Dust evolution and planetesimal formation

Polarimetry of DG Tau and CW Tau disks with ALMA: clues on the dust properties

Francesca Bacciotti (INAF - Osservatorio Astrofisico di Arcetri)

Polarimetric data of rotating disk/jet systems around Class II sources were obtained with ALMA with the aim of determining the magnetic field configuration for the jet launch. The observed properties, however, support dust self-scattering as the origin of the polarization. In this case we derived from the data information on the distribution of the dust, on the grain size population, and on hidden substructures. This complements the results obtainable from  unpolarized emission, which can to be of great interest in studies of disk evolution.

Planet-induced Meridional Flows of Sub-mm-sized Dust Grains in Protoplanetary Disks

Jiaqing Bi (University of Victoria)

Dust gaps and rings appear ubiquitous in bright protoplanetary disks. Disk-planet interaction with dust-trapping at the edges of planet-induced gaps is one plausible explanation. However, the sharpness of some observed dust rings indicate that dust grains have settled to a thin layer in some systems. We test whether or not dust around gas gaps opened by planets can remain settled by performing three-dimensional, dust-plus-gas simulations of protoplanetary disks with an embedded planet. We find planets that open gas gaps 'puff up' small, sub-mm-sized grains at the gap edges, where the dust scale-height can reach 80% of the gas scale-height. We attribute this dust 'puff-up' to the planet-induced meridional gas flows previously identified by Fung & Artymowicz. We thus emphasize the importance of explicit 3D simulations to obtain the vertical distribution of sub-mm-sized grains around planet gaps. We caution that the gas-gap-opening planet interpretation of well-defined dust rings is only self-consistent with large grains exceeding mm in size.

Protoplanetary Disk Rings as Sites for Planetesimal Formation

Daniel Carrera (Iowa State University)

ALMA images show that axisymmetric dust rings are a ubiquitous feature of protoplanetary disks. These rings are likely caused by gas pressure bumps that concentrate solid particles. The resulting overdensity of dust may trigger planetesimal formation by the streaming instability (SI). In this talk I will present the first 3D simulations of planetesimal formation in the presence of a pressure bump modeled after ALMA observations. We use a large simulation domain with the bump in the center and a realistic dust/gas ratio of Z=0.01. We find that, for cm-size particles, even a small pressure bump produces enough particle concentration to trigger planetesimal formation by the SI; a bump does not need to fully stop particle drift or form a particle trap for the SI to be efficient and converting a large fraction of the solid content into planetesimals. For mm-size particles we find tentative evidence that planetesimal formation may not occur, but additional work with higher resolution is needed. Ultimately, however, our results suggest that for cm-sized particles, planetesimal formation in pressure bumps is an extremely robust process.

How dust fragmentation may be beneficial to planetary growth

Joanna Drazkowska (LMU Munich)

In the pebble accretion scenario, the outcome of planet formation is determined by the sizes and flux of the pebbles drifting through the protoplanetary disk. Many authors studying the pebble-driven planet growth set the size of pebbles and the pebble flux as arbitrary parameters, independent of the protoplanetary disk model. In this talk, I will present what comes out of connecting a self-consistent dust evolution model considering full size distribution obtained in detailed dust coagulation simulation and embryo growth by pebble accretion.

Impact of magneto-rotational instability on grain growth in protoplanetary disks

Munan Gong (Max Planck Institute for Extraterrestrial Physics)

Grain growth in protoplanetary disks is the first step towards planet formation. One of the most important pieces in the grain growth model is calculating the collisional velocity between two grains induced by the turbulent motion of the gas. The collisional velocities in previous works are obtained based on the assumption that the turbulence is hydrodynamic with the Kolmogorov power spectrum. However, realistic protoplanetary disks are magnetized, and turbulent motions can be induced by the magneto-rotational instabilities (MRI).To understand the impact of MRI turbulence on grain growth, we carry out 3D MHD simulations of the MRI as well as driven turbulence, for a range of physical and numerical parameters. We find that the convergence of the turbulence α-parameter does not necessarily imply the convergence of the energy spectrum. The MRI turbulence is largely solenoidal, for which we observe a persistent kinetic energy spectrum with a -4/3 slope, shallower than the -5/3 slope in the Kolmogorov turbulence. This power-law slope appears to be converged in terms of numerical resolution, and to be due to the bottleneck effect. The turbulence autocorrelation time is nearly constant at large scales, limited by the shearing timescale, and shows a power-law drop at small scales, with a slope close to -1. The deviation from the standard picture of the Kolmogorov turbulence can change the collisional velocities of small grains by orders of magnitude. To understand the dust growth in protoplanetary disks, better computational techniques are needed to achieve the large dynamical range necessary for resolving the turbulence spectrum in the future.

Self-­scattering of non­-spherical dust grains

Florian Kirchschlager (University College London (UCL))

The understanding of millimetre polarisation has made a leap forward since high-resolution imaging of ALMA came available. Amongst other effects, self-scattering (i.e., scattering of thermal dust emission on other grains) is thought to be the origin of millimetre polarisation. This opens the first window to a direct measurement of dust grain sizes in regions of optically thick continuum emission as it can be found in protoplanetary disks. However, the newly derived values of grain sizes are one order of magnitude smaller than those obtained from more indirect measurements as well as those expected from theory, and we see the origin of this contradiction in the application of a perfect sphere as dust model in today’s self-scattering simulations. We have therefore studied self-scattering of non-spherical dust grains using discrete dipole approximation simulations. Combined with the intrinsic polarised emission of non-spherical grains, we find significant deviations of the resulting self-scattering polarisation when comparing non-spherical to spherical grains from nano- to millimetre size. In particular, tremendous deviations are found for the polarisation signal of grains when observed beyond the Rayleigh-regime, i.e. for >100 μm size grains observed at 870 μm wavelength. Furthermore, we find that for some conditions the emerging polarisation signal is even flipped in orientation by 90 degree. These results show clearly that the perfect compact sphere is an oversimplified model and that it is necessary to re-evaluate the dust grain sizes derived from (sub-)mm polarisation in order to understand the dust-growth process in protoplanetary disks.

Dust dynamics during protoplanetary disk formation

Ugo Lebreuilly (CEA)

Dust is an essential component of protoplanetary disks. The dust grains are the seeds of planet formation, they control the coupling with the magnetic field through the values of the non-ideal magnetohydrodynamics resistivities and their continuum emission is extremely useful to observe the disks in details (e.g. as HL Tau in ALMA Partnership et al., 2014).  In the diffuse interstellar medium (ISM) dust grains represent a small fraction of the mass (about 1 %, Mathis et al.1977 ; Weingartner & Draine 2001) and their size distribution is well modeled by the MRN power law (Mathis et al. 1977). The MRN distribution assumes grains with sizes comprised between a few nanometers and less than a micron. In the dense parts of the ISM, there are increasing evidences of the presence of larger grains formed via grain growth. The coreshine effect could for example be explained by the presence of 10 micron grains in the densest parts of molecular clouds (Pagani et al., 2010). Even larger 100 microns grains could be present in Class 0 et Class I young stellar objects (Kataoka et al., 2015, 2016; Pohl et al., 2016; Sadavoy et al., 2018a,b, 2019, Galametz et al. 2019). In this presentation, I will review recent results that we presented in Lebreuilly et al. 2020. In this work, we described in details the conditions for a gas and dust decoupling using the RAMSES code (Teyssier 2002) and its dust module (Lebreuilly et al., 2019). These conditions are required to form protoplanetary disks that are initially significantly enriched in large dust grains. When the largest dust grains of the dust size distribution have a typical size of 10-100 microns, the dust-to-gas ratio can increase by a factor of a few in the central regions of the protostellar collapse, e.g. in the protoplanetary disk. These variations have strong implications for planet formation as protoplanetary disks with 2-3% dust grains might trigger the streaming instability more easily (Youdin & Goodman 2005) and because a large dust-to-gas ratio tends to favor dust growth over the radial drift of large grains (Laibe, 2014).

Earliest compositional bifurcation of planetary building blocks

Tim Lichtenberg (University of Oxford)

Recent astronomical and geochemical evidence indicate early spatial and temporal fragmentation of the planet formation process, whose physical origins remain disputed. Here, we use a coupled numerical model to investigate how the build-up and evolution of the circumsolar disk influence the timing and internal evolution of forming protoplanets. We find that the orbital drift of the water iceline can generate two temporally and spatially distinct bursts of planetesimal formation, which sample different source regions of interstellar materials and experience limited intermixture. Driven by internal radiogenic heating, these two planetary reservoirs compositionally evolve in divergent geophysical modes and recover accretion chronology, thermo-chemical pattern in planetary materials, and mass divergence of inner and outer Solar System. Our numerical experiments suggest that the earliest interplay between disk dynamics and geophysical evolution of accreting planetesimals initiated the chemical fractionation and isotopic dichotomy of the Solar System.

ALMA Characterization of Dust Grains in Sz91 Transitional Disk

Karina Maucó (Universidad de Valparaíso)

We obtained ALMA Band 4 observations (2.1 mm) of the Sz91 transitional disk located in the Lupus III molecular cloud. We detected a ring of dust in the continuum peaking at ~90 au from the central star, consistent with previous observations. The north side of the disk seems to be brighter than the south side (something also found in previous ALMA data at band 7). We also found a bright blob NE, although we still need to estimate how significant this blob is with respect to the general emission from the dust ring. We estimated the spectral index adding band 7 and band 6 archive observations in our analysis and found an alpha value ~3.4. Brightness temperatures for the disk at these wavelengths imply that the emission is optically thin. In this regime, we found a dust opacity coefficient of betta ~1.4. Given these values, and comparing Sz91 with the sample of Pérez et al. (2018), Sz91 is located among the disks with the highest beta coefficients. This implies that a significant fraction of small dust must be present on the disk. However, these values are estimated assuming a power law index of the dust size distribution of 3.5, similar to that of the ISM. Using a lower p coefficient, which is expected if there is grain growth, will increase the maximum grain size for the same betta value. Observations at longer wavelengths (cm) are needed in order to properly probe the spectral index of this source.

Collisions between granular micro-aggregates: conditions for dust agglomeration

Belén Planes (CONICET)

Collisions between dust aggregates are important in many astrophysical environments. In planetary formation it is crucial to understand how a system evolves through collisions, as it is currently unknown how dust aggregates grow to form planetesimals, and eventually, larger bodies like planets. We use complex numerical simulations in micro-scale to study how variations in impact velocity but also in porosity and in the mass difference between the projectile and the target affect the result of the collision. Our interest is to find the threshold values that separate agglomeration from fragmentation. We found that the impact velocity necessary to fracture the target varies with the porosity and the mass ratio between the aggregates, contrary to previous studies that assume it independent of these variables. From our results we conclude that dust growth due to collisional processes could be more feasible than in previous estimates.

Are Small Dust Grains Actually Coupled to the Gas in Protoplanetary Disks?

Evan Rich (University of Michigan)

Dust evolution in protoplanetary disks from small dust grains to pebbles is key to the planet formation process. When modeling and interpreting these protoplanetary disks, a common assumption is that small dust grains (~ 1 micron) are strongly coupled to the gas. We can now independently measure the gas's height and the hight of small dust grains in protoplanetary disks and test the dust gas coupling assumption. Utilizing near-IR polarized light and sub-mm ALMA observations, we measured the scale height and flare angle of the small dust grains and CO gas for three protoplanetary disks. We found that in two of the three protoplanetary disks we studied, the gas and the small dust grains have different flare angles, contrary to what is commonly assumed. We computed Monte Carlo Radiative Transfer modeling of a simple protoplanetary disk to show that the flare angle discrepancy is not an observational effect. Finally, simple analytical modeling of protoplanetary disks indicates several disk mechanisms could be responsible, including a difference in gas to dust ratios. This study is the first observational investigation of heights of gas and small dust grains in protoplanetary disks. Future observations with ALMA and near-IR scattered light instruments are needed to see if this trend continues in a larger sample of protoplanetary disks.

Revealing dust disks substructures from multi-wavelength continuum emission

Anibal Sierra (Departamento de Astronomía, Universidad de Chile)

Using dust continuum observations from ALMA in the disks around AS209, GMAur, HD163296, IMLup, and MWC480,  I present results on the radial distribution of solid mass and maximum grain size that can explain the spatially-resolved spectrum of each disk, including or not dust scattering at long wavelengths. In general, in this work we find that the dust surface density and grain size profiles decrease from the inner disks to the outer disks, with local maxima at the bright rings locations, as expected from dust trapping models. The inferred grain sizes from the inner to outer disks vary from ~1 cm to 1 mm.

Planetesimal Formation in Turbulent Protostellar Disks

Jake Simon (Iowa State University)

If indeed planet formation begins much earlier than previously thought, then the formation of planetary constituents, planetesimals, must also occur at an early stage, likely when the protostellar disk is still actively accreting gas onto the protostar through turbulent angular momentum transport processes. In this contribution, I present a series of simulations designed to address the influence of this turbulent angular momentum transport on planetesimal formation via the streaming instability mechanism.  We find that unless there is a significant enhancement of dust to counteract the effects of turbulence, this turbulence acts destructively to prevent the formation of planetesimals.  Thus, in order for planetesimal formation to occur rapidly (as observations suggest), either turbulence must be weak or planetesimal formation must occur preferentially in areas where dust is highly concentrated (i.e., rings). I will conclude my contribution by discussing the implications of these results for the newly emerging paradigm of early stage planet formation.

Optical Depth of Dust Rings in Protoplanetary Disks

Sebastian Stammler (University Observatory, LMU Munich)

High angular resolution radio observations of protoplanetary disks revealed prominent substructures. Many of these young disks show bright dust rings. A puzzling observation is that all of the more closely observed rings seem to share the same optical depth. This indicates that there might be a common mechanism that causes this feature. In this talk I will show that self-regulated planetesimal formation in dust rings can limit the optical depth to the observed values.

Planetesimal formation in the dust ring through the gravitational instability

Sanemichi Takahashi (NAOJ)

We investigate the planetesimal formation through the gravitational collapse of the dust ring. Resent observations found that the dust ring-gap structures are formed in many protoplanetary disks. If these rings are formed by the dust concentration, planetesimals can be formed in the rings.  Especially, the rings formed in the disk around the HL Tau may have large dust mass so that the self-gravity is important. In the case that the rings are formed by the secular gravitational instability (secular GI), which is one of the promising mechanisms of the multiple ring structures, the rings are formed through the contraction due to the self-gravity.  As a result, we expect that the ring can be massive enough to be self-gravitating. Massive ring structures can be unstable against the self-gravitating contraction in the azimuthal direction. When this instability grows in the dust rings, the planetesimal formation through the gravitational collapse is expected. We estimate the mass of the planetesimals formed by the gravitational collapse of the dust rings, especially for the rings formed through the growth of the secular GI.  We give the initial disk model and evaluate the line mass of the dust rings from the dust surface density and the most unstable wavelength of the secular GI. We perform the linear stability analysis of the gravitational collapse for each ring. The mass of the planetesimal is obtained from the most unstable wavelength of the gravitational instability times the line mass of the dust rings. In this talk, we discuss the radial distribution of the mass of planetesimals and the dependence of the instability on the ring thickness.

The effect of dust porosity on millimeter-wave scattering polarization in protoplanetary disks

Ryo Tazaki (University of Amsterdam)

Dust coagulation in a protoplanetary disk is the first step in planetesimal formation. However, the pathway from dust aggregates to planetesimals remains unclear. Both numerical simulations and laboratory experiments have suggested the importance of dust structure in planetesimal formation, but it is not well constrained by observations. We study how the dust structure and porosity alter polarimetric images at millimeter wavelengths by performing 3D radiative transfer simulations. Aggregates with different porosities and fractal dimensions are considered. As a result, we find that dust aggregates with lower porosity and/or higher fractal dimension are favored to explain the observed millimeter-wave scattering polarization of disks. Although we cannot rule out the presence of aggregates with extremely high porosity, a population of dust particles with relatively compact structure is at least necessary to explain polarized-scattered waves. In addition, we show that particles with moderate porosity show a weak wavelength dependence of the scattering polarization. Finally, we discuss implications for dust evolution and planetesimal formation in disks.

Observations of edge-on protoplanetary disks with ALMA: measuring vertical settling of dust grains

Marion Villenave (IPAG)

Disks seen from the edge are of particular interest as they provide a unique point of view to determine their vertical extent. In this talk, I will present ALMA continuum high angular resolution observations of 12 edge-on protoplanetary disks located in nearby star-forming region. Six sources are resolved along their minor axis in at least one millimetric band, providing direct information on the vertical distribution of the millimeter sized particles. The comparison of a generic radiative transfer disk model with our data shows that at least three disks are consistent with a small millimeter dust scale height, of a few au (measured at r = 100 au). This is in contrast with the more classical value of h_gas ∼ 10 au derived from scattered light images and from gas line measurements. These results confirm, by direct observations, that large (millimeter) grains are subject to significant vertical settling in protoplanetary disks.

Formation of giant planet via disk gravitational fragmentation

Eduard Vorobyov (Department of Astrophysics, University of Vienna)

The core accretion theory has difficulty in explaining the entire spectrum of orbital distances of known exoplanets, in particular of those found at tens and hundreds of astronomical units from the host star. I demonstrate that exoplanets on orbits of tens of au can form via disk gravitational fragmentation. I use the numerical hydrodynamics code FEOSAD, which computes the joint dynamics of gas and dust in a protoplanetary disk including dust-to-gas friction and dust growth. The gas-dust clumps first form at distances on the order of a hundred au and have masses on the order of tens of Jupiter's, but random close encounters between them lead to their inward migration and tidal truncation. The inward-migrating clump forms a compact dust-enriched protoplanetary seed with a mass on the order of Jupiter, while losing most of its outer diffuse atmosphere via stellar tidal torques. The inward migration usually stops at a few tens of astronomical units from the star. This mechanism can explain the existence of gas giants on wide orbits is such systems as HR 8799.