isothermal simulations. Methods. We performed two-dimensional hydrodynamical simulations of the binary star systems under the assumption of coplanarity of the disk, host star and binary companion. We used the grid-based, staggered mesh code FARGO with an additional energy equation to which we added radiative cooling based on opacity tables. Results. The eccentric binary companion perturbs the disk around the primary star periodically. Upon passing periastron, spirals arms are induced that wind from the outer disk towards the star. In isothermal simulations this results in disk eccentricities up to edisk ≈ 0.2, but in more realistic radiative models we obtain much smaller eccentricities of about edisk ≈ 0.04-0.06 with no real precession. Models with varying viscosity and disk mass indicate that disks with less mass have lower temperatures and higher disk eccentricity. Conclusions. The fairly high disk eccentricities, as indicated in previous isothermal disk simulations, implied a more difficult planet formation in the γ Cephei system caused by the enhanced collision velocities of planetesimals. We have shown that under more realistic conditions with radiative cooling the disk becomes less eccentric and thus planet formation may be made easier. However, we estimate that the viscosity in the disk has to very low, with α ≲ 0.001, because otherwise the disk's lifetime will be too short to allow planet formation to occur along the core instability scenario. We estimate that the periodic heating of the disk in eccentric binaries will be observable in the mid-IR regime. [ABSTRACT FROM AUTHOR]

ABSTRACT The aim of this paper is to investigate the spatial structure of a protoplanetary disc whose dynamics is governed by magnetorotational turbulence. We perform a series of local three-dimensional chemoradiative magnetohydrodynamic simulations located at different radii of a disc which is twice as massive as the standard minimum mass solar nebula of Hayashi. The ionization state of the disc is calculated by including collisional ionization, stellar X-rays, cosmic rays and the decay of radionuclides as ionization sources, and by solving a simplified chemical network which includes the effect of the absorption of free charges by μm-sized dust grains. In the region where the ionization is too low to ensure good coupling between matter and magnetic fields, a non-turbulent central 'dead zone' forms, which ranges approximately from a distance of 2 to 4 au from the central star. The approach taken in this work allows for the first time to derive the global spatial structure of a protoplanetary disc from a set of physically realistic numerical simulations. [ABSTRACT FROM AUTHOR]

I. Hubeny Does anyone from the panel have a theme question to start with today?V. Trimble It's another one-liner: From an active galaxy meeting many years ago when people talked about spiral structure. I was reminded by Dr. Rucinski's talk of Lodewijk Woltjer's remark: “The larger our ignorance, the stronger the magnetic field.” [ABSTRACT FROM PUBLISHER]

I. Hubeny Today, the discussion will be open to the general audience. In Sessions C, D, and E, we have talked about models and modelling techniques so I expect the discussion will focus on these topics. [ABSTRACT FROM PUBLISHER]

M. Richards: Several talks today have expressed fuzzy boundaries to describe the objects called “stars.” Is the following classification correct? Are stars restricted to objects that have masses greater than 0.089 solar masses and begin making energy with hydrogen burning? Do we include the stellar remnants: the white dwarfs and neturon stars? Do we include the brown dwarfs because they burn lithium or deuterium. We know that planets are not in this group since they have no energy production. [ABSTRACT FROM PUBLISHER]

We study the effects of diffusion on the non-linear corotation torque, or horseshoe drag, in the two-dimensional limit, focusing on low-mass planets for which the width of the horseshoe region is much smaller than the scaleheight of the disc. In the absence of diffusion, the non-linear corotation torque saturates, leaving only the Lindblad torque. Diffusion of heat and momentum can act to sustain the corotation torque. In the limit of very strong diffusion, the linear corotation torque is recovered. For the case of thermal diffusion, this limit corresponds to having a locally isothermal equation of state. We present some simple models that are able to capture the dependence of the torque on diffusive processes to within 20 per cent of the numerical simulations. [ABSTRACT FROM AUTHOR]

We investigate a massive ( g cm at 1 au) protoplanetary disc model by means of 3D radiation magnetohydrodynamic simulations. The vertical structure of the disc is determined self-consistently by a balance between turbulent heating caused by the magnetorotational turbulence and radiative cooling. Concerning the vertical structure, two different regions can be distinguished: a gas-pressure-dominated, optically thick mid-plane region where most of the dissipation takes place, and a magnetically dominated, optically thin corona which is dominated by strong shocks. At the location of the photosphere, the turbulence is supersonic (), which is consistent with previous results obtained from the fitting of spectra of young stellar objects. It is known that the turbulent saturation level in simulations of MRI-induced turbulence does depend on numerical factors such as the numerical resolution and the box size. However, by performing a suite of runs at different resolutions (using up to grid cells) and with varying box sizes (with up to 16 pressure scaleheights in the vertical direction), we find that both the saturation levels and the heating rates show a clear trend to converge once a sufficient resolution in the vertical direction has been achieved. [ABSTRACT FROM AUTHOR]

We investigate the influence of radiative transport on the growth of the magnetorotational instability (MRI) in accretion discs. A general dispersion relation describing the growth of small disturbances on a homogeneous background shear flow is provided. It includes compressibility and radiative effects in the flux-limited diffusion approximation. All the effects of radiation transport can be subsumed into one single parameter, an effective speed of sound. Radiative diffusion does not affect the region where the MRI operates, but it alters the growth rates of the MRI either by increasing or decreasing them. We report about corresponding numerical simulations, which are also used for a first investigation of the non-linear stage of the MRI in gas-pressure dominated accretion discs with radiation transport included. [ABSTRACT FROM AUTHOR]

To explain important properties of extrasolar planetary systems (e.g. close-in hot Jupiters, resonant planets) an evolutionary scenario which allows for radial migration of planets in disks is required. During their formation protoplanets undergo a phase in which they are embedded in the disk and interact gravitationally with it. This planet-disk interaction results in torques (through gravitational forces) acting on the planet that will change its angular momentum and result in a radial migration of the planet through the disk. To determine the outcome of this very important process for planet formation, dedicated high resolution numerical modeling is required. This contribution focusses on some important aspects of the numerical approach that we found essential for obtaining successful results. We specifically mention the treatment of Coriolis forces, Cartesian grids, and the FARGO method. [ABSTRACT FROM AUTHOR]

We study the torque on low-mass planets embedded in protoplanetary discs in the two-dimensional approximation, incorporating non-isothermal effects. We couple linear estimates of the Lindblad (or wave) torque to a simple, but non-linear, model of adiabatic corotation torques (or horseshoe drag), resulting in a simple formula that governs type I migration in non-isothermal discs. This formula should apply in optically thick regions of the disc, where viscous and thermal diffusion act to keep the horseshoe drag unsaturated. We check this formula against numerical hydrodynamical simulations, using three independent numerical methods, and find good agreement. [ABSTRACT FROM AUTHOR]

As of today over 40 planetary systems have been discovered in binary star systems. In all cases the configuration appears to be circumstellar, where the planets orbit around one of the stars, the secondary acting as a perturber. The formation of planets in binary star systems is more difficult than around single stars due to the gravitational action of the companion on the dynamics of the protoplanetary disk. In this contribution we first briefly present the relevant observational evidence for planets in binary systems. Then the dynamical influence that a secondary companion has on a circumstellar disk will be analyzed through fully hydrodynamical simulations. We demonstrate that the disk becomes eccentric and shows a coherent precession around the primary star. Finally, fully hydrodynamical simulations of evolving protoplanets embedded in disks in binary star systems are presented. We investigate how the orbital evolution of protoplanetary embryos and their mass growth from cores to massive planets might be affected in this very dynamical environment. We consider, in particular, the planet orbiting the primary in the system γ Cephei. [ABSTRACT FROM AUTHOR]

The formation of resonant planets pairs in exoplanetary systems involves planetary migration inside the protoplanetary disc: an inwards migrating outer planet captures in Mean Motion Resonance an inner planet. During the migration of the resonant pair of planets, the eccentricities are expected to rise excessively, if no damping mechanism is applied on the inner planet. We express the required damping action to match the observations, and we show that the inner disk can play this role. This result applies for instance to the system GJ 876: we reproduce the observed orbital elements through a fully hydrodynamical simulation of the evolution of the resonant planets. [ABSTRACT FROM AUTHOR]

In this paper, we investigate the influence of radiative transport on the growth of the magnetorotational instability (MRI) in accretion discs. The analysis is performed by the use of analytical and numerical means. We provide a general dispersion relation together with the corresponding eigenfunctions describing the growth rates of small disturbances on a homogeneous background shear flow. The dispersion relation includes compressibility and radiative effects in the flux-limited diffusion approximation. By introducing an effective speed of sound, all the effects of radiation transport can be subsumed into one single parameter. It can be shown that the growth rates of the vertical modes – which are the fastest growing ones – are reduced by radiative transport. For the case of non-vertical modes, the growth rates may instead be enhanced. We quantify the effects of compressibility and radiative diffusion on the growth rates for the gas-pressure dominated case. The analytical discussion is supplemented by numerical simulations, which are also used for a first investigation of the non-linear stage of the MRI in gas-pressure dominated accretion discs with radiation transport included. [ABSTRACT FROM AUTHOR]

We perform numerical simulations of a disc–planet system using various grid-based and smoothed particle hydrodynamics (SPH) codes. The tests are run for a simple setup where Jupiter and Neptune mass planets on a circular orbit open a gap in a protoplanetary disc during a few hundred orbital periods. We compare the surface density contours, potential vorticity and smoothed radial profiles at several times. The disc mass and gravitational torque time evolution are analysed with high temporal resolution. There is overall consistency between the codes. The density profiles agree within about 5 per cent for the Eulerian simulations. The SPH results predict the correct shape of the gap although have less resolution in the low-density regions and weaker planetary wakes. The disc masses after 200 orbital periods agree within 10 per cent. The spread is larger in the tidal torques acting on the planet which agree within a factor of 2 at the end of the simulation. In the Neptune case, the dispersion in the torques is greater than for Jupiter, possibly owing to the contribution from the not completely cleared region close to the planet. [ABSTRACT FROM AUTHOR]

We investigate the gravitational interaction of a Jovian-mass protoplanet with a gaseous disc with aspect ratio and kinematic viscosity expected for the protoplanetary disc from which it formed. Different disc surface density distributions are investigated. We focus on the tidal interaction with the disc with the consequent gap formation and orbital migration of the protoplanet. Non-linear two-dimensional hydrodynamic simulations are employed using three independent numerical codes. A principal result is that the direction of the orbital migration is always inwards and such that the protoplanet reaches the central star in a near-circular orbit after a characteristic viscous time-scale of ∼10[sup 4] initial orbital periods. This is found to be independent of whether the protoplanet is allowed to accrete mass or not. Inward migration is helped by the disappearance of the inner disc, and therefore the positive torque it would exert, because of accretion on to the central star. Maximally accreting protoplanets reach about 4 Jovian masses on reaching the neighbourhood of the central star. Our results indicate that a realistic upper limit for the masses of closely orbiting giant planets is ∼5 Jupiter masses, if they originate in protoplanetary discs similar to the minimum-mass solar nebula. This is because of the reduced accretion rates obtained for planets of increasing mass. Assuming that some process such as termination of the inner disc through a magnetospheric cavity stops the migration, the range of masses estimated for a number of close orbiting giant planets as well as their inward orbital migration can be accounted for by consideration of disc–protoplanet interactions during the late stages of giant planet formation. [ABSTRACT FROM AUTHOR]

The evolution of a system consisting of a protoplanetary disc with two embedded Jupiter-sized planets is studied numerically. The disc is assumed to be flat and non-self-gravitating; this is modelled by the planar (two-dimensional) Navier–Stokes equations. The mutual gravitational interaction of the planets and the star, and the gravitational torques of the disc acting on the planets and the central star are included. The planets have an initial mass of one Jupiter mass M[sub Jup] each, and the radial distances from the star are one and two semimajor axes of Jupiter, respectively. During the evolution a joint wide annular gap is created by the planets. Both planets increase their mass owing to accretion of gas from the disc: after about 2500 orbital periods of the inner planet it has reached a mass of 2.3 M[sub Jup], while the outer planet has reached a mass of 3.2 M[sub Jup]. The net gravitational torques exerted by the disc on the planets result in an inward migration of the outer planet on time-scales comparable to the viscous evolution time of the disc. The semimajor axis of the inner planet remains constant as there is very little gas left in its vicinity to induce any migration. When the distance of close approach eventually becomes smaller than the mutual Hill radius, the eccentricities increase strongly and the system may become unstable. If disc depletion occurs rapidly enough before the planets come too close to each other, a stable system similar to our own Solar system may remain. Otherwise the orbits may become unstable and produce systems like υ And. [ABSTRACT FROM AUTHOR]

ABSTRACTWe study the structure and dynamics of the gap created by a protoplanet in an accretion disc. The hydrodynamic equations for a flat, two-dimensional, non-self-gravitating protostellar accretion disc with an embedded, Jupiter-sized protoplanet on a circular orbit are solved. To simulate possible accretion of mass on to the protoplanet we continually remove mass from the interior of the planet's Roche lobe, which is monitored. First, it is shown that consistent results independent of numerical issues (such as boundary or initial conditions, artificial viscosity or resolution) can be obtained. Then, a detailed parameter study delineates the influence of the disc viscosity and pressure on the magnitude of the accretion rate. We find that, even after the formation of a gap in the disc, the planet is still able to accrete more mass from the disc. This accretion occurs from regions of the disc that are radially exterior and interior to the planet's orbital radius. The rate depends on the magnitude of the viscosity and vertical thickness of the disc. For a disc viscosity α=10-3 and vertical thickness H/r=0.05 we estimate the time-scale for the accumulation of one Jupiter mass to be of the order of a hundred thousand years. For a larger (smaller) viscosity and disc thickness this accretion rate is increasing (decreasing). For a very small viscosity α5×10-4 the mass accretion rate through the gap on to the planet is markedly reduced, and the corresponding accretion time-scale becomes larger than the viscous evolution time of the disc. [ABSTRACT FROM AUTHOR]

Miniplate osteosynthesis is an osteoplastic procedure that achieves secure stabilization of the midface, even following extensive fractures of the midface and the floor of the anterior cranial fossa. This procedure can be considered a satisfactory operative treatment in terms of cosmetic and functional results. Correlative osteology is discussed and clinical experiences are reviewed. [ABSTRACT FROM AUTHOR]

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