Building Blocks of Planets 2020 - Abstract Booklet - Tuesday 14.4.2020

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
solar system 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 drag 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 protoplanetary disk 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 cosmic dust 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 Stars
                                         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 star-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 planetesimals 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

Particle-Fluid Hybrid Methods in the PLUTO 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 viscosity. 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 Planetesimal 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 Kuiper Belt 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 asteroid 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.