Role and origin of protoplanetary disk substructures

Tightly wound spirals driven by buoyancy resonances

Jaehan Bae (Carnegie Institution for Science)

Spiral arms are one of the most common types of substructures revealed in recent high-resolution observations of protoplanetary disks. They are often interpreted as an outcome of the gravitational interaction between the disk and embedded planets. In this talk, I will show that planets can generate a family of tightly wound spirals through buoyancy resonances, in addition to those driven by the Lindblad resonances. Buoyancy resonances produce predominantly vertical motions whose magnitude is of order of 100 m/s for Jovian-mass planets, sufficiently large to detect using molecular line observations with ALMA. Based on the morphology and magnitude of the perturbation, we propose that the tightly wound spirals seen in the velocity map of TW Hya could be driven by a (sub-)Jovian-mass planet at 90 au.

A young cousin to our solar system?

Gesa H.-M. Bertrang (MPIA (Heidelberg))

Observations of protoplanetary disks present usually a steady-state-view on the actually very dynamical process of planet formation. We went further and gathered a multi-epoch data set, combining new observations and archival data, on one of the closest Herbig disks, HD169142. This allowed us to not only trace this highly structured disk at unprecedented spatial resolution, but more so, to discover moving structures and a shadow, pointing towards a massive (1-10Mj) Jupiter on an only 12au wide orbit around its host star; an orbit, comparable to our own gas giants’. This is the best constrained out of now 5 proto-planetary candidates harbored in this disk. Combining our optical observations with very recent ALMA data, we even directly observe a ~4.5au wide dust trap indicating more dynamical interactions in this disk.  With our approach of multi-epoch, multi-wavelength observations together with radiative transfer and hydrodynamical models, we are able to trace not the steady-state but the dynamical process of planet formation in this candidate of a young version of multi-planet systems such as HR8799 or our own Solar System.

Mapping Icy Grains in the Circumstellar Disk of AB Aurigae

Sarah Betti (University of Massachusetts Amherst)

The formation of planets within circumstellar disks is highly dependent on local disk conditions, (circum)stellar activity, and the distribution and phase of materials necessary for formation. One important key to interpreting observational signatures of planet formation is understanding the role icy grains play in the formation of planets. Icy grains aid grain coagulation and thus planetesimal growth, making them an important component of giant planet formation.  We use LBT LMIRCam data to map the transitional disk around the young (~4 Myr) Herbig Ae/Be star AB Aur in scattered light at the 3.08 μm ice feature.  We present preliminary reference differential imaging (RDI) subtracted images of the disk at 3.08μm and at bracketing wavelengths (K and L’) and use radial and azimuthal profiles to probe the sub-structure and asymmetries of icy dust grains within the disk.

Observational Predictions for the Early Stages of Planet Formation

Diana Blanco (CSU Northridge / UC Santa Cruz)

Within circumstellar planet forming disks, dust grains are thought to grow, coagulate, and finally condense into planetesimals, seeding the formation of planets. High angular resolution observations at sub-millimeter/millimeter wavelengths of disks surrounding newly born stars have revealed that the morphological structure of these systems is composed of large-scale azimuthally symmetric rings, gaps, and holes as well as asymmetric spiral arms, arcs, and crescents. Despite the low level of exoplanet detections in these systems, theoretical studies predominantly label existing substructures as signposts of yet undetected planets and classify disk features as a result of planet-disk interactions. Substructures within disks are predicted prior to planet formation and are in fact necessary to catalyze the formation of planetesimals. Throughout this work, we investigate the development, evolution and possible appearance of these structures via 3D hydrodynamical radiative numerical simulations which follow the evolution of gas and dust in a protoplanetary disk (PPD). We discuss the predicted spatial structure in the solid component of the disk due to the vertical shear instability. In light of recent detections of analogous structures in an ever increasing sample of PPDs, we present observational predictions with current and future radio interferometers whose synergy is ultimately integral in understanding the formation of planets.

Fingerprints of giant planets in the composition of solar twins

Richard Booth (Imperial College London)

The Sun shows a ∼10 per cent depletion in refractory elements relative to nearby solar twins. It has been suggested that this depletion is a signpost of planet formation. The exoplanet statistics are now good enough to show that the origin of this depletion does not arise from the sequestration of refractory material inside the planets themselves. This conclusion arises because most sun-like stars host close-in planetary systems that are on average more massive than the Sun's. Using evolutionary models for the protoplanetary discs that surrounded the young Sun and solar twins, we demonstrate that the origin of the depletion likely arises due to the trapping of dust exterior to the orbit of a forming giant planet. In this scenario, a forming giant planet opens a gap in the gas disc, creating a pressure trap. If the planet forms early enough, while the disc is still massive, the planet can trap ≳100 M⊕ of dust exterior to its orbit, preventing the dust from accreting on to the star in contrast to the gas. Forming giant planets can create refractory depletions of ∼ 5-15 per cent, with the larger values occurring for initial conditions that favour giant planet formation (e.g. more massive discs that live longer). The incidence of solar twins that show refractory depletion matches both the occurrence of giant planets discovered in exoplanet surveys and `transition' discs that show similar depletion patterns in the material that is accreting on to the star.

The observational impact of dust trapping in self-gravitating discs

James Cadman (University of Edinburgh)

The gravitational instability (GI) may play an important role in the very early evolution of protoplanetary discs by providing a source of viscosity, generating conditions suitable for accelerated dust growth, and potentially forming giant gaseous protoplanets through the direct gravitational collapse of disc material. It is now possible to observe young discs with telescopes such as ALMA. However a self-gravitating phase is likely to be brief (t<10^5 yrs) as a disc will rapidly evolve toward a lower mass, gravitationally stable state, thus limiting our prospects of observing these systems. In this work we use Monte Carlo radiative transfer models to analyse how dust-trapping in the spiral regions of self-gravitating discs results in enhanced emission, and we place constraints on the disc properties required to drive spiral features detectable with ALMA. We also analyse the Elias 27, WaOph 6 and IM Lup disc observations from the DSHARP survey, and make predictions about the nature of the spiral substructure which has been observed.

Magnetically-induced Substructures in Outer Protoplanetary Disks

Can Cui (University of Cambridge)

It has been revealed that the majority of protoplanetary disks posses annular substructures. While the planet-disk interaction is the most popular explanation, there is another category which is planet-free. In this work, we perform global 3D simulations with ambipolar diffusion to model the outer part of the disk and to study the magnetic flux concentration. With fine resolution to resolve the MRI, we find that the disk is weakly MRI turbulent. Magnetic fields can induce annular substructures self-consistently, and the magnetic flux concentration is generic and stochastic. The angular momentum transport is dominated by MHD disk winds, with minor contribution from the MRI turbulence. The plausible mechanisms that drive the magnetic flux concentration are also discussed.

Asymmetric Discs Resulting From Planet Formation

Daniel Cummins (Imperial College London)

High-resolution imaging of protoplanetary discs has revealed their wealth of substructure, and how these relate to planet formation processes is a major question in planet formation theory. A small fraction of discs show large-scale crescent-shaped features, which have been interpreted as large quantities of dust trapped in anticyclonic vortices. Such regions of high dust-to-gas ratios are expected to promote planet formation processes, so understanding their formation and evolution is of primary interest. Gas-only hydrodynamics simulations have demonstrated that the thermal feedback from a planetary embryo undergoing rapid formation by pebble accretion can trigger the generation a large-scale vortex. However, the ability for such a vortex to trap dust and the impact this has on the culpable planet are yet to be investigated. I will present results from gas and dust hydrodynamics simulations containing a rapidly accreting planet, showing the efficiency with which dust grains accumulate in the resulting vortex, and discuss the consequences this has for the growth of the planetary embryo.

Dust Depleted Inner Disks in a Large Sample of Transition Disks through Long-baseline ALMA Observations

Logan Francis (University of Victoria)

Transition disks with large inner dust cavities are thought to host massive companions. However, the disk structure inside the companion orbit and how material flows toward an actively accreting star remain unclear. We present a high-resolution continuum study of inner disks in the cavities of 38 transition disks. Measurements of the dust mass from archival Atacama Large Millimeter/Submillimeter Array observations are combined with stellar properties and spectral energy distributions to assemble a detailed picture of the inner disk. An inner dust disk is detected in 18 of 38 disks in our sample. Of the 14 resolved disks, 8 are significantly misaligned with the outer disk. The near- infrared excess is uncorrelated with the mm-dust mass of the inner disk. The size–luminosity correlation known for protoplanetary disks is recovered for the inner disks as well, consistent with radial drift. The inner disks are depleted in dust relative to the outer disk, and their dust mass is uncorrelated with the accretion rates. This is interpreted as the result of radial drift and trapping by planets in a low α (∼10−3) disk, or a failure of the α-disk model to describe angular momentum transport and accretion. The only disk in our sample with confirmed planets in the gap, PDS 70, has an inner disk with a significantly larger radius and lower inferred gas-to-dust ratio than other disks in the sample. We hypothesize that these inner disk properties and the detection of planets are due to the gap having only been opened recently by young, actively accreting planets.

Generating Vortices with Slowly-growing Gas Giant Planets

Michael Hammer (University of Arizona)

Gap-opening planets can generate dust-trapping vortices that may be responsible for some recently-discovered crescent-shaped dust asymmetries in transition discs. Although this model can explain some of these features well, most previous numerical studies of vortices have neglected the time it takes to grow a planet to Jupiter-size, a process that may last more than 1000 orbits. In our work, we incorporate more realistic planet formation timescales into two-fluid (gas and dust) hydrodynamical simulations, which we use to generate synthetic ALMA images of planet-induced vortices. Unlike with instant planet growth, we find that planets generate vortices that are elongated instead of compact. Unlike more compact vortices, the dust in these elongated vortices circulates instead of settling at the center, frequently creating two observational signatures: (1) an elongated azimuthal extent and an (2) off-center peak. With α < 10^-4, we find that Saturn-mass planets in discs with H/R <= 0.06 are more likely to be accompanied by dust asymmetries compared to Jupiter-mass planets because they can trigger multiple generations of vortices in succession. We also find that vortices with H/R >= 0.08 survive >6000 planet orbits regardless of planet mass because they are less affected by the planet's spiral waves.

A Tale of Two Grains: Understanding HL Tau In A Nonideal Way

Xiao Hu (University of Virginia)

Recent high resolution observations unveil ring structures in circumstellar disks. The origin of these rings has been widely investigated under various theoretical scenarios. With prescribed magnetic diffusion profiles calculated from dust evolution, we find rings and gaps can be formed in gaseous structure, using global 3D non-ideal MHD simulations including effects from both Ohmic dissipation and ambipolar diffusion. Disk ionization structure varies across the snowline and leads to different accretion rates. Similar scenario is repeated in 2d global MHD simulations, coupled with self-consistent ray-tracing radiative transfer, thermochemistry, and non-ideal MHD diffusivities. We found that dust size plays a crucial role in the ring formation around the snowlines of protoplanetary disks (PPDs) through the accretion process. Disk ionization structures and thus tensorial conductivities depend on the size of grains. When grains are significantly larger than PAHs, the non-ideal MHD conductivities change dramatically across each snow line of major volatiles, leading to a sudden change of the accretion process across the snow lines and the subsequent formation of gaseous rings/gaps there.

Spiral Structures Observed in CO Emission around RU Lup and GM Aur

Jane Huang (University of Michigan)

While complex millimeter continuum structures have now been mapped by ALMA for a number of disks, comparatively few of these have been accompanied by deep gas observations. I will present our recent CO observations of RU Lup and GM Aur, obtained as part of DSHARP follow-up and as part of the MAPS ALMA Large Program, respectively. In contrast to their largely axisymmetric, multi-ringed millimeter continuum, the CO emission of both of these disks reveal large-scale, non-Keplerian spiral arms. The irregular kinematics and morphological resemblance to some younger embedded systems suggest that RU Lup's and GM Aur's spirals could arise due to continuing interactions with their environments.

New dust substructure trends in DSHARP: A super-resolution analysis with Frankenstein

Jeff Jennings (University of Cambridge) 

The DSHARP survey evidenced the ubiquity of annular substructure in the mm dust distribution of protoplanetary discs. Intriguingly, a significant amount of super-resolution information can be further extracted from these datasets, yielding tighter constraints on known substructures and identification of new ones. In this talk I overview results from a super-resolution analysis of all 20 DSHARP sources using Frankenstein (“frank”), which nonparametrically fits the visibilities to reconstruct a disc's 1D brightness profile. This analysis identifies several new trends in disc structure, including: substructure in all of the sample's most compact discs (DoAr 33, HT Lup, SR 4, WSB 52); substructure interior to 10 AU in larger discs (GW Lup, Sz 114, Elias 20, Elias 24); inner holes indicative of transition discs (HD 143006, Sz 129);substructure in the cores of spiral discs (Elias 27, IM Lup, WaOph 6); and new features in the survey's most structured discs (AS 209, HD 142666, HD 143006, HD 163296).

Planet Formation via Self-Sustaining Vortices

Kundan Kadam (Konkoly Observatory, Budapest, Hungary )

Starting with the iconic image of HL Tauri, showing a grand-design ring structure, ALMA has revolutionized the way planet formation may be observed in the cradles of protoplanetary disks. Since then, disk substructure and asymmetries such as gaps, rings, horseshoes, spiral arms, and inner holes seem to be a norm in such systems. One of the fundamental questions raised by these observations is that how the planets are formed so early in these young disks which are typically less than a million years old. In this presentation, I will talk about our latest findings related to new aspects of planetary system formation. With the help of numerical hydrodynamic simulations including the dust evolution as well as dust-dependent viscosity, we found that a protoplanetary disk can form a cascade of self-sustaining, small-scale vortices. These vortices collect large amounts of dust and are secularly stable, thus offering sites for overcoming traditional barriers to dust growth. The phenomenon of self-sustaining vortices can explain rapid formation of planetary systems, while being consistent with several observational constraints put forward by recent observations.

Substructures in disks around Very Low Mass Stars

Nicolas Kurtovic (Max Planck Institute for Astronomy)

The discovery of giant planets orbiting very low mass stars (VLMS) and the recent observed substructures in disks around VLMS is challenging planet formation models. Specifically, radial drift of dust particles is a catastrophic barrier in these disks, which prevents the formation of planetesimals and therefore planets. In this work, we aim to estimate if structures such as cavities, rings and gaps are common in disks around VLMS, and to test models of structure formation in these disks. We also aim to compare the radial extent of the  gas and dust emission  in disks around VLMS, which can give us insights about radial drift.
We studied six disks around VLMS in the Taurus star forming region using ALMA Band 7 (340GHz) at a resolution of0.1''. Our observations resolve the disk dust continuum in all disks. In addition, we detect the 12CO (J=3-2) emission line in all targets and 13CO (J=3-2) in five of the six sources. The angular resolution allows the detection of dust substructures in three of the six disks, which we studied by using uv-modeling. The other three disks with no observed structures are the most compact and faintest in our sample, with the radius enclosing the 90% of the continuum emission varying between 13~21au. When taking the 12CO emission to determine the gas disk extension Rgas, the ratio of Rgas/Rdust in our sample varies from 2.3 to 6.0, which is consistent with models of radial drift being very efficient around VLMS in the absence of substructures. Given our limited angular resolution, substructures were directly detected only in the most extended disks, which represent 50% of our sample, and there are hints of unresolved structured emission in one of the bright smooth sources. Our observations do not exclude giant planet formation on the substructures observed.

Dust rings from multi-band ALMA observations in protoplanetary disks

Feng Long (Center for Astrophysics | Harvard & Smithsonian)

Concentric gaps and rings are frequently seen in the continuum images of disks, while their origins are still heavily debated and their implications for planet formation are also inconclusive. We present ALMA observations at 1.3 and 3 mm for three disks, selected from our previous ALMA Taurus structure survey. At comparable spatial resolution of 0.1 arcsec, dust substructures identified at 1.3 mm are now also observed at corresponding locations at the longer 3 mm wavelength. The measured spectral index increases outward and exhibits local minima that correspond to the peaks of dust rings, indicative of the changes in grain properties across the disks and dust substructures. The low optical depths in our dust rings suggest that grains in the rings may have grown to millimeter sizes. The similar dust emission morphology, as well as disk radial extension at both wavelengths, is consistent with the scenario of dust trapping in local pressure maxima. A direct comparison of ring width in testing dust trapping models is however challenged by the narrowness of the rings.

Wide, deep cavities in gas and dust: simulations versus observations

Dhruv Muley (University of Victoria)

In recent years, high-resolution imaging of gas and dust has enabled observers to probe protoplanetary-disk substructure with unprecedented accuracy. Of particular interest for such studies have been transition disks, defined by the existence of large interior cavities. Using hydrodynamical simulations, we investigate the hypothesis that these may have been carved by super-Jupiter companions. We find that planets above 3 Mj reliably become eccentric via higher-order Lindblad resonances; when such planets are allowed to accrete, they clear a wide swathe of the disk. The resulting disks also show a wide separation between the gas gap (visible in 12/13-CO) and the outer dust-trapping pressure maximum (probed millimeter continuum), which without eccentricity would require a brown-dwarf (>15 Mj) companion. Recent observations show many transition cavities with this wide separation, incommensurate with the low occurrence rate of brown-dwarf companions; it thus stands to reason that many transition cavities may indeed be carved by eccentric, accreting super-Jupiters.

Observational consequences of planet migration

Pooneh Nazari (Leiden University)

Recent observations of protoplanetary discs show spectacular structures such as rings and gaps. Some of these structures are due to planets but the effect of planet migration has not been considered fully. This talk is about how planet migration can affect these structures and whether such effects are detectable using synthetic multi-wavelength ALMA observations. Three morphologies that a migrating planet can produce depending on its speed are identified and it is discussed whether it is possible to distinguish a migrating planet from other mechanisms producing similar effects.

ATCA reveals dust traps at the edge of cavities in transition discs

Brodie Norfolk (Swinburne University)

The origin of the inner dust cavities observed in transition discs remains unknown. The segregation of dust and size of the cavity is expected to vary depending on which clearing mechanism dominates grain evolution. A multi-wavelength survey of TDs  comparing 8.8mm observations from ATCA and sub-mm observations from ALMA has revealed that emission from both wavelengths peak at the same radius from the star. This suggests that the both grain populations are trapped at a local maxima in the gas density induced by either a dead zone or companion.

A Kinematic and Multiwavelength Study of the Spiral Structure in Elias 2-27

Teresa Paneque (ESO)

Elias 2-27 is a young Class II system that harbours a massive protoplanetary disk, with an estimated disk-to-star mass ratio of 0.3. The disk has a characteristic spiral arm structure and an inner dust gap, however the origin of this morphology is still unclear. We present the analysis of new multi-wavelength ALMA continuum data at 0.89mm and 3.3mm, together with the kinematical study of CO isotopologue tracers (13CO and C18O J=3-2). Additionally we perform SPH simulations to compare with the observational results. Our results show tentative dust-trapping signatures at the location of the spiral arms and a highly perturbed gas emitting layer. We determine the spiral structure is likely to be caused by gravitational instabilities.

Characterizing Young, Cool M-Stars and their Planet-Forming Disks

Jamila Pegues (Center for Astrophysics | Harvard & Smithsonian)

M-stars are the most common hosts of planetary systems in the local Galaxy.  Observations of protoplanetary disks around these cool stars are remarkable tools for understanding the environment within which their planets form.  We present a small sample of protoplanetary disks around M-stars (spectral types M4-M5).  Using spectrally and spatially resolved ALMA observations of a range of molecular lines, we measure the dynamical masses of these stars and characterize the chemistry in their disks.  We find that dynamical masses for a combined sample of M-stars exceed fiducial stellar evolutionary model predictions, and we use this discrepancy to constrain the nature of young, cool M-stars. We then find that similar patterns of chemistry exist between our M-star disk sample and solar-type disks, and we investigate hydrocarbons as one important possible exception.  Finally, we discuss future observations, which are crucial for obtaining a holistic view of the chemistry of planet formation around the "coolest" stars.

Five years since HL Tau: What have we learnt?

Daniel Price (Monash University)

I will review our attempts over the last 5 years to understand resolved ALMA imaging of protoplanetary discs using 3D dust-gas hydrodynamics. Since our initial work explaining HL Tau in Dipierro+2015 we have worked to bring simulations and observations ever closer. One of the most exciting outcomes has been finding direct evidence for embedded planets, helping to solve some of the argument about what causes rings and gaps. I will also discuss our attempts to model most of the large-cavity transitional discs in an attempt to explain their nature.

Interpreting high spatial resolution line observations of planet-forming disks with gaps and rings: The case of HD 163296

Christian Rab (Center for Astrochemical Studies ,MPE)

We modelled the dust and gas disk of HD 163296 with the radiation thermo-chemical disk code ProDiMo.  With this model, we study the impact of thermo-chemical processes on observables such as channel maps, radial intensity profiles and rotation velocity. The 12CO 2-1 radial intensity profiles show clear peaks at the position of the dust gaps. At least in two dust gaps, those peaks are not produced by temperature changes in the gaps but by the absorption of line emission from the back side of the disk by the dust. However, temperature changes within the gaps can have a significant impact on the derived rotation velocity profiles; if the dust gaps are very deep (depletion factor > 100). For shallower dust gaps, the measured deviation from the rotation velocity is mostly dominated by the density gradient and hence indicate the presence of gas depletion within the dust gap as proposed by Teague+ 2018.

Are Gravitational Unstable Protoplanetary Discs Rare?

Sahl Rowther (University of Warwick)

We carry out three dimensional SPH simulations to show that a migrating giant planet strongly suppresses the spiral structure in self-gravitating discs.
We present mock ALMA continuum observations which show that in the absence of a planet, spiral arms due to gravitational instability are easily observed. Whereas in the presence of a giant planet, the spiral structures are suppressed by the migrating planet resulting in a largely axisymmetric disc with a ring and gap structure. Our modelling of the gas kinematics shows that the planet's presence could be inferred, for example, using optically thin 13C16.  Our results show that it is not necessary to limit the gas mass of discs by assuming  high dust-to-gas mass ratios in order to explain a lack of spiral features that would otherwise be expected in high mass discs.

Outward Planet Migration in a Low Viscosity HL Tau Disk

Jess Speedie (University of Victoria)

Context: Under the assumption that the observed gaps and rings in the dust disk of HL Tau are carved by planets, the onus falls to theoretical planet formation models to explain how sufficiently massive planets came to have such large orbital radii (tens of au). Typically, a disk viscosity described by the alpha parameter is assumed to be alpha=1e-2–1e-3. In this case, there exists two barriers to forming massive planets at large orbital radii: the rapid depletion of column density (planet-building material) in the disk’s outer regions, and the well-known Type I migration problem.
Methods: We employ a semi-analytic theoretical planet formation model of HL Tau to explore how planets can overcome these two barriers under the assumption of a lower disk viscosity (alpha=1e-4). We individually grow and evolve hundreds of planetary embryos, starting from different orbital radii throughout the disk, over the course of ~2 million years. Planets grow by core accretion and migrate according to disk torques, self-consistently calculated based on our state-of-the-art astrochemistry model.
Results: A low disk viscosity leads to a sustained high column density in the outer regions of the disk, allowing for increased planet growth. In tandem, the lowered viscosity translates the mass-dependence of the outward-directed corotation torque downward to dominate at lower planet masses, and propel planetary embryos to large orbital radii. There, they are trapped and slowly return inwards while accreting to become Super-Earths. We develop an analytic approximation to describe the planet mass at which the corotation torque takes its maximum, and find that it scales as ~alpha^{2/3}.
Conclusions: Our work emphasizes the influence of disk viscosity on planet growth and migration, and describes a mechanism by which Super-Earth mass planets can find themselves stranded many tens of au away from their host star. Future work is needed to determine whether these planets can induce a significant enough change in the pressure gradient to subsequently carve gaps in the disk’s dust component.

Observing the Kinematics of Gaseous Substructures

Richard Teague (Center for Astrophysics | Harvard & Smithsonian)

Not only has ALMA revolutionized our view of the dust in protoplanetary disks, it has offered an unparalleled view of the gaseous component of the disk. In particular, it is now possible to map out the kinematical structure of these disks at an unprecedented precision, allowing for the detection of subtle perturbations attributed to planet-disk interactions. I will present sub-mm observations of the planet formation environment which show a stunning level of substructure in the gas, both in terms of the gas temperature and dynamics. Using novel techniques, I will demonstrate how we can extract the 3D velocity structures of these disks, tracing large-scale flows which can transport volatile material to sites of active planet formation. Furthermore, I will present multi-band observations of spiral substructure, showing that not only do these observations enable us to trace the dynamical structure of the spirals themselves, but probe the local physical conditions and thus directly confront numerical simulations of planet-disk interactions.

Dynamics of giant planets in protoplanetary discs: the case of PDS70

Claudia Toci (Università degli Studi di Milano)

Thanks to instruments such as ALMA and SPHERE we obtained more and more high-resolution images of protoplanetary discs. These images show a large number of gaps and ring-like structures (see e.g. Andrews et al. 2018). Although several explanations for these structures are possible, a natural explanation is the presence of one or more planets embedded in the parental disc, actively shaping the surrounding material (e.g. Lin & Papaloizou 1979, Dipierro et al.2015, Rosotti et al.2017, ..). The disc surrounding PDS 70, where two giant planets have been directly detected and many features are observed (Keppler et al. 2018, Mueller et al. 2019, Haffert et al. 2019, Christiaens et al. 2019) is then an ideal target to study planet-disc interaction. Its inner region (R<50 au, where the planets are) is poor in dust and gas while the outer region shows a dust and gas ring. Sub – mm observations (Isella et al. 2019,…) suggest the presence of a circumplanetary disc surrounding each planet. Near-infrared observations show several features close to the theoretical positions of the planets and also to a broader and brighter feature (Mueller et al. 2018). Here, I will present an analysis of 3D SPH simulations devised to represent ideal cases for the study of planet-disc interaction for the PDS 70 disc, assuming two embedded giant planets. We model both dust and gas, aiming to understand the physical origin of features observed in mm and near-infrared light images. In our simulations, planets are initialised with lower mass and are free to grow and migrate to eventually reach a mean motion resonance, dynamically stable.To compare our models with the observations, we post-processed our near-infrared synthetic images in order to account for observational biases known to affect high-contrast images. I will discuss the formation and evolution of the dust and gas ring and I will show that the features observed at infrared and millimetre wavelengths can be explained by the accretion stream onto the outer planet, without requiring a circumplanetary disc around planet c. Our successful reproduction of the observations indicates that planet-disc dynamical interactions alone are sufficient to explain the observations of PDS 70.

SCExAO/CHARIS High-Contrast Imaging of the HD 34700 Protoplanetary Disk

Taichi Uyama (Caltech IPAC/NExScI)

We present Subaru/SCExAO+Coronagraphic High Angular Resolution Imaging Spectrograph (CHARIS) broadband (JHK-band) integral field spectroscopy of HD 34700 A. CHARIS data recover HD 34700 A's disk ring and confirm multiple spirals discovered by Monnier et al. We set limits on substellar companions of ∼12 MJup at 0"3 (in the ring gap) and ∼5 MJup at 0"75 (outside the ring). Our injection test and comparison with the practical CHARIS results set a robust constraint on a potential substellar-mass companion predicted by Monnier et al. (2019). The data reveal darkening effects on the ring and spiral. Spiral fitting resulted in very large pitch angles (∼30°-50°); a stellar flyby of HD 34700 B or infall from a possible envelope is perhaps a reasonable scenario to explain the large pitch angles.

What can we learn from DS Tau?

Benedetta Veronesi (Università degli Studi di Milano)

Recent high resolution and high fidelity observations of protoplanetary discs are resolving in details protoplanetary discs.  In particular, recent surveys have shown that the majority of these discs are characterised by rings and gaps. What can we learn from these features? Are these discs hiding planets? Multiple theories have been proposed in order to explain such features, involving e.g. chemistry (condensation at snow-lines..) and dynamics (embedded companions). I will present the results we have obtained in order to model the protoplanetary disc orbiting around DS Tau, an M-type star, in the Taurus star-forming region located at a distance of 159 pc. This disc is showing a wide gap (∼30 au) in the continuum at 1.3 mm (Long et al. 2018) and at 2.9 mm (Long et al. 2020). We performed 3D-hydrodynamical (PHANTOM, Price et al. 2018) and radiative transfer (MCFOST, Pinte et al. 2006, 2009) simulations in order to reproduce the observed gap shape at 1.3 mm. In this modeling, we assumed that the observed gap is carved by a planet between one and five Jupiter masses. We fit the shape of the radial intensity profile along the disc major axis varying the planet mass, the dust disc mass, and the evolution time of the system. As best fit model in the 1.3 mmc case, we obtain that if the observed gap is carved by an embedded planet, that planet should have a mass of Mp=3.5 MJup.
Finally, we also computed the CO-isotopologues channel maps of the system with the parameters of the best fitting model, in order to study the expected signature of the planet in the gas kinematics. We found that a planet mass of 3.5 MJup would be able to produce a kink detectable by ALMA with a reasonable beam size and velocity resolution.

A highly non-Keplerian protoplanetary disc: Spiral structure in the gas disc of CQ Tau

Lisa Wölfer (Max-Planck Institut für extraterrestrische Physik)

In the past years, high angular resolution observations have revealed that circumstellar discs appear in a variety of shapes with diverse substructures being ubiquitous. This has given rise to the question of whether these substructures are triggered by planet-disc interactions. Besides direct imaging, one of the most promising methods to distinguish between different disc shaping mechanisms is to study the kinematics of the gas disc. Including kinks in the iso-velocity contours, the deviations of the rotation profile from Keplerian velocity can be used to probe perturbations in the gas pressure profile that might be caused by embedded (proto-) planets. In this work analyze the gas brightness temperature and kinematics of the transitional disc around the intermediate mass star CQ Tau in order to resolve and characterize substructure in the gas, caused by possible perturbers. For our analysis we use spatially resolved ALMA observations of the three CO isotopologues 12CO, 13CO and C18O (J=2-1). We further extract robust line centroids for each channel map and fit a number of Keplerian disc models to the velocity field. The gas kinematics of the CQ Tau disc present non-Keplerian features, showing bent and twisted iso-velocity curves in 12CO and 13CO. Significant spiral structures are detected between 30-180 au in both the brightness temperature and the rotation velocity 12CO after subtraction of an azimuthally symmetric model, which may be tracing planet-disc interactions with an embedded planet or low-mass companion. We identify three spirals, two in the brightness temperature and one in the velocity, spanning a large azimuth and radial extent. Together with the observed large dust and gas cavity, these spiral structures support the idea of a massive embedded companion in the CQ Tau disc.

Temperature dip in rings and formation of more rings

Shangjia Zhang (UNLV)

Substructures are ubiquitous in protoplanetary disks. Often when these disks are observed in higher resolution, more substructures are revealed. Here we devise a new mechanism to form multiple rings followed by one existing ring. Most of the previous mechanisms of forming substructures hinge on spatial changes in pressure, either by varying density or temperature. Our substructure formation mechanism starts with temperature variation due to the distinct opacity between small and big grains dominant in different regions. We confirm previous studies with self-consistent thermal models that shadowing and illumination effects can lead to mild temperature variation. With a self-consistent multi-species dust model, we find that the abrupt transition of opacity can lead to higher temperature variation along the ring. Coupling with 1D dust evolution model, we find that dust can be trapped at the pressure bump outer to the initial ring, thus results in double rings in dust continuum observation. The separation of two rings is close and determined by $\alpha/St$. They can be misinterpreted as a low mass planet carves a narrow gap. This can possibly explain double rings observed in several protoplanetary disks.