Building Blocks of Planets 2020 Abstract Booklet

- Tuesday 14.4.2020 -

Scattering-induced intensity reduction: large mass content with small grains in the inner region of the TW Hya disk Takahiro Ueda

Dust continuum observation is one of the best methods to constrain the properties of protoplanetary disks. Recent theoretical studies have suggested that the dust scattering at the millimeter wavelength potentially reduces the observed intensity, which results in an underestimate in the dust mass. We investigate whether the dust scattering indeed reduces the observed continuum intensity by comparing the ALMA archival data of the TW Hya disk at Band 3, 4, 6, 7 and 9 to models obtained by radiative transfer simulations. We find that the model with scattering reproduces the observed SED of the central part of the TW Hya disk best while the model without scattering is still consistent within the conservative errors of the absolute fluxes. To explain the intensity at Band 3, the dust surface density needs to be ∼ 10 g cm^−2 at 10 au in the model with scattering, which is 26 times more massive than previously predicted. The model without scattering needs 2.3 times higher dust mass than the model with scattering because the model without scattering needs lower temperature. At Band 7, even though the disk is optically thick, scattering reduces the intensity by ∼ 35% which makes the disk looks optically thin. Our study suggests the TW Hya disk is still capable of forming cores of giant planets at where the current planets exist.

Tracing dust evolution from cores to disks Leonardo Testi

I will discuss observational evidence for dust evolution from cores too disks. I will focus on the uncertainties and pitfalls, what we think we have learned and what we perhaps need to revisit with a new perspective. I will mostly, but not only, focus on how ALMA observations have changed our perspective and highlight some of the future directions that are required to progress in the observational characterization of dust evolution.

Dust evolution as disk mass estimate Riccardo Franceschi

The dust content is fundamental for the structure and observation of protoplanetary disks. Moreover, observations of dust emission have been used in most of disk mass estimates, assuming a constant dust-to-mass ratio. However, dust properties should be changing in an evolving disk, depending on the structure of the disk and the grain size distribution, both uncertain quantities. Dust particles are subjected to radial drift, gas and turbulent mixing, and all these processes depends on grain size. This causes a grain size differentiation along the disk structure that, along consideration on grain growth, can be used to derive the total disk surface density profile. This approach can be checked with multiwavelength observation of dust line location for different sized grain in several disks, and could provide constraint to disk mass estimates.

Edge-on observations of young disks Marion Villenave

To form giant planets in lifetime, small micron sized particles must grow rapidly to larger grains. To do so, they need to settle efficiently towards the disk midplane, and likely concentrate into dust traps. Currently observational constraints on vertical settling, which is intrinsically related to grain growth, are incomplete. During this talk, I aim to present new observational constraints on vertical settling efficiency. I will compare optical/infrared scattered light with millimeter observations of several edge-on disks, to probe the difference in vertical extent between micron-sized and larger millimeter-sized dust grains. I will show that the most edge-on disks of our sample, well resolved vertically, are compatible with having a millimeter dust scale height of about 1au at 100 au. Compared to a gas scale height estimated to about 10 au at 100 au, this result indicates very efficient vertical settling. This increasing dust density in the midplane is expected to enhance the efficiency of planet formation. Dust models after Planck Vincent Guillet

Dust models are key to study the nature of interstellar grains and the processes that govern the evolution of their properties through the interstellar medium. In this talk, I will focus on the importance of dust models for the analysis of polarization observations. I will show how far-infrared and submillimeter observations by Planck and BLASTPol have ruled out historical dust models, and forced to a revision that is still ongoing. In the context of protoplanetary disks, I will advocate for the use of physical dust models to confidently interpret polarization patterns by aligned grains when the wavelength is of the order of the grain size.

Polarization as a tool for characterizing particles Characterizing Cosmic dust Olga Muñoz

The IAA Cosmic Dust Laboratory (Muñoz et al., JQSRT, 2010) has produced an important number of experimental phase functions and degree of linear polarization curves of cosmic dust analogues. The studied samples comprise a wide range of sizes (from sub-micron up to mm-sized), shapes and compositions. I will discuss our current efforts to constraint the parameter space (size, shape and refractive index) of cosmic dust grains by direct comparison of laboratory data with astronomical observations.

Size and Structures in Disks around Very Low Mass Nicolas Kurtovic

Most of the stars in our galaxy are M-dwarfs, which are commonly hosts of planetary systems, which we know are formed in protoplanetary disks. Although planet formation models predict very efficient radial drift in the disks of these objects, millimeter wavelength observations have revealed the existence of circumstellar disks around them, suggesting the existence of strong pressure bumps. In this talk I will show our observations of 5 disks around VLMS in Taurus. With 0.1'' angular resolution we resolve the emission in all the disks, and we find evidence of substructure in 2 of them. By observing the molecular line emission 12CO and 13CO we find the gas radii being 3~6 times more extended than dust radii, larger than ratios observed in disks hosted by solar type stars or more massive. With these observations the total number of disks around VLMS in Taurus increases to 6, with clear evidence of substructure in at least 3 of them.

Formation of multiple dust rings and gaps due to intermittent planet migration in protoplanetary disks Gaylor Wafflard-Fernandez

Recent observations of spatially resolved protoplanetary disks, in particular with the radio interferometer ALMA, reveal a large diversity of substructures in the dust thermal emission (sequences of dark rings (gaps) and bright rings, asymmetries, spirals, ...). A key challenge for protoplanetary disks and planet formation models is to be able to make a reliable connection between these observed substructures and the supposed existence of planets impacting the dust content of protoplanetary disks. The observation of N dark rings of emission is often interpreted as evidence for the presence of N planets which clear dust gaps around their orbit and form dust- trapping pressure maxima in the disk. In general, these models assume planets on fixed orbits. We choose here to take into account the gravitational interaction between a planet and the gas content of a protoplanetary disk. We thus consider the large-scale inward migration of a single planet in a massive disk. In many circumstances, the migration of a partial gap-opening planet with a mass comparable to Saturn is found to run away intermittently. By means of 2D gas and dust hydrodynamical simulations, we show that intermittent runaway migration can form multiple dust rings and gaps across the disk. Each time migration slows down, a pressure maximum forms beyond the planet gap that traps the large dust. Post-processing of our simulations results with 3D dust radiative transfer calculations confirms that intermittent runaway migration can lead to the formation of multiple sets of bright and dark rings of continuum emission in the (sub)millimeter beyond the planet location.

Signatures of planet formation and orbital evolution in the cold dust emission of protoplanetary discs Clément Baruteau

The classical picture of protoplanetary discs being smooth, continuous structures of gas and dust has been challenged by the growing number of spatially resolved observations. These observations tell us that radial discontinuities and large-scale asymmetries are common features of the emission of protoplanetary discs, which are often interpreted as signatures of the presence of (unseen) planetary companions. During this seminar, I will report on our recent and ongoing work on how the formation and orbital evolution of planets impact the dust emission in protoplanetary discs, mainly at radio wavelengths. Through gas and dust hydrodynamical simulations post-processed with dust radiative transfer calculations, I will show that recent ALMA observations strongly suggest the presence of several planets in the discs around MWC 758 and HD 169142.

Mdust-Mstar & Roust-Mstar relations: Models vs. Observations Paola Pinilla

Demographic surveys of protoplanetary disks, in particular with ALMA, have provided access to a large range of disk dust masses and radii around stars with different stellar types and in different -forming regions. These surveys found a power-law relation between Must (and Rdust) and Mstar that steepens in time, but which is also flatter for transition disks and disks with sub-structures. In this talk, I will present the results of dust evolution models that focus on investigating the effect of particle traps on these observed relations. I will explain what are the required conditions to reproduce the observed trends, in particular I will focus on what we can learn about the origins of the pressure traps in protoplanetary disks.

Characterizing the dust content of protoplanetary disk substructures using multiwavelength (sub-)mm observations Enrique Macias

A key piece of information to understand the origin and role of disk substructures is their dust content. In particular, disk substructures associated with gas pressure bumps can work as dust traps, accumulating grains, and increasing their growth. This kind of substructures could therefore play a crucial role in the planetary formation process. In this talk I will present the multi-wavelength analyses of protoplanetary disk substructures using (sub-)mm observations taken with ALMA and VLA. Using these data, we estimate the radial variations in the dust density and dust particle size distribution, showing strong evidence that most ring substructures are efficiently trapping large dust particles. We find dust sizes between 1 mm and 1 cm, as well as some evidence of flatter power-law size distributions than in the interstellar medium. Additionally, this multi-wavelength study allows us to robustly constrain the dust temperature in the disk midplane, showing that most ring substructures do not coincide with the snowlines of the most important volatiles in the disk. Finally, our analyses show that, due to the high optical depths of disk substructures, observations at wavelengths longer than ~3 mm are crucial to characterize their dust content and obtain accurate estimates of the disk dust mass.

Effects of scattering, temperature gradients, and settling on the derived dust properties of protoplanetary disks Anibal Sierra Morales

Although dust scattering can be a very important opacity source in protoplanetary disks observed at millimeter wavelengths, it is usually neglected in the analysis of dust continuum observations. Here we discuss the expected emission and spectral indices when scattering is taken into account. We find that for large albedo (ω > 0.6) the emergent intensity decreases in the optically thick regime with respect to the pure absorption case, and increases at optical depths between 10^{-2} and 10^{-1}. The spectral indices in the optically thick regime are also modified and decrease to values below 2 for maximum grain sizes between 100 μm and 1 mm. In addition, we find that vertical temperature gradients decrease the spectral indices with respect to the isothermal case. Dust settling also has important effects in the optically thick regime, where the emission mainly traces the dust grains in the upper layers of the disk. For a dust surface density larger than 3.21 g cm^{−2}, large grains at the disk mid plane could be hidden. The shape of the spectral energy distribution is also modified when scattering is included. Finally, we find that scattering can explain the observed excess emission reported at λ = 7 mm in several disks if the disks are optically thick at 1.3 mm and the grains have sizes between 300 μm < a_{max} < 1 mm. In this case, the slope of the SED changes and the excess is obtained when the emission is interpreted as a pure absorption case.

Grain size distribution in the HD163296 disk Andrea Isella

I will present new measurements of the grain size distribution in the HD 163296 planet forming disk resulting from high angular resolution observations of the dust continuum emission recorded at wavelengths spanning from 0.8 mm to 1 cm. The observations resolve variation of the spectral index of the dust emission across the rings and gaps that characterize this system and, in doing that, inform about the interplay between disk substructures and dust evolution. Finally, I will compare these results with those obtained from the analysis of the polarization of the dust continuum emission.

Size Matters: The particle size distribution in HL Tau from VLA and ALMA images Carlos Carrasco-Gonzalez

Particle size distributions in protoplanetary are usually estimated through measurements of the dust opacity at different millimeter wavelengths assuming optically thin emission and dust opacity dominated by absorption. However, Atacama Large Millimeter/submillimeter Array (ALMA) observations have shown that these assumptions might not be correct in the case of protoplanetary disks, leading to overestimation of particle sizes and to underestimation of the disk’s mass. We have presented an analysis of high-quality ALMA and VLA images of the HL Tau protoplanetary disk, covering a wide range of wavelengths (0.8 mm - 1 cm), and with a physical resolution of ̃7.35 au. We describe a procedure to analyze a set of millimeter images without any assumption about the optical depth of the emission, and including the effects of absorption and scattering in the dust opacity. This procedure allows us to obtain the dust temperature, the dust surface density, and the maximum particle size at each radius. In the HL Tau disk, we found that particles have already grown to a few millimeters in size. We detect differences in the dust properties between dark and bright rings, with dark rings containing low dust density and small dust particles. Different features in the HL Tau disk seem to have different origins. Planet-disk interactions can explain substructure in the external half of the disk, but the internal rings seem to be associated with the presence of snow lines of several molecules.

- Wednesday 15.4.2020 -

Simple physical models for the streaming instability Jonathan Squire

The streaming instability – a promising mechanism for clumping mid-sized grains into in disks – has remained puzzling since its discovery by Youdin & Goodman in 2005. Based on the recently discovered "Resonant Drag Instability" framework, in this talk I will attempt to rectify this by developing simple, physically motivated models for how it works and why it clumps grains. The models are based on the physics of gaseous epicyclic motions and dust-gas drag forces, and explain key features such as its sudden change in properties as the dust-to-gas ratio surpasses one, the faster growth of the similar "settling instability" for small grains, and the spatial structure of the fastest-growing modes. As well as improving general theoretical understanding of a commonly studied mechanism, we hope that the models could see use in diagnosing and understanding grain clumping in more realistic nonlinear simulations, and in developing models for dust-induced turbulence.

Kinematic detections of protoplanets Christophe Pinte

We still do not understand how planets form, or why extra-solar planetary systems are so different from our own solar system. Recent observations of protoplanetary discs have revealed rings and gaps, spirals and asymmetries. These features have been interpreted as signatures of newborn protoplanets, but the exact origin is unknown, and remains poorly constrained by direct observation. In this talk, we show how high spatial and spectral resolution ALMA observations can be used to detect embedded planet in their discs. We report the kinematic detections of Jupiter-mass planets in the discs of HD 163296 and HD 97048. For HD 97048, the planet is located in a gas and dust gap. An embedded planet can explain both the disturbed Keplerian flow of the gas, detected in CO lines, and the gap detected in the dust disc at the same radius. While gaps appear to be a common feature in protoplanetary discs, we present a direct correspondence between a planet and a dust gap, indicating that at least some gaps are the result of planet-disc interactions.

3D global simulations of the Vertical Shear instability Marcelo Fernando Barraza Alfaro

Turbulence is a key ingredient in the disk evolution and planet formation. However, the origin of the low level of turbulence recently observed in protoplanetary disks is not yet well understood. The Vertical Shear Instability (VSI) is a candidate to be responsible for the hydrodynamic turbulence in the outer regions of the disk. Via 3D global hydrodynamical simulations, we study the evolution of the VSI in an isothermal disk, with and without an embedded planet. We post-process the outputs of the simulations to study the observability of the VSI. We produce synthetic observations of radiative transfer calculations of the gas line emission. Further, we investigate if kinematic signatures of hydrodynamical turbulence are present in our predictions, and if they are observable in the near future with ALMA. In this talk, I will present preliminary results on this project.

Observations of Class 0/I Protostars: Disk Diversity at Early Times Dominique Segura-Cox

Circumstellar disks are fundamental to the low-mass star and planet formation processes, yet their properties are only beginning to be unveiled in detail during the earliest Class 0 and I phases due to the dense gas and dust envelopes present at early times. Multiple recent high-resolution continuum studies using different interferometers (VLA, ALMA, PdBI) of Class 0/I sources show strong evidence for relatively small disks (R<50 au) as the most common outcome of the early star-formation process. On the other hand, some large disks do exist in the Class 0 and I phases. I will discuss my recent work showing dust rings already in the process of formation in a large (R~85 au) Class I disk. This large ringed Class I disk also has a larger-scale (~1500 au) gas streamer feeding material from the envelope to the disk. Disk material could be continually replenished during the Class I stage through ongoing accretion from the envelope, when zones of dust grain growth are already developing in disk rings. Several ringed disks in the Class II phase have strong evidence of already sizable planets shepherding the dust into tight rings, sending the message that to understand the first steps of planet formation we must look towards disks in the younger embedded phases. The size diversity of the disks in the young phases and the dynamical connection from disk to envelope are necessary topics for future studies to determine the conditions for grain growth and ultimately planets.

HD simulations of protoplanetary disks. Part I. Resolution Study for VSI. Lizxandra Flores Rivera

Winds in disks can be driven by thermal on the surface of the disk but their interaction with other instabilities in the disk has not been well investigated yet. In this first part of the project, we focus on diagnosing the Vertical Shear Instability (VSI) at high numerical resolution and comparing our configuration using two different numerical schemes. We also aim to predict at what scale height does the UVs photons may influence the photoevaporation at the surface of the disk. We use a global isothermal accretion disk setup, 2.5D (2 Dimensions, 3 Components) which covers a radial domain from 0.5 to 5.0 AU and an extended meridional domain of about 180 degree in theta. We reach a high resolution of 203 cells per scale height where the VSI operates until our density floor up to 10 in scale height with respect to the midplane. We determine we need 50 cells per scale height as a lower limit to solve for the VSI. Body modes generate faster for 101 cells per scale height and higher resolution, responsible for the energy saturation as it becomes more turbulent until reaching steady state. We conclude that our model of VSI turbulence shows converged properties and is now suitable to introduce photoevaporation processes in the future for more realistic simulations.

The RoSSBi code: a multi-fluid scheme for Keplerian disks Clément Surville

Disk evolution and planet formation involve several processes: fluid instabilities, dust-gas interactions, shocks and heat transport, gravitational effects, among the most dominant. Moreover, recent direct imaging of young disks imposes additional constrains on the global scale structures, which challenge the theoretical models. I designed the RoSSBi code in that framework to produce accurate numerical experiments of Keplerian disks. I will present the main ingredients of this Finite Volume scheme, that make it unique and efficient to conduct global models of dusty disks, and to tackle the most recent challenges in the field.

Particle-Fluid Hybrid Methods in the Code Andrea Mignone

I will present the implementation of a new particle module describing the physics of dust grains coupled to the gas via drag forces. The proposed particle-gas hybrid scheme has been designed to work in Cartesian as well as in cylindrical and spherical geometries. The numerical method relies on a Godunov- type second-order scheme for the fluid and an exponential midpoint rule for dust particles which overcomes the stiffness introduced by the linear coupling term. Besides being time-reversible and globally second-order accurate in time, the exponential integrator provides energy errors which are always bounded and it remains stable in the limit of arbitrarily small particle stopping times yielding the correct asymptotic solution. Such properties make this method preferable to the more widely used semi-implicit or fully implicit schemes at a very modest increase in computational cost. Coupling between particles and grid quantities is achieved through particle deposition and field-weighting techniques borrowed from Particle-In-Cell simulation methods. In this respect, we derive new weight factors in curvilinear coordinates that are more accurate than traditional volume- or area-weighting. A comprehensive suite of numerical benchmarks is presented to assess the accuracy and robustness of the algorithm in Cartesian, cylindrical and spherical coordinates. Particular attention is devoted to the streaming instability which is analyzed in both local and global disk models. The module is part of the PLUTO code for astrophysical gas- dynamics and it is mainly intended for the numerical modeling of protoplanetary disks in which solid and gas interact via aerodynamic drag.

Multi-Species Protoplanetary Disks Pablo Benitez-Llambay

In order to unravel the processes driving the evolution of protoplanetary disks it is critical to accurately model and solve numerically the self-consistent dynamics of gas and dust species. Several fundamental processes in protoplanetary disks in which dust dynamics plays an important role are usually investigated in the realm of monodisperse dust distributions. In this talk, I will present and describe an asymptotically stable numerical scheme to solve the momentum transfer between multiple species that conserves momentum to machine precision. I will also discuss its implementation in the publicly available code FARGO3D and show how this implementation correctly describes the self-consistent aerodynamic coupling between gas and multiple dust species. This framework and its implementation in a publicly available code open up new opportunities for investigating a wide range of fundamental processes occurring in multi-species protoplanetary disks and planet formation, including, for example, resonant drag instabilities and the structure and observational signatures of protoplanetary disks.

On streaming instability in pressure maxima Guillaume Laibe

Spatially resolved observations suggest that young planets may cohabit with millimeter dust grains in young discs. In that respect, can streaming instability develop at specific locations and catalize the formation of planetary cores while preserving a population of pebbles in the rest of the disc? We will discuss the potential role played by pressure maxima in this scenario.

Gas accretion damped by dust back-reaction Matías Gárate

In protoplanetary disks, accretion can be driven by turbulent . However, in regions with high concentrations of solids, the dust back-reaction can slow down, and even reverse the gas accretion flow. We find that at the water snowline, which acts as a traffic jam for solids due to the change in composition and sticking properties, the dust back-reaction can stop the gas accretion, and enhance the concentration of large particles, but only if the dust reservoir is large enough, if the disk has an initially high dust-to-gas ratio, and if the viscous turbulence is low.

Dust and gas drag in disks Jean-François Gonzalez

In protoplanetary disks, gas and dust are coupled via the aerodynamic drag. The drag of dust on gas, or back- reaction, is often neglected in situations where the dust-to-gas ratio is small. We will revisit the expressions for the radial velocities of both phases and show that the effect on the gas motion is stronger than usually assumed. We will then give illustrations with practical cases.

Simulations of the Onset of Collective Motion of Sedimenting Particles Vincent Carpenter

Niclas Schneider and Gerhard Wurm have conducted experiments at the University of Duisburg-Essen in which they drop hollow glass beads through a rotating chamber filled with air, and have observed a transition in the sedimentation behavior of the particles. For average dust to gas ratios above 0.08 (across the entire chamber), individual particles that are located in closely packed groups sediment faster than isolated particles, with an amount of addition speed that depends on how closely packed the groups are. We attempt to replicate these experiments with numerical hydrodynamics simulations using the Pencil Code, both to validate the code and to allow for detailed exploration of the mechanisms involved in triggering the collective motion. This work is ongoing; here we discuss the current status of the project and present the latest results, indicating agreement with the experiment.

Protoplanetary Disk Rings as Sites for Formation Daniel Carrera

ALMA images have shown that axisymmetric dust rings are a ubiquitous feature of young protoplanetary disks. These rings must be caused by pressure bumps in the gas profile; a small bump can induce a traffic jam-like pattern in the dust density, while a large bump may halt dust migration entirely. The increased dust concentration may trigger planetesimal formation by the streaming instability. Here we present the first large scale simulations of planetesimal formation in the presence of a pressure bump. We model a large 3D shearing box with a solar-like metallicity of Z = 1%, including the particle back-reaction and self-gravity. Starting with a uniform pressure profile, we simulate the gradual growth of a Gaussian pressure bump. We find that even a small pressure bump can naturally lead to the formation of of planetesimal formation by the streaming instability. A pressure bump does not need to fully halt particle migration for the SI to become efficient. Therefore, it seems likely that dust rings are planetesimal factories. Importantly, this is the first time that the SI has been shown to work in a simulation with no initial enhancement in metallicity. Overcoming that concern helps cement the SI as the leading model of planetesimal formation.

Linear and Nonlinear Evolution of Multi-species Streaming Instability Chao-Chin Yang

- Thursday 16.4.2020 -

Evolution of the Water Snowline in Magnetized Protoplanetary Disks Shoji Mori

Dust Settling Instability in Protoplanetary Disks Leonardo Krapp

The streaming instability has been identified as a promising mechanism to concentrate solids and promote planetesimal formation in the midplane of disks. It has been demonstrated in Squire & Hopkins (2018) that a related settling instability (here DSI) occurs as particles sediment towards the midplane. However, the ability of the DSI to concentrate solids and generate turbulence is yet to be addressed. To shed light on this aspect, we present a systematic study of the saturated state of the DSI by performing a series of numerical simulations with the multi-fluid version of the FARGO3D code. We furthermore have extended the existing linear analysis to more realistic scenarios including particle size distributions and background disk turbulence. Our findings suggest that particle clumping is too weak to trigger planetesimal formation during the settling of particles, but the DSI could generate weak levels of turbulence in otherwise nearly laminar regimes.

Disk structures from the variation of disk ionisation Timmy Delage

Disk ionisation is key in understanding how the magneto-rotational instability (MRI) operates to drive the turbulence in protoplanetary disks. In particular, ionisation drives the so-called Dead Zones. Previous works have shown that a Dead Zone can efficiently trap dust particles at its outer edge when implemented into dust/gas evolution models. Therefore, dead zone trapping can be a promising mechanism to explain the current ALMA observations of transition disks. However, those works treated the dead zone outer edge as a free parameter neglecting that it is actually constrained by the disk ionisation. In this talk, I will discuss a new method to account for ionisation in the context of combining Dead Zone and dust/gas evolution models. The necessity of it will be motivated by conducting a parametric analysis on what parameters can influence the Dead Zone outer edge. I will show that disk structures (stellar and disk mass) as well as dust properties can have a significant impact on it.

Dust growth in hydrodynamic models of protoplanetary disks Joanna Drazkowska

Dust growth is often neglected when building models of protoplanetary disks due to its complexity and computational cost. Nonetheless, it may play a significant role in shaping the evolution of the protoplanetary disk and it is the first step towards planet formation. I will demonstrate the consequences of including dust coagulation, fragmentation, and back-reaction in 2-D (r-phi) hydrodynamic models of the protoplanetary disk.

Ice Lines in Protoplanetary Disks Sebastian Stammler

Growth and dynamics of pebbles at the ice line Katrin Ros

The growth of millimetre-sized to centimetre-sized pebbles is an important step towards the formation of planetesimals and planets. Around ice lines dust growth processes are influenced by the presence of condensible vapour released when icy particles drift radially inwards and sublimate. Turbulent diffusion leads to outwards transport of part of this vapour, which is then deposited on solids there. Experimental results have shown that the nucleation of new ice on bare dust grains requires a higher vapour pressure than the deposition of vapour on already icy grains, thus favouring the growth of already icy particles. In this talk I will discuss the impact of these processes on pebble growth at the water ice line, showing that icy pebble growth might be facilitated in a narrow region outside of the ice line, whereas bare dust grains diffuse out over the disc. I will also highlight the possible connection to the observed dark rings near ice lines in protoplanetary discs.

How streaming instability and Kelvin-Helmholtz instability can regulate planetesimal formation Konstantin Gerbig

The formation of planetesimals is an exciting yet poorly understood problem in planet formation theory. A prominent scenario for overcoming dust growth barriers in protoplanetary disks is the gravitational collapse of local over-dense regions, producing approximately 100 km sized objects. In recent years, the streaming instability has been shown to generate clumps with sufficiently high particle concentrations for collapse to occur. However, the diffusive properties of the surrounding gas constitute an often overlooked barrier for the onset of gravitational collapse and planetesimal formation. In fact, even in the absence of external turbulence, drag instabilities like the Kelvin-Helmholtz instability and the streaming instability itself induce turbulent diffusion, which can prevent collapse on small scales. In this talk, I will briefly review the effect of Kelvin-Helmholtz stability and streaming instability on the particle-layer of protoplanetary disks, and then present our recent results and relate the characteristic scale set by these instabilities directly to the requirements for planetesimal formation in the gravitational collapse scenario.

Influence of grain growth in thermal structures of protoplanetary discs Sofia Savvidou

The thermal structure of a protoplanetary disc is regulated by the opacity that dust grains provide. However, previous works have often considered simplified prescriptions for the dust opacity in hydrodynamical disc simulations, for example by considering only a single particle size. Instead we perform 2D hydrodynamical simulations of protoplanetary discs where the opacity is self-consistently calculated for the dust population, taking into account the particle size, composition and abundance. We first compare simulations utilizing single grain sizes to two different multi-grain size distributions at different levels of turbulence strengths, parameterized through the α-viscosity, and different gas surface densities. We then discuss how the two grain size distributions, one limited by fragmentation only and the other determined from a more complete fragmentation-coagulation equilibrium, compare to each other and with discs that only include micrometer sized dust. We investigate the dependency of the water iceline position on the α-viscosity, the initial gas surface density at 1 AU and the dust-to-gas ratio. The inclusion of the feedback loop between grain growth, opacities and disc thermodynamics brings to light significant differences with disc models utilising single grain sizes and will allow for more self-consistent simulations of accretion discs and planet formation in future work.

How gas accretion changes the shape and depth of gaps in protoplanetary discs Camille Bergez-Casalou

The accretion of gas onto giant planets has a large impact on the structure of their surrounding disc. We study this influence to characterize the evolution of the disc and planetary mass in unison. We perform isothermal hydrodynamical simulations with the Fargo2D1D code which allows us to simulate a full disc, ranging from 0.1 to 260 AU. The gas accretion routine is based on recipes from the literature (Kley 1999, Machida et al 2010), using a sink cell approach. We started by comparing the influence of gas accretion onto the gap shape. For our fiducial parameters, we find that the gap shapes of an accreting and a non accreting planet are very similar, making gas accretion hard to observe. On the other hand, we find that gas accretion has a non negligible impact on gas accretion onto the star: a planet with a high accretion rate can reduce the accretion onto the star by a factor 3. We focused then our investigation on the influence of the viscosity and aspect ratio of the disc on gas accretion. At low viscosity, the Rossby Wave Instability is triggered and creates vortices influencing the gas accretion rate onto the planet and onto the star. As gap opening is one of the key processes for gas accretion, we compared the gap opening mass in our simulations to different existing criteria (Crida et al 2006, Fung et al 2014, Kanagawa et al 2015). We find that, depending on the viscosity, gas accretion has a strong influence on the gap opening mass. This implies that if a planet has a high gas accretion rate at low viscosity, then it is harder for the planet to carve a gap (i.e. a larger mass is needed to clear a gap, where we defined a gap like in Crida et al 2006). Studying the impact of gas accretion on the disc is important to help constraining gas accretion via observation.

A massive disk around a 20 Msun YSO Josep Miquel Girart

ALMA very high angular resolution (polarization) observations of a massive YSO, GGD 27 MM1, have allowed to resolve and study in detail the properties of the accretion disk around this YSO. We derived the density and temperature structure of the disk. We find that the disk is compact (R disk ≃ 170 au) and massive (≃5 M☉), at about 20% of the stellar mass of ≃20 M☉. We compare these properties with those found in low mass disks and discuss about the feasibility of planet formation in this disk.

The polarized gate to planet formation Gesa H.-M. Bertrang

Dust grains, the building blocks of planets, are of more complex nature than usually assumed in planet formation models. I will present a way to characterize dust grains in more detail, getting access to grain size, shape, and porosity, and at the same time, characterizing magnetic fields in protoplanetary disks by applying the powerful toolbox of polarimetry.

Polarization Observations and Dust Growth in Young Disks Sarah Sadavoy

Grain growth in young protostellar disks is an important first step in planet formation. Observations of dust polarization from self-scattering processes offer an unique opportunity to constrain grain sizes robustly in these young systems, and thanks to the development of high resolution polarization capabilities, such observations are now possible. In this presentation, I will provide an overview of recent observational studies of dust polarization toward young disks from ALMA and the VLA. In particular, I will describe the multitude of young disks with polarization signatures consistent with self-scattering processes and the implications of these signatures for dust grain growth at early times (< 0.5 Myr). I will also discuss why some young disks do not show this polarization signatures and how we can use these non-detections to still study dust grain growth and planet formation. Dust Feedback and Instabilities in Multi-Dimensional Disks Hui Li We present the latest development in studying dust coagulation and feedback in protoplanetary disks, using both 2D and 3D two-fluid disk simulations. We will discuss the interplay among streaming instability, vertical shear instability and Rossby wave instability in 3D disks. Furthermore, we will present results on how dust coagulation can impact the outcome of dust evolution in rings and vortices in disks. Implications for interpreting observations are discussed.

Evolution Of MU69 from a Binary Planetesimal Into Contact By Kozai-Lidov Oscillations And Nebular Drag Wladimir Lyra

The New Horizons flyby of the cold classical object MU69 showed it to be a con- tact binary. The existence of other contact binaries in the 1–10km range, possibly including 67P/Churyumov–Gerasimenko, raises the question of how common these bodies are and how they evolved into contact. Here we consider that the pre-contact lobes of MU69 formed as a binary em- bedded in the Solar nebula, and calculate its subsequent orbital evolution in the presence of gas drag. We find that the sub-Keplerian wind of the disk brings the drag timescales for 10 km bodies to un- der 1 Myr for quadratic-velocity drag, which is valid in the belt. In the Kuiper belt we find that a combination of nebular drag and Kozai-Lidov oscillations is a promising channel for collapse. We analytically solve the hierarchical three-body problem with nebular drag and implement it into a Kozai cycles plus tidal friction model. The permanent quadrupoles of the pre-merger lobes make the Kozai oscillations stochastic, and we find that when gas drag is included the shrinking of the semimajor axis more easily allows the stochastic fluctuations to bring the system into contact. Evolution to contact happens very rapidly (within 104 yr) in the classical Kozai region up to ≈ 95◦, and within 3 Myr in the drag-assisted non- classical region beyond it. The synergy between J2 and gas drag widens the window of contact to 80◦–100◦ initial inclination, over a larger range of semimajor axes than Kozai and J2 alone. As such, the model predicts a low occurence of binaries in the asteroid belt, and an initial contact binary fraction of about 10% for the cold classicals in the Kuiper belt. The speed at contact is the orbital velocity; if contact happens at pericenter at high eccentricity, it deviates from the escape velocity only because of the oblateness, independently of the semimajor axis. For MU69, the oblateness leads to a 30% decrease in contact velocity with respect to the escape velocity, the latter scaling with the square root of the density. For mean densities in the range 0.3-0.5 gcm−3, the contact velocity should be 3.3 − 4.2 m s−1, in line with the observational evidence from the lack of deformation features and estimate of the tensile strength.