Thinkshop 2020

11 May 2020, 09:00 → 15 May 2020, 18:00 Europe/Berlin

Lecture Hall (Leibniz Institute for Astrophysics Potsdam (AIP))

Note that because of the global COVID-19 outbreak, the meeting is postponed to an indefinite time in the future.

Protoplanetary disk dynamics are crucial in defining the ecosystem for planet formation. The shift away from turbulence as the main agent for driving disk evolution towards disk winds has recently gained momentum in the community.

At the same time, both the absence of significant levels of turbulence and the presence of large-scale outflows are gaining observational support. Yet, we are still lacking clear diagnostics for magnetic fields during the T Tauri phase.

The task of extracting useful knowledge about the conditions in planet-forming disks, for instance from molecular-line ALMA data, is paramount. It demands an approach based on forward modelling combined with synthetic observation to understand the relevant dynamical and chemical processes shaping the detected emission features.

Highlighting recent developments, we aim to orient the workshop around this chemo-dynamical link with a special focus on the potential role played by magnetic fields.

Scientific Organizing Committee

Zhaohuan Zhu 朱照寰 Roman Rafikov Ilaria Pascucci

Joan Najita Meredith Hughes Jeremy Goodman

Barbara Ercolano Ilsedore Cleeves Pablo Benítez-Llambay

Confirmed Invited Speakers

• Riouhei Nakatani (RIKEN, Tokyo)
• Gesa Bertrang (MPIA, Heidelberg)
• participating SOC members

Local Organizing Committee

Katrin Böhrs (AIP) Doris Lehmann (AIP) Yori Fournier (AIP)

Philipp Weber (NBIA) Oliver Gressel (AIP) Martín Pessah (NBIA)

Monday, 11 May

12:45 → 13:00 Welcome and Introduction

13:00 → 14:40 Magnetic Winds & Level of Turbulence

13:00 Magnetic flux transport in protoplanetary discs

Magnetic effects are important for accretion disc dynamics. In PPDs, much still needs to be understood in how non-ideal MHD effects (Ohmic, Hall and ambipolar diffusion), which are relevant because of the low ionisation levels found in much of the disc, affect the radial accumulation and retention of magnetic flux needed for processes such as the launching of a magnetic disc wind. I aim to present results from semi-analytic local models incorporating all three non-ideal effects, and how they inform our understanding of the flux transport problem. I will also be presenting some recent numerical simulations exploring the interplay of these non-ideal effects with the magnetic field geometry, and the resulting implications on the flux transport process.

Speaker: Mr Philip Leung

13:25 A systematic exploration of magnetised disc wind solutions in protoplanetary discs

The recent developments in our understanding of the chemical composition, the ionisation equilibrium and the dynamics of protoplanetary discs has led to the conclusion that magnetised disc wind (MDW) are probably playing an important role in shaping the long term evolution of these objects. Most of our understanding of these winds comes from global direct numerical simulations which include complex microphysics and which explore only a very limited subspace of parameters. Hence, it is very difficult to draw firm conclusions about the long-term evolution of discs subject to MDW.

In this contribution, I will present a systematic exploration of MDW solutions, which can then be used in secular models to predict the evolution of a disc, in a way similar to the "alpha disc" model. I will also discuss the solutions topology (top/down symmetry, midplane/surface accretion layers, laminar stress, etc...) which have often been mis-interpreted in the literature.

Speaker: Geoffroy Lesur (Institute of planetology and astrophysics of Grenoble/CNRS)

13:50 Are Protoplanetary Disks Truly Laminar?

Recent years have seen tremendous progress in our understanding of angular momentum transport in protoplanetary disks. It is now thought that accretion is driven primarily by a large-scale vertical field threading the disk, either through magnetically launched winds or large-scale magnetic stresses within the disk plane. However, it remains an open question as to just how laminar these accreting disks are. Even if not the primary source of angular momentum transport, turbulence may still play a crucial role in the multitude of processes involved in forming planets.

In this talk, I will present both theoretical and observational evidence that protoplanetary disks are not necessarily laminar and in some cases can still harbor vigorous turbulence. From ALMA observations of molecular line broadening, we now know of at least one source that presents a clear signature of turbulence. I will present these compelling new observations and compare them to earlier predictions of magnetically driven turbulence. I will also present a number of numerical experiments that indeed show magnetically driven turbulence is still present throughout large regions of the disk, even in the presence of a large-scale magnetic field. I will conclude with an outlook for future observational and theoretical studies and what the results so far imply for our understanding of planet formation and disk evolution.

Speaker: Prof. Jacob Simon (Iowa State University)

14:15 Constraining the Radial Turbulence Parameter $\alpha$ from the Distribution of Water and Refractory species in an Evolving Solar Nebula

The overall structure of the evolving protoplanetary disk is set by the interaction of accretion and dispersal processes such as turbulence, winds and photoevaporation (Ercolano & Pascucci 2017), with finer structural details (as gaps, cavities, spirals and warps) in the disk provided by planet formation (eg. Van der Marel et al. 2015). Recent meteoritic evidence from Kruijer et al. (2017) strongly suggest the presence of two distinct isotopic reservoirs in the <1 Myr solar nebula likely created when Jupiter's formation opened a gap in the disk and separated the nebula into two regions. In this talk, we explore the interdependence of disk structure, accretion and chemistry. We model the radial motions of water vapor and small icy solids in the nebular gas under different assumptions of turbulence and track the bulk water content across heliocentric distance. We then include the formation of a proto-Jupiter within our models and back-track the structure of the solar nebula at various times from spatial and temporal cosmochemical data and constrain the variation of the turbulence paramater $\alpha$. We find that our solar nebula likely had a more turbulent inner disk and a weakly turbulent outer disk, and thus may have been wind-driven. We also find that the solar nebula was likely a transitional disk by 4 Myr. Moreover, we speculate that $\Sigma(r)$ can be a diagnostic of $\alpha(r)$ and therefore, the mechanism of angular momentum transport in the disk.

Speaker: Anusha Kalyaan (Clemson University)

14:40 → 15:00 Shotgun poster session

15:00 → 15:45 Coffee break

15:45 → 17:00 Collapse & Disk Formation

15:45 The regulation of angular momentum by non-ideal MHD

Magnetic fields play a major role in the regulation of angular momentum during the protostellar collapse, hence for the formation of the protoplanetary disk. The magnetic braking is able to slow the rotation and extract the angular momentum from the disk. However, this process is tampered by a decoupling between neutral and charged particles, especially the ions through the ambipolar diffusion. The decoupling is heavily impacted by the chemistry at stake, and particularly by the grain size distribution. I will show how a modification of the grain size distribution enhances the decoupling between the magnetic field and the gas, and how it modifies the distribution and transport of angular momentum.

Speaker: Pierre Marchand (American Museum of Natural History)

16:10 Observational Constraints on Envelope Ionization in Class I Protostars

Ionization rate is one of the most important parameters controlling both the chemical and dynamical processes in protoplanetary disks. What few observational constrains on ionization currently exists suggest overall low ionization, limiting the processes able to take place. I will present new NOEMA observations which, when combined with chemical modeling, are indicative of enhanced ionization rates in the envelopes of three Class I protostars. I will then discuss the potential impact of this early enhancement on the evolution of protoplanetary disks.

Speaker: Kamber Schwarz (University of Arizona)

16:35 Cosmic-rays: a possible explanation for the difference of mean disk sizes in star-forming regions

Surveys of protoplanetary disks in star-forming regions of similar age revealed
significant variations in average disk mass between some regions. Disks in the Orion Nebular Cluster (ONC) and Corona Australis (CrA) are on average a factor of a few smaller than disks observed in Lupus, Taurus, Chamaeleon I or Ophiuchus. We aim for an understanding of the physical mechanism behind this spread by testing the influence of cosmic-ray ionization rates on the formation process of protoplanetary disks. We run non-ideal magnetohydrodynamical protostellar collapse simulations assuming different cosmic-ray ionization rates. We compute the resitivities for ambipolar diffusion and Ohmic dissipation with a chemical network. Consistent with previous results, our models demonstrate that a higher cosmic-ray ionization rate leads to stronger magnetic braking, and hence to the formation of smaller disks. Considering recent findings that protostars act as forges of comsic rays, we show that a high average cosmic-ray ionization rate in
star-forming regions like the ONC or CrA can explain the detection of smaller disks in these regions. Our results show that on average a higher cosmic-ray ionization rate leads to the formation of smaller disks. Therefore, smaller disks in regions of similar age can be the consequence of different levels of ionization, and may not exclusively be caused by disk truncation via external photoevaporation. We strongly encourage observations that allow measuring the cosmic-ray ionization degrees in different star-forming regions to test our hypothesis.

Speaker: Michael Küffmeier (Zentrum für Astronomie Heidelberg (ZAH))

17:00 → 17:45 Monday afternoon discussion

Tuesday, 12 May

09:00 → 10:15 Observational Signatures & Constraints

09:00 Disk Spreading and Surface Accretion Flows in Protoplanetary Disks

Despite its importance for star and planet formation, the physical process(es) that drive disk accretion remain frustratingly unclear. The magnetorotational instability is questioned both theoretically and observationally and disk winds are increasingly invoked. I will describe how the evolution of gas disk sizes from Class I to Class II indicates that some mechanism produces significant disk spreading (i.e., angular momentum transport within the disk) during the Class II phase. I will also show results from high resolution mid-infrared spectroscopy of a Class I source that may provide evidence for an alternative angular momentum transport mechanism, “surface accretion flows,” which have been reported in MHD simulations of magnetized disks.

Speaker: Dr Joan Najita (National Optical-Infrared Astronomy Research Laboratory)

09:25 Observed sizes of planet-forming disks trace viscous spreading

The evolution of protoplanetary disks is set by the conservation of angular momentum, where the accretion of material onto the central star is balanced by viscous expansion of the outer disk or by disk winds extracting angular momentum without changing the disk size. Studying the time evolution of disk sizes allows us therefore to distinguish between viscous stresses or disk winds as the main mechanism of disk evolution. Observationally, estimates of the disk gaseous outer radius are based on the extent of the CO rotational emission, which, during the evolution, is also affected by the changing physical and chemical conditions in the disk.
We have used physical-chemical models to study how the extent of the CO emission changes with time in a viscously expanding disk. We find that the gas outer radius ($R_{\rm CO,\ 90\%}$) measured from our models matches the expectations of a viscously spreading disk: $R_{\rm CO,\ 90\%}$ increases with time and for a given time $R_{\rm CO,\ 90\%}$ is larger for a disk with a higher viscosity $\alpha_{\rm visc}$. However, in the extreme case where the disk mass is low ($ \leq 10^{-4}\ \mathrm{M}_{\odot}$) and $\alpha_{\rm visc}$ is high ($\geq 10^{-2}$), $R_{\rm CO,\ 90\%}$ will instead decrease with time as a result of CO photodissociation in the outer disk.
We find that most observed gas outer radii in Lupus can be explained using a viscously evolving disk that starts out small $(\simeq 10\ \mathrm{AU})$ and has a low viscosity $(\alpha_{\rm visc} = 10^{-4} - 10^{-3})$.

Speaker: Mr L. Trapman (Leiden University)

09:50 Protoplanetary Disk Chemodynamics as a Scale for Weighing Young Stars and Planets

Much of our understanding of star and planet formation hinges on the accuracy of stellar masses and ages derived from pre-main sequence evolutionary track models. Consequently, errors in evolutionary track models could propagate through much of our understanding of star and planet formation. Moreover, there remain few constraints on pre-main sequence evolutionary tracks owing in large part to the lack of pre-main sequence stars with precisely measured masses. Fortunately, Keplerian rotation in protoplanetary disks provides an avenue towards directly measuring the masses of young stars, and therefore could provide large samples of well measured masses with which to constrain evolutionary tracks. I will present my efforts using radiative transfer forward modeling of ALMA spectral line observations, in conjunction with precise distance measurements from Gaia that are used to break the stellar mass-distance degeneracy, to directly measure pre-main sequence stellar masses with precisions as high as 2%. I will also present efforts to extend these methods to measure stellar masses for embedded protostars, where the traditional method of estimating masses from evolutionary tracks can be impossible, as well as an attempt to directly measure the mass of a young purported protoplanet.

Speaker: Patrick Sheehan (Northwestern University)

10:15 → 10:45 Coffee break

10:45 → 12:20 Disk Thermodynamics & Magnetothermal Winds

10:45 Atomic oxygen and warm CO in magnetothermal winds

Forbidden lines of atomic oxygen at 6300 and 5577 Angstroms and rovibrational lines of CO near 4.7 microns are relatively strong and well-observed in T Tauri stars. The oxygen lines have been touted as the ``smoking gun" of photoevaporative winds, but after considering the requirements for their excitation, we conclude that they are at least as likely to form in magnetothermal winds such as those modeled recently by Wang and collaborators. The CO line profiles are more often single- rather than double-peaked, with rotational temperatures of 300-1000 K, and are probably excited by IR or UV pumping rather than collisions. These lines are difficult to explain by photoevaporative winds, and are more naturally produced by magnetothermal ones.

Speaker: Prof. Jeremy Goodman (Princeton University)

11:10 The Inner Regions of Protoplanetary Disks: 3D Radiation Magneto-Hydrodynamical Models

Many planets orbit within an AU of their stars, raising questions about their origins. Particularly puzzling are the planets found near the silicate sublimation front. We investigate conditions near the front in the protostellar disk around a young intermediate-mass star, using the first global 3-D radiation non-ideal MHD simulations in this context.

The results show magnetorotational turbulence around the sublimation front at 0.5 AU. Beyond 0.8 AU is the dead zone, cooler than 1000 K and with turbulence orders of magnitude weaker. A local pressure maximum just inside the dead zone concentrates solid particles, allowing for efficient growth. Over many orbits, a vortex develops at the dead zone's inner edge, increasing the disk's thickness locally by around 10%.

We synthetically observe the results using Monte Carlo transfer calculations, finding the sublimation front is bright in the near-infrared. The models with vertical magnetic flux develop extended, magnetically-supported atmospheres that reprocess extra starlight, raising the near-infrared flux 20%. The vortex throws a non-axisymmetric shadow on the outer disk.

Radiation-MHD models of the kind we demonstrate open a new window for investigating protoplanetary disks' central regions. They are ideally suited for exploring young planets' formation environment, interactions with the disk, and orbital migration, in order to understand the origins of the close-in exoplanets.

Speaker: Dr Mario Flock (MPIA)

11:35 Electric heating in laminar protoplanetary disks

Observations point toward weak levels of turbulence in protoplanetary disks, and theoretical studies now focus on magneto-thermal winds rather than MRI turbulence as the main driver of mass accretion. Although MHD turbulence might be quenched, laminar magnetic structures may still transport angular momentum and induce substantial accretion heating inside the disk. Using steady-state radiative MHD models, I will show how magnetic fields can impact the thermal structure of the inner disk despite extremely low ionization fractions.

Speaker: William Béthune (University of Tübingen)

12:00 Hot and thick discs: stability of Initial Conditions and preliminary results

Both the Core Accretion and the Gravitational Instability models for giant planet formation predict the presence of circumplanetary discs (CPDs) during the last formation phases (Alibert et al. 2005, Ward & Canup 2010). These discs are found to be continuously fed by an influx of gas from the protoplanetary disc (Tanigawa et al. 2012). Magnetic fields generated by the disc itself could play a key role in modeling this accretion flow (Gressel et al. 2013). In the early stages of a giant planet's life, the magnetic field generated by the planet could be even stronger, thus potentially important depending on how it couples with the surrounding flow (Yadav et al. 2017 and Cauley et al. 2019), and possibly dominant.
In the Core Accretion scenario, CPDs are expected to be very hot and thick when forming. For such discs, regardless of their nature (CPD or PPD), standard thin-disc approximations can not be used to set ICs and new numerical and analytical methods have to be investigated, to ensure especially equilibrium at boundaries. Here we present a study of equilibrium initial conditions for thick and hot disc simulations with the meshless finite mass (MFM) method in the GIZMO code (Hopkins 2015; Deng et al.2019), which can be used by any code utilizing a particle-based representation of the fluid. 
Time permitting, we will show preliminary results obtained after setting a CPD initial condition with these methods and adding magnetic fields to our simulation.

Speaker: Marco Cilibrasi (University of Zurich)

12:20 → 13:30 Lunch break

13:30 → 15:30 Disk Thermodynamics & Magnetothermal Winds

13:30 Radiation Hydrodynamics Simulations of Photoevaporating Protoplanetary Disks with Multi-Metallicity Chemistry

Recent observations have found shorter lifetimes of protoplanetary disks (PPDs) in low-metallicity environments than in the solar neighborhood (Yasui et al. 2009, 2010). It suggests a more efficient disk dispersal with decreasing metallicity. Prior studies have shown that photoevaporation is one of the essential disk-dispersing mechanisms that can yield sufficient mass-loss rates consistent with observed disk lifetimes. Ercolano & Clarke (2010) have demonstrated that EUV/X-ray photoevaporation potentially explains the shorter disk lifetimes for low-metallicity PPDs.
In our studies, we implement photoelectric heating due to FUV as well as photoionization heating due to EUV/X-ray and examine the effects on thermochemical structures PPDs. We perform a suite of radiation hydrodynamics simulations, varying disk metallicities, to study the effects of metallicity on thermochemical structures and photoevaporation. Our simulations self-consistently solve hydrodynamics, radiative transfer, and nonequilibrium chemistry. We also consistently determine grain temperatures with 2D radiative transfer.
The results show increasing mass-loss rates as metallicity decreases at sub-solar metallicities owing to the reduced opacity of the disk. It is consistent with the observational trend that the lifetimes are shorter in low metallicity environments. At even lower metallicities, dust-gas collisional cooling remains efficient compared to FUV photoelectric heating. The disk temperatures are too low to drive strong photoevaporation regardless of FUV heating. For further lower metallicities, dynamical time is shorter than the heating or cooling timescale, and thus the atmosphere of PPDs becomes effectively adiabatic. Overall, our results show metallicity significantly affects the thermochemical structures and dynamics of the PPD atmosphere.

Speaker: Riouhei Nakatani (RIKEN)

13:55 Global axisymmetric simulations of photoevaporation and magnetically driven protoplanetary disk winds

Photoevaporation and magnetically driven winds are two independent mechanisms that remove mass from protoplanetary disks. In addition to accretion, the effect of these two principles acting concurrently could be significant, and the transition between them has not yet been extensively studied and quantified.
In order to contribute to the understanding of disk winds, we present the phenomena emerging in the framework of two-dimensional axisymmetric, nonideal magnetohydrodynamic simulations including extreme-ultraviolet (EUV) and X-ray driven photoevaporation. Of particular interest are the examination of the transition region between photoevaporation and magnetically driven wind, the possibility of emerging magnetocentrifugal wind effects, and the morphology of the wind itself, which depends on the strength of the magnetic field.
We used the PLUTO code in a two-dimensional axisymmetric configuration with additional treatment of EUV and X-ray heating and dynamic ohmic diffusion based on a semi-analytical chemical model.
We determine that the transition between the two outflow types occurs for values of the initial plasma beta β ≥ 107, while magnetically driven winds generally outperform photoevaporation for stronger fields. In our simulations we observe irregular and asymmetric outflows for stronger magnetic fields. In the weak-field regime, the photoevaporation rates are slightly lowered by perturbations of the gas density in the inner regions of the disk. Overall, our results predict a wind with a lever arm smaller than 1.5, consistent with a hot magnetothermal wind. Stronger accretion flows are present for values of β < 107.

Speaker: Peter Rodenkirch (Institute for Theoretical Astrophysics Heidelberg)

14:20 TACOs, thermally-assisted (magneto-)centrifugal outflows

Laminar outflows driven by large-scale magnetic fields likely play an important role in the evolution and dispersal of protoplanetary disks, and in setting the conditions for planet formation. Extending our previous non-ideal MHD model with radiative transfer as well as a simplified thermochemistry, we follow the dual aim of studying the influence of thermal driving and, at the same time, laying the foundation for synthetic observations. Our simulations develop magnetocentrifugal outflows that are primarily driven by magnetic tension forces. As a consequence, the mass-loss rate in the wind only increases moderately when including thermochemical effects. For typical field magnitudes, magnetic dissipation heating remains sub-dominant compared with thermochemical and irradiation heating. We, moreover, follow the evolution of the entrained vertical magnetic flux and find it to diffuse out of the disk on secular timescales as a result of non-ideal MHD. Based on line-radiative post processing, we demonstrate that velocity spectra and moment 1 maps of O and CS (as well as other molecules) show significant, potentially observable differences between models with and without outflows. In particular the shape of the line profiles, and velocity asymmetries in the moment 1 maps could enable the identification of outflows emanating from the surface of a disk.

Speaker: Oliver Gressel (AIP)

14:45 Radiation-Hydrodynamical Models of X-ray Photoevaporation in Carbon Depleted Circumstellar Discs

Transition discs provide an important tool to probe various mechanisms that might influence the evolution of protoplanetary discs and therefore the formation of planetary systems. One of these mechanisms is photoevaporation due to energetic radiation from the central star, which can in principal explain the occurrence of discs with inner cavities. Current models, however, fail to reproduce a subset of the observed transition discs, namely objects with large measured cavities and vigorous accretion. For these objects the presence of (multiple) giant planets is often invoked to explain the observations. In our work, we explore the possibility of X-ray photoevaporation operating in discs with different gas-phase depletion of carbon and show that the influence of photoevaporation can be extended in such low-metallicity discs. As carbon is one of the main contributors to the X-ray opacity, its depletion leads to larger penetration depths of X-rays in the disc and results in higher gas temperatures and stronger photoevaporative winds. We present radiation-hydrodynamical models of discs irradiated by internal X-ray+EUV radiation assuming carbon gas-phase depletions by factors of 3,10 and 100 and derive realistic mass-loss rates and profiles. Our analysis yields robust temperature prescriptions as well as photoevaporative mass-loss rates and profiles which may be able to explain a larger fraction of the observed diversity of transition discs.

Speaker: Lisa Wölfer (Max-Planck-Institut für extraterrestrische Physik)

15:10 Thermal evolution of protoplanetary disks: from beta-cooling to decoupled gas and dust temperatures

Thermal processes can play an important role in dynamics, chemistry, and dust growth of protoplanetary disks. Using numerical hydrodynamics simulations in the thin-disk limit, we explore different approaches to computing the disk thermal structure: a simplified beta-cooling approach, in which the rate of disk cooling is proportional to the local dynamical time, a fiducial model with equal dust and gas temperatures calculated taking viscous heating, irradiation, and radiative
cooling into account, and also a more sophisticated approach allowing for decoupled dust and gas temperatures. We found that the gas temperature may significantly exceed that of dust in the outer regions of young protoplanetary disks. The outer envelope, however, shows an inverse trend with the gas temperatures dropping below that of dust. Models with a constant beta-parameter fail to reproduce the disk evolution with more sophisticated thermal schemes.
We discuss whether the temperature decoupling is important for the gas dynamics,
chemical evolution, and dust growth in young protoplanetary disks.

Speaker: Eduard Vorobyov (University of Vienna)

15:30 → 16:00 Coffee break

16:00 → 17:15 Observational Signatures & Constraints

16:00 The polarizing puzzle of dusty disks

From a theoretical point-of-view, magnetic fields are crucial to the evolution of planet-forming disks. However, profound observational constraints are pending. Presently, the number of cutting-edge polarization observations presenting inconclusive data increases continuously. In very recent years, polarization at mm-wavelengths, the classical tracer of magnetic fields, emerged as highly ambiguous, and the pressing demand for comprehensive tools to analyze these new observations is growing.

I will present an overview on the sources of continuum polarization with focus on the impact of grain alignment, scattering, and grain porosity on the polarization measurement – as well as a potential solution to this dusty ambiguousness, linearly polarized gas emission.

Speaker: Gesa H.-M. Bertrang (MPIA)

16:25 What can we learn from ALMA polarimetric observations of protoplanetary disks?

Polarimetric observations of protoplanetary disks at millimeter wavelengths have been dramatically developing owing to its high-sensitivity and high-resolution observations with ALMA. However, the mechanisms of the polarization are under discussion. The proposed mechanisms so far are the self-scattering and the grain alignment, but the alignment directions are possibly with magnetic fields, with radiation fields, or with gas-drag directions. In this talk, I first review the possible mechanisms that may produce the millimeter polarization, and then present the case studies of different disks. On the lopsided disk of HD 142527, we found that the magnetic fields are dominated by the toroidal components at least in the south regions. On the ring-gap disk of HD 163296, by modeling the self-scattering polarization, we found that the dust scale height is lower in the inner regions, which indicates the low turbulence of gas in the inner disk and is consistent with the concept of magnetic dead zone. I also discuss possible constraints on the grain dynamics on AS 209 and HL Tau by modeling the gas-flow aligned dust grains.

Speaker: Akimasa Kataoka (National Astronomical Observatory of Japan)

16:50 ALMA chemical survey of Disk-Outflow sources in Taurus

ALMA-DOT is a small campaign devoted to the chemical characterization of disk-outflow sources in Taurus. The sample currently consists of six Class I sources known to drive powerful outflows. The high angular resolution and sensitivity of ALMA allowed us to characterize their chemical composition by separating the disk emission from the outflow and envelope contamination. Six molecules - from carbon monoxyde to the simple organic formaldehyde and methanol - have been surveyed in each target. I present the results of the survey, shedding light on (i) the timescale for the formation of dust and gas sub-structures, (ii) the morphological interplay of dust and gas, (iii) the role of disk-feeding filaments, (iv) the distribution and formation mechanism of simple organic molecules, and (v) the chemical composition of molecular outflows. Young sources like those probed by ALMA-DOT offer the best laboratory to study how soon are planets formed and how are complex molecules delivered to the (assembling) planetary atmospheres.

Speaker: Antonio Garufi

17:15 → 17:45 Tuesday afternoon discussion

Wednesday, 13 May

09:00 → 10:15 Planet Disk Interaction

09:00 40 years of disk-planet interaction: some new developments in an old problem

Gravitational coupling between protoplanetary disks and embedded planets is an old problem ascending to the seminal studies of Goldreich & Tremaine (1980) and Lin & Papaloizou (1979). It is widely recognized as playing a key role in many areas of exoplanetary science: determination of the planetary architectures, disk evolution, planetary accretion, and so on. In this talk I will describe several key theoretical advances that took place in this field in recent years. They provide a better understanding of the ways in which density waves get excited in disks by planets, how they propagate through the disk, and how they dissipate, linearly and non-linearly, driving global disk evolution. I will particularly focus on recent advances in incorporating realistic disk thermodynamics in studies of disk-planet coupling, their impact on the numerical studies of this phenomenon, and the observational manifestations of massive planets in protoplanetary disks, including both scattered light imaging and recent ALMA observations of fine structures in disks.

Speaker: Roman Rafikov (University of Cambridge)

09:25 Investigating a new paradigm for type II migration

Understanding the origins and dynamics of massive planets during the planet-formation process is essential to understanding how the structures of individual planetary systems came to be. Massive planets have the ability to open gaps in their host disk, and the radial movements of these gap-opening planets is typically referred to as type II migration. In the classical view, a protoplanetary disk accretes onto its star on a viscous timescale, carrying this gap inwards with the planet moving with the gap. This theory assumes that the planet remains in a state of quasi-equilibrium at the center of the gap. However, a non-zero torque from the disk must be applied to the planet for it to move radially, meaning the planet is not necessarily located at equilibrium. This implies that while the gap is in motion, and the planet is being dragged along with it, the location of the planet is not necessarily at the center of the gap. In addition, if we define the location of the gap center to be the radial position of a planet with a fixed orbit (i.e., a non-migrating planet), we also consider the possibility that the equilibrium position of the planet differs from this center. We explore these properties involving the gap-planet interaction of an evolving protoplanetary disk with 2D simulations using the FargOCA hydrodynamics code. We accomplish this by fixing the orbital radius of a massive planet ($M_p/M_* = 0.001$) until its disk has reached a steady state. At this stage we release the planet from its fixed orbit, and allow it to migrate freely. We then analyze the planet's displacement from equilibrium, as well as its displacement from the gap center, varying disk properties such as aspect ratio and viscosity, and explore their effects on the planet migration rate.

Speaker: Dylan Kloster (OCA)

09:50 Radiative effects in protoplanetary disks: planet-disk interaction

Recent ALMA observations revealed concentric annular structures in several young, class-II objects. Some have been modeled numerically with a single embedded planet assuming a locally isothermal equation of state, a method often used in the irradiation-dominated outer disk regions. We compare locally isothermal and radiative disks similar to HD 163296 and AS 209 with embedded planets and
show that the disk thermodynamics can impact the number of rings and the contrast of spirals produced by a planet. Radiative effects can suppress features visible in locally isothermal simulations. These results suggest the need for multiple planets to explain the ring-rich structures in such systems.

Speaker: Alexandros Ziampras (University of Tuebingen)

10:15 → 10:45 Coffee break

10:45 → 12:15 Embedded Solids & Planet Formation

10:45 A massive planet disruption as the source of dust in TW Hydra protoplanetary disc

The protostar TW Hydra features the best studied and one of the most unusual protoplanetary discs. Its dust disc has a cliff-like rollover at 52 AU which coincides with a suspected sub-Neptune mass planet recently detected as an azimuthally elongated AU-scale excess in ALMA 1.3 mm continuum (Tsukagoshi +19). Here we build detailed models of dust growth, dynamics and synthetic disc emission to investigate the origin of TW Hydra's peculiarities.

We show that the standard scenario in which the dust in TW Hydra disc is primordial accounts neither for the dust morphology nor the excess emission. We propose an alternative model in which the primordial dust is long consumed by the star or locked in planets; the dust currently observed in the system is ejected by the suspected ALMA planet. We show that in this model the mm-sized dust particles are blown inside the planetary orbit, naturally explaining the dust disc morphology and its relation to the 1.3 mm excess. Further, dust lost by the planet performs a characteristic U-turn relative to the planet producing an azimuthally elongated emission feature similar to the one observed by ALMA. Finally, the disruption scenario provides an attractive explanation for why one of the oldest protoplanetary discs happens to be tens times more massive in terms of dust than most discs a fraction of TW Hydra's age.

We consider two scenarios for the nature of the dust-loosing planet. In the first, a dusty pre-collapse gas envelope of a massive core growing in the Core Accretion framework is disrupted, e.g., as a result of a catastrophic encounter. In the second, a massive dusty gas giant planet formed in the Gravitational Instability scenario is disrupted by the energy release in its massive core. In the latter case all of TW Hydra protoplanetary disc, including its gaseous component, may originate in such a disruption; the planet mass has to be no larger than 2 Jupiter masses and it must be 5-10 times more abundant in metals than the Sun.

If these ideas are correct then future observations of TW Hydra, and potentially other discs, will allow us to study planet formation in an entirely new way -- by analysing the flows of dust and gas recently belonging to giant planets. Reverse engineering of mass loss from the planets may inform us about their density structure and elemental composition before the disruption.

Speaker: Sergei Nayakshin (University of Leicester)

11:10 Evolution and growth of dust grains in protoplanetary disks with magnetically driven disk wind

Magnetically driven disk winds (MDWs) are one of the promising mechanisms of dispersal processes of protoplanetary disks (Suzuki et al. 2010, Bai 2013). When the MDWs play a key role, the gaseous component of protoplanetary disks evolves in a different manner from that of the classical viscous evolution. As a result, the subsequent planet formation is also affected by the MDWs. In this work, we investigate the effects of the MDWs on the radial drift of solid particles with a size of 0.1$\mu$m - 1km. We propose that the MDWs is a possible solution to the ``radial drift barrier'' of collisionally growing dust grains, which is a severe obstacle to the planet formation (e.g., Nakagawa et al.1986).
In order to study the evolution of dust grains in the disks, we calculate the advection and collisional growth of dust particles in evolving protoplanetary disks under the 1+1 D (time + radial distance) approximation. We solve a coagulation equation of solid particles under a single-size approximation (Sato et al. 2016) for various conditions of turbulent viscosity, the mass loss by the MDW, and the magnetic braking by the MDW.
We found that significant grain growth occurs in the inner region of the protoplanetary disks. The grown dust particles are larger than the km-sized bodies and they are no longer caught by the radial drift barrier. The mechanism of such successful dust growth is separated into two parts: (1) the increase of the equilibrium size of the dust particles caused by the convergent flow of the dust mass and dispersal of the gas component, (2) the unstable dust growth driven by the feedback loop between the size, radial drift velocity, and surface density of the dust component. The disk evolution owing to the MDWs strongly supports the former part of the growth mechanism. When the equilibrium size of the dust particles reaches the size that the Stokes number of the dust particles exceeds unity, the dust size evolution shift to the unstable mode (i.e., the latter part).
Because of the successful growth of dust particles, the ring-like structure containing the planetesimal sized bodies can be formed at the inner part of the protoplanetary disks. We will discuss the effects of such the ring-like structure on the subsequent planetary system formation and the disk observations.

Speaker: Tetsuo Taki (National Astronomical Observatory of Japan)

11:35 Planet Formation By Chondrule Accretion

Recently performed nested-grid, high-resolution hydrodynamic and radiation-hydrodynamics simulations of gas and particle dynamics in the vicinity of Mars- to Earth-mass planetary embryos (Popovas et al 2018MNRAS.479.5136P and 2019MNRAS.482L.107P) have provided quantitatively robust estimates of accretion rates for planet embryos formed inside a pressure trap. The simulations extended from the resolved surfaces of the embryos to several vertical disk scale heights, with a vertical dynamic range exceeding 1e5. Heating due to the accretion of solids caused vigorous convective motions, however even convection driven by a nominal accretion rate one Earth mass per Myr did not significantly alter the pebble accretion rate. Ray-tracing radiative transfer showed that rocky planet embryos embedded in protoplanetary disks can retain hot and light atmospheres throughout much of the evolution of the disks.

Importantly, the results showed that particles larger than the chondrules ubiquitously observed in meteorites are not required to explain the accretion of rocky planets such as Earth and Mars within the lifetime of the disk. Due to cancellation effects, accretion rates of a given size particles are nearly independent of disk surface density, while proportional to the dust-to-gas ratio. As a result, accurate growth times for specified particle sizes may be estimated. For 0.3-1 mm size particles, and assuming a dust-to-gas ratio of 1:100, the growth time from a small seed is ~1.5 million years for an Earth mass planet at 1 AU and ~1 million years for a Mars mass planet at 1.5 AU.

The magnitude and robustness of the accretion rate estimates hinges on the assumption of the embryo residing in a pressure trap. A vertically projected dust to gas ratio of 1:100 is thus a lower limit, with continued trapping of mm-size particles expected to accelerate accretion. This mechanism is therefore a prime candidate to explain rapid formation of rocky planets, leaving open only the question of by which mechanism the accretion is quenched, thus determining the final mass.

I will discuss provocative scenarios where this question is resolved, including implications for the formation of gas dwarfs and gas giants.

Speaker: Åke Nordlund (Niels Bohr Institute, Copenhagen)

12:15 → 13:30 Lunch break

13:30 → 15:15 Planet Disk Interaction

13:30 Observational signatures of tightly-wound spirals driven by buoyancy resonance

Spiral waves are one of the most fundamental outcomes of planet-disk interaction. In addition to the well-known Lindblad resonance, buoyancy resonance, which occurs when the vertical buoyancy frequency of disk gas matches with an integer multiple of the planet's orbital frequency, can excite spiral waves. Based on three-dimensional global hydrodynamic simulations and synthetic ALMA line observations, we will show that buoyancy spirals can produce observable kinematic signatures. One of the main characteristics of buoyancy spirals is their tightly-wound morphology (i.e., a small pitch angle compared with Lindblad spirals). The strength and observability of buoyancy spirals depend sensitively on the disk thermodynamics. This highlights the importance of using more realistic thermodynamics in hydrodynamic simulations.

Speaker: Jaehan Bae (Carnegie Institution of Washington)

13:55 On the migration of giant mass planets in 3D disks accreting from surface layers

The classic view of a viscous disk, where viscosity is generated by strong turbulence driven by the magneto rotational instability, is challenged by modern magneto-hydrodynamic simulations. Disks are probably much less viscous than previously thought. Nevertheless, disks cannot be in-viscid, a minimum viscosity is set for example by the so-called vertical shear instability (VSI). In addition, disk winds remove angular momentum from thin surface layers of the proto-planetary disk, promoting fast radial transport of gas towards the central star in these layers. This radial transport accounts for the observed stars' accretion rate.
In a classical viscous disk with radial transport corresponding to observed stellar accretion rate, giant planets migrate towards the star and easily become hot Jupiters with short orbital period. However, the majority of observed giant planets have distances of 1-3AU from their parent star.
This contradiction has been investigated looking at a variety of migrations mechanisms, but no general mechanism to reduce planet migration has been found.
The new paradigm of disks with small bulk viscosity and fast radial advection in surface offers a different perspective of the problem.
Consequently, we perform 3D numerical simulations using the FARGOCA code. We simulate the effect of disk wind by imposing a loss of angular momentum generating a desired mass flux in a thin surface layer.
We show that planets migration is only marginally affected by the fast gas in the thin layers and that the migration speed is mainly regulated by the bulk viscosity of the disk. However, the migration rate measured with a bulk viscosity of alpha=1.e-4 (typically of the order of that generated by the VSI in the outer disk) is still too fast to understand the observed radial distribution of extra-solar planets. Decreasing further the viscosity seems necessary for the understanding of the observations.

Speaker: Elena Lega

14:20 Giant planet migration in low-viscosity disks

Giant planet migration (a.k.a. Type-II migration) should occur with a migration speed proportional to the disk's viscosity. This has been verified for alpha-disks with alpha>1.e-4 (Robert et al., 2018). But what happens in disks with vanishing viscosity? Does Type-II migration stalls? A variety of behaviors have been observed in the literature for migration in low-viscosity disks. Migration seems to be very stochastic, with very fast migration episodes (e.g. McNally et al., 2018). However, most simulations so far have been conducted in 2D disks. We find that low-viscosity 3D disks are much more stable than 2D disks of equivalent viscosity when they are perturbed by a planet, because they are submitted to the constraint of vertical hydrostatic equilibrium. Consequently giant planet migration in 3D disks behaves more regularly. Nevertheless, the presence of vortex formed at the outer edge of the gap opened by the planet affects the planet evolution. We have investigated in details the influence of a vortex on planet migration and on the growth of the planet's orbital eccentricity as well as the feedback of the eccentricity on migration.

Speaker: Mr Alessandro Morbidelli (OCA)

14:45 Non-equilibrium dynamics of massive embedded planetary systems

Recently, young planets with masses around $10\,M_\text{Jupiter}$ which are still embedded in a disk have been observed, e.g. in the PDS 70 system. At this mass range, the planet-disk interaction is non-linear and the planets are attributed with having carved the observed gap into their parent disk. One possible scenario for the formation of large gaps is outward migration in 2:1 mean motion resonance (MMR) where the inner planet is more massive than the outer one. This process is known to strongly excite the planets' eccentricities which in turn leads to eccentric gaps. The latter could be an observable feature of such systems.

We perform 2D, vertically integrated hydrodynamics simulations to study the migration and dynamics of the embedded planetary system employing a viscous $\alpha$-disk model. To avoid artificial wave-damping boundary conditions we choose large outer disk radii and work in the center of mass frame of the planetary system. In addition to the often used locally isothermal equation of state, we run simulations with radiative cooling, viscous heating and irradiation from the star from which temperature distributions in the perturbed disk can be extracted.

The simulations exhibit the expected smooth 2:1 MMR outward migration. For sufficiently high surface densities of the order of the minimum mass solar nebula we additionally observe epochs of fast migration. During a sequence which we call migration jumps, the outer planet undergoes fast outward migration traveling tens of au outward. It stays at the large distance for some kyr before returning back into the 2:1 MMR via fast inward migration. The whole sequence only takes 10-20 kyr. Meanwhile the inner planet remains relatively unaffected. A migration jump causes strong perturbations in the disk including pronounced spiral arms and asymmetric features including vortices and mass accumulation in the Lagrange points inside the gap region. The latter might be an observational indication for this process.

Due to the large mass of the embedded planets, the features created in the disk are strong and synthetic observations of the simulations might help to identify whether these mechanisms are at play in the disks we observe. In addition, migration jumps of massive planets can be expected to cause strong scattering of dust particles and small bodies in the radial range on which the jumps occur. Thereby, they might play an important role for the dust and small body distribution in the earlier stages of the systems lifetime.

Speaker: Mr Thomas Rometsch (University of Tübingen)

15:15 → 15:30 Coffee break

15:30 → 16:40 Disk Microphysics and Ionization

15:30 A Post-Processing Pipeline for Proto-Planetary Disk Simulations (PPPPPDS)

I will present a new post-processing pipeline for (magneto-)hydrodynamic simulations of protoplanetary accretion disks and results from its first application. By combining publicly available radiative transfer and astrochemistry tools, we process snapshots from radiative, non-ideal MHD simulations of thermally-assisted centrifugal outflows from disks (Gressel et al. 2020) to search for observational signposts of outflows which are accessible from current observatories. In particular, we compare synthetic observations from models with and without outflows to determine which transitions and chemical species can be used to distinguish between the two classes of models. We find that the shape of the line profiles, and velocity asymmetries in moment 1 maps, can discriminate between disks with and without outflows. By combining the synthetic observations with the full simulation data, we can also pinpoint where emission from a particular line or species is coming from in the outflow and/or disk, which can help us better understand existing and future observations of disks and outflows.

Speaker: Jon Ramsey (Virginia Initiative on Cosmic Origins, Department of Astronomy, University of Virginia)

15:55 Consistent Microphysics in Protoplanetary Disk Simulations

Consistent time-dependent modeling of microphysics, especially thermochemistry and radiation-matter interactions, is desired by the studies on protoplanetary disks. In this talk, I will introduce our GPU-accelerated numerical infrastructures for consistent microphysics co-evolved with (magneto-)hydrodynamic simulations. Their applications in the studies of protoplanetary disk dispersal processes will be elaborated, including photoevaporation and magnetized wind-driven accretion mechanisms. Formation of disk substructures is also explored using our system, emphasizing the necessity of non-ideal MHD consistently coupled with microphysics.

Speaker: Dr Lile Wang (Center for Computational Astrophysics, Flatiron Institute)

16:20 Modelling microphysics in MHD: ambipolar diffusion, chemistry, and cosmic rays

From molecular clouds to protoplanetary disks, non-ideal magnetic effects are important in many astrophysical environments. Indeed, in star and disk formation processes, it has become clear that these effects are critical to the evolution of the system. The efficacy of non-ideal effects are, however, determined by the complex interplay between magnetic fields, ionising radiation, cosmic rays, microphysics, and chemistry. In order to understand these key microphysical parameters, we present a one-dimensional non-ideal magnetohydrodynamics code and apply it to a model of a time-dependent, oblique, magnetic shock wave. By varying the microphysical ingredients of the model, we find that cosmic rays and dust play a major role, and that, despite the uncertainties, the inclusion of microphysics is essential to obtain a realistic outcome in magnetic astrophysical simulations.

Speaker: Dr Tommaso Grassi (USM/LMU)

Thursday, 14 May

09:00 → 10:15 Observational Signatures & Constraints

09:00 Measuring 3D Velocities in Planet Forming Disks

I will present a novel method to extract azimuthally averaged 3D velocity profiles from ALMA data. Application of this to the well studied source HD 163296 reveals a highly dynamical disk, hosting large flow structures indicative of meridional flows likely driven by three embedded protoplanets. These flows provide an efficient transport mechanism of volatile-rich gas in the disk atmosphere towards the planet-forming midplane. In addition, we find tentative evidence of a slow disk wind in the outer 100 au of the disk, like connected to the previously detected large scale wind described in Klaassen et al. (2013). I will further demonstrate how application of this method to multiple molecular species will allow us to map the dynamical structure of a protoplanetary disks in the (r, z) plane, allowing us to directly search for characteristic flow structures which will help us to distinguish between potentially active instabilities. I will end with an outlook to how extensions of these methods can be used to search for embedded planets by searching for localized deviations from the background rotation.

Speaker: Dr Richard Teague (Center for Astrophysics | Harvard & Smithsonian)

09:25 Doppler flips and wiggles in long baseline observations of protoplanetary disks with ALMA

Protoplanets and circumplanetary disks are elusive yet they are cornerstones to the most popular interpretations for observed protoplanetary disk structures. The gaseous velocity field also bears the imprint of planet–disk interactions, with non-Keplerian fine structure in the molecular-line channel maps (e.g., wiggles or kinks). Such features could in principle be connected to the perturber by comparison with hydrodynamical simulations, however, there is a more direct way of pinpointing the protoplanet’s location: identifying the place where the non-Keplerian velocities undergo an abrupt sign reversal, a.k.a. a "Doppler flip", in velocity centroid maps. In this talk, I will discuss the kinematic signatures of planet formation and present recent observations of the young disk in HD 100546 in CO emission and dust continuum. The high-resolution 1.3 mm continuum observation reveals fine radial and azimuthal substructures in the form of a complex maze of ridges and trenches sculpting a dust ring. Near these dust structures, we pick up a conspicuous Doppler flip. The 12CO channel maps are modulated by wiggles that deviate from Keplerian kinematics and which are somewhat connected to the Doppler flip signal.

Speaker: Sebastián Pérez (Universidad de Santiago de Chile)

09:50 Measuring dust-gas coupling via the gas rotation curve

ALMA is showing that most proto-planetary discs are highly sub-structured and that the most frequent structure consists in azimuthally symmetric "gaps and rings". Rings have attracted a lot of attention since they might be the signature of young planets. But rings are extremely important for another reason: they provide us with a privileged window inside disc physics. Indeed, as shown by the DSHARP team, their finite dust width shows that there must be some level of dust diffusion, or else the dust would only collect at the pressure maximum. However, the DSHARP team was only able to place a lower limit, and not to measure, the amount of diffusion, because they did not have information on the gas distribution. I will show how the analysis of the gas rotation curve, another breakthrough enabled by ALMA observations of molecular lines, is a powerful way to measure the width of rings in the gas and therefore allows us to measure the dust-gas coupling, which controls the dust ring width. Formally, the relevant parameter is the ratio between the Shakura-Sunyaev α and the dust Stokes number St. I will also discuss the impact of the disc 3D structure on this analysis and show that the measurement of ring width is robust towards the details of the vertical structure. At the moment, there are only two objects with good enough S/N to perform these measurements. In these objects, I will report gas widths larger than in the dust, consistently with the idea of dust trapping. I will show how the data point to a relatively high degree of dust-gas coupling (typical α/St ~ 0.1). Scenarios with very low levels of turbulence and high levels of grain growth can therefore be rejected. Future constraints on the dust grain size in the rings will help in breaking the degeneracy between St and alpha.

Speaker: Giovanni Rosotti (Leiden University)

10:15 → 10:45 Coffee break

10:45 → 12:15 Disk Microphysics and Ionization

10:45 Interpreting emission line diagnostics of disc winds

Winds from planet forming discs can be photoevaporative or magnetically driven. Both types of wind can remove mass from the disc and affect the surface density evolution of the planet making material. A basic difference between these two types of wind is that magnetic winds, unlike photoevaporative winds can also remove angular momentum and thus drive accretion in the system. Indeed a departure from the classic alpha disc model is gaining momentum in the field and is one of the main topics of this workshop. Unfortunately, both photoevaporation and MHD numerical modes have yet to be observationally constrained and their relative contribution to the evolution of discs at various ages is still uncertain. In this contribution I will discuss currently observed disc wind diagnostics and present past and current efforts in modelling these lines using state-of-the-art theoretical models as well as analytical prescriptions.

Speaker: Prof. Barbara Ercolano (USM/LMU, Munich)

11:10 Chemical networks for Protoplanetary Disk Gas Dynamics

I will discuss a few applications requiring coupling chemistry with gas dynamics in protoplanetary disks. The most common application is to obtain the level of ionization, which determines the coupling between gas and magnetic fields. In the bulk disk, as far as ionization is concerned, equilibrium chemistry holds unless sub-micron sized grains are depleted. This allows magnetic diffusivities to be obtained from a pre-computed look-up table based on a complex chemical network, although magnetic diffusivities could have non-trivial dependence on magnetic field strengths due to small dust grains. More interesting applications involve the transport of chemical species over dynamical timescales, such as those important for heating/cooling in the disk atmosphere, which requires explicitly evolving a (reduced) chemical network with gas dynamics. We further show that a complex network can be reduced to a network with ~20-30 species with ~50-60 gas-phase reactions that still reasonably reproduces the abundances of most major species of interest in the disk atmosphere of the bulk midplane region. However, the intermediate layer is more complex, which may pose a challenge to chemo-dynamical studies.

Speaker: Xuening Bai (Tsinghua University)

11:35 The role of cosmic rays in protoplanetary disks and young exoplanetary systems

The influence of magnetic fields in protoplanetary disk evolution depends sensitively on the level of ionisation present. Protoplanetary disks are thought to be only very weakly ionised which provides imperfect coupling to magnetic fields and influences disk dynamics. Understanding the sources of ionisation, such as cosmic rays, present in the disks underpins our overall understanding of how these systems evolve and form planets.

However, young solar-type stars are very magnetically active and drive stronger stellar winds that may shield protoplanetary disks from galactic cosmic rays, thus losing an important source of ionisation. At the same time, the increased magnetic activity of young stars suggests that protoplanetary disks, and young exoplanetary systems, are bombarded by stellar cosmic rays, or stellar energetic particles. I will present recent results from our model of cosmic ray transport in these systems and the chemical signatures that we might expect from cosmic rays.

Speaker: Donna Rodgers-Lee (Trinity College Dublin)

12:15 → 13:30 Lunch break

13:30 → 15:00 Embedded Solids & Planet Formation

13:30 The impact of ice mantles on dust evolution in dynamically evolving protostellar discs

The snowlines of various volatiles are often associated with dust evolution in protoplanetary discs and may be identified in observations due to their impact on dust properties. In the vicinity of snowlines icy mantles of dust grains sublimate, which can lead to a different regime of dust growth. Dynamical effects of icy grains crossing snowlines may be reflected in the distribution of volatiles in the gas phase.
In this work, we present the FEOSAD model of protostellar disc with dust evolution, updated to include evolution of icy mantles. The chemical part of the model accounts for time-dependent absorption and desorption of main disc volatiles (H$_2$O, CO$_2$, CH$_4$, and CO) on two evolving populations of dust grains: small and grown dust. This 2D hydrodynamic code allows to consider the feedback of ice mantles on dust evolution through variable fragmentation velocity.
We discuss if the dynamical effects when calculating the snowline positions are important for dust growth. We analyse the impact of ice mantles on dust evolution in protostellar discs and discuss the role of ices in the process of planet formation.

Speaker: Tamara Molyarova (Institute of Astronomy, Russian Academy of Sciences)

13:55 Pebble accretion in turbulent discs

Planets are born in the mid-plane of accretion discs around young protostars. This process takes place most likely in weakly ionised regions, where the evolution of the environment is driven by internal turbulence and the gas flow is not laminar but has stochastic components. Turbulence can be generated purely hydrodynamically via different instabilities, like the vertical shear instability (VSI). A fast pathway to the formation of giant planetary cores has been recently identified in pebble accretion, though a realistic investigation of this process in a turbulent environment was necessary. We tested the solid accretion of a large range of dust sizes on to different planetary core masses in a VSI turbulent global 3D disc and compared it with a laminar disc. Furthermore, we tested the influence of a realistic equation of state and radiative cooling on the efficiency of pebble accretion. We found that turbulence decreases slightly the solid accretion efficiency with respect to analytical calculations of laminar discs, having a ~2% efficiency of pebble-like particles for a 5 Earth-mass planet at 5 au. The introduction of radiative transfer can affect this result significantly by changing the aspect ratio of the disc, thus the pebble isolation mass.

Speaker: Giovanni Picogna (Universitäts-Sternwarte München, LMU)

14:20 Dust Settling Instability in Protoplanetary Disks

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 (and streaming) instability (here SSI) occurs as particles sediment towards the midplane. However, the ability of the SSI 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 SSI 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 SSI could generate weak levels of turbulence in otherwise nearly laminar regimes.

Speaker: Dr Leonardo Krapp

15:00 → 15:30 Coffee break

15:30 → 17:05 Massive Disks & Instabilities

15:30 3D global simulations of the Vertical Shear instability

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.

Speaker: Mr Marcelo Fernando Barraza Alfaro (Max Planck Institute for Astronomy, Heidelberg)

15:55 Global simulations of magnetised self-gravitating protoplanetary disks

In the early stages of a protoplanetary disk, when its mass is a significant fraction of its star's, turbulence generated by gravitational instability (GI) should feature significantly in the disk's evolution. At the same time, the disk may be sufficiently ionised for magnetic fields to play some role in the dynamics.
Though usually neglected, the impact of magnetism on the GI may be critical, with consequences for several processes: the efficiency of accretion, spiral structure formation, fragmentation, and the dynamics of solids. In this paper,
we report on the first global three-dimensional magnetohydrodynamical simulations of a self-gravitating protoplanetary using the meshless finite mass (MFM)
Lagrangian technique.
We confirm that GI spiral waves trigger a dynamo that amplifies
an initial magnetic field to nearly thermal amplitudes (plasma $\beta <10$), an order of magnitude greater than that generated by the magneto-rotational instability alone.
We also determine the dynamo's nonlinear back reaction on the gravitoturbulent flow: the saturated state is substantially hotter, with an associated larger Toomre parameter and weaker, more `flocculent' spirals.
But perhaps of greater import is the dynamo's boosting of accretion via a significant Maxwell stress; mass accretion is enhanced
by factors of several relative to either pure GI or pure MRI.
Our simulations use ideal MHD, an admittedly poor approximation in protoplanetary disks, and thus future studies should explore the full gamut of non-ideal MHD. In preparation for that, we exhibit a small number of Ohmic runs that reveal that the dynamo, if anything, is stronger in a non-ideal environment.
This work confirms that magnetic fields are a potentially essential ingredient in gravitoturbulent young disks, possibly controlling their evolution, especially
via their enhancement of (potentially episodic) accretion.

Speaker: Prof. Lucio Mayer (University of Zurich)

16:20 Synthetic ALMA emission of massive protostellar discs.

Massive protostellar discs are the sibling circumstellar structures of protoplanetary accretion discs. They form, evolve as a scaled-up version of the surroundings of low-mass stars and both formation mechanisms are unified within the so-called burst mode of star formation. This picture naturally links the development of gravitational instabilities in centrifugally balanced accretion discs to the formation of gaseous clumps and stellar companions which will influence the future evolution of massive protostars in the Hertzsprung–Russell diagram. We perform molecular line emission plus dust continuum radiative transfer calculations and compute synthetic images of disc structures modelled by the gravito-radiation-hydrodynamics simulation of a forming stars, in order to investigate the Atacama Large Millimeter/submillimeter Array (ALMA) observability of circumstellar gaseous clumps and forming multiple systems. We show that substructures are observable regardless of their viewing geometry or can be inferred in the case of an edge-viewed disc. The observation probability of the clumps increases with the gradually increasing efficiency of gravitational instability at work as the disc evolves. Our results motivate further observational campaigns devoted to massive accretion discs as around the protostars S255IR-NIRS3 and NGC 6334I-MM1, whose recent outbursts are a probable signature of disc fragmentation and accretion variability.

Speaker: Dr Dominique Meyer (University of Potsdam)

16:45 Dust dynamics in self-gravitating disks – could planet formation start in young disks?

I will present the results on the evolution of dust particles in self-gravitating disks residing in a gravitoturbulent state, when heating due to shocks of density waves balances cooling. It is well known that density structures in the gaseous component of the disk induced by self-gravity (gravitational instability) can trap dust efficiently enough, so that the dust component itself undergoes further gravitational collapse due to its own self-gravity. Previous results both in global and local shearing box studies indicate that over-pressure regions related to spiral density waves can be very efficient at collecting dust particles, creating significant local over-densities of particles. The degree of such concentrations depends on two parameters: the size of dust particles and the rate of gas cooling. In recent years, increasing observational evidence indicates that large-scale vortices (e.g., induced by planetary gaps) and rings are most preferable sites of dust trapping.

Motivated by this, we studied effects of vortices on the evolution of dust particles using local shearing box simulations of self-gravitating protoplanetary disks, including also the dust-back reaction on gas and self-gravity of the dust component itself. In contrast to non-self-gravitating disks, vortices in self-gravitating disks tend to be smaller-scale (of the order of local Jeans scale) and short-lived structures. We found that these types of structures are nevertheless quite efficient at trapping small and intermediate-sized dust particles with friction times comparable to, or less than, the local orbital period of the disk. This can lead to significant over-densities in the solid component of the disk, with density enhancements comparable to, and even higher, than those within spiral density waves; increasing the rate of gravitational collapse of dust into bound structures (planetesimals). I will also discuss the resulting surface density structure of dust trapped in such vortices in connection with recent observations of disks.

Speaker: George Mamatsashvili (Helmholtz-Zentrum Dresden-Rossendorf)

17:10 → 18:00 Thursday afternoon discussion

Friday, 15 May

09:00 → 10:15 Transition Disks & Irregular Structure

09:00 Ring/gaps formation driven by MHD winds in protoplanetary discs

Rings and gaps have been observed in a wide range of proto-planetary discs, from young systems like HLTau to older discs like TW Hydra. Recent disc simulations have shown that magnetohydrodynamic (MHD) turbulence (in both the ideal or non-ideal regime) can lead to the formation of rings and be an alternative to the embedded planets scenario. In this talk, I will investigate the way in which these ring form in this context and seek a generic formation process, taking into account the various dissipative regimes and magnetisations probed by the past simulations. I will show that a linear instability, driven by MHD winds, might occur and spontaneously form these rings/gaps structures. Given its robustness, the process identified could have important implications for a wide range of accreting systems threaded by large-scale magnetic fields. To make connection with observations, I will finally analyze the dust distribution around rings/gaps structures and characterize its emission.

Speaker: Antoine Riols (IPAG Grenoble)

09:25 Kinematic signatures of planet-induced warps

Observations of structures in discs such as gaps and kinks give us a tantalising glimpse of possible planets forming there. Recent numerical simulations show that planets that are misaligned with respect to the protoplanetary disc may cause the disc to warp and even break into distinct planes. These effects occur even with small misalignments and are therefore likely to be reasonably common. We have performed chemical models of a disc warped by a planet and used this to predict the kinematic signatures of planet-induced warps. We highlight the most useful diagnostics, and discuss how observations can indicate the presence of a planet at various radii within a protoplanetary disc.

Speaker: Alison Young (University of Leicester)

09:50 An Origin of Misaligned Disks and Planetary Orbits: Angular Momentum Accretion in Star Formation Process

I try to explain the primordial origin of misalignment between the disk rotation and host star’s rotation from the context of the disk formation. Theoretical and observational investigations have provided convincing evidence for the formation of molecular cloud cores by the gravitational fragmentation of filamentary molecular clouds, which has important implication for the origin of the stellar initial mass function. On the other hand, the size and total angular momentum of a protoplanetary disk are supposed to be directly related to the rotational property of the parental molecular cloud core where the central protostar and surrounding disk are born. Our recent analysis concludes that both the mass function and angular momentum distribution of molecular cloud core are the natural outcome of transonic turbulence with Kolmogorov spectrum in parental filamentary molecular clouds. The implication of this identification is non-homogeneous angular momentum distribution inside a molecular cloud core. The actual angular momentum accretion onto a young stellar object in the core should create misalignment of disk surrounding the star. We show the probability distribution of the misalignment as a function of disk mass. This finding may explain the origin of misaligned planets created in those disks.

Speaker: Prof. Shu-ichiro Inutsuka (Nagoya University)

10:15 → 10:45 Coffee break

10:45 → 12:15 Transition Disks & Irregular Structure

10:45 What are the dynamical signatures of vortices?

The large scale crescent shape structures detected in protoplanetary disks have sometimes been interpreted as vortices. Vortices are of particular interest to understand planet formation as they are known to concentrate dust and could participate to planetesimal formation. We study the multiple fingerprints of such large Rossby vortices and propose observational predictions to test the if these crescents could be interpreted as vortices.

We performed 2D hydro-simulations where a vortex forms at the edge of a gas depleted region. We derived idealized line-of-sight velocity maps, varying orientation relative to the observer. The signal of interest, as a small perturbation to the dominant axisymetric component in velocity, may be isolated in observational data using a proxy for the dominant quasi-Keplerian velocity. We propose that the velocity curve on the observational major axis be such a proxy. Applying our method to the disk around HD 142527 as a study case, we predict line-of-sight velocities scarcely detectable by currently available facilities. We show that corresponding spirals patterns can also be detected with similar spectral resolutions.

Speaker: Heloise Meheut (UCA/CNRS)

11:10 The time variable dynamics of snow-lines

Snow-lines are regions of protoplanetary discs where volatiles transition from the solid-phase to the gas phase. They play an important role in the chemical evolution of protoplanetary discs and perhaps planet/planetesimal formation. The majority of work treats these transitions as passive, uncoupled from the dynamics. I will argue that snow-lines in the outer regions of protoplanetary discs (where the dis cooling is optically thin), are thermally unstable. Namely, condensation leads to an increase in the solid abundance which leads to increased cooling and more condensation (or vica-versa). I will demonstrate a dynamical simulation that actively couples the condensation physics, to the radiative transfer and hence the disc's temperature, dust dynamics and growth. I will use this simulation to show that snow-lines are not static, but dynamically evolve in otherwise stationary discs and drive the dynamics in the outer regions of protoplanetary discs. We find the CO snow-line can move 10s AU on timescales of a few 1e5 years, creating further structures and rings in the disc and even multiple snow-lines. This thermal instability at snow-lines is likely important for the chemical, thermal and dynamical evolution of protoplanetary discs. It perhaps even plays a role in explaining ringed ALMA discs and planet/planetesimal formation.

Speaker: James Owen (Imperial College London)

11:35 Winds in transition disks

Protoplanetary disks (PPD) have been widely observed around young stars and are supposed to host planetary formation. Among these disks stand the transition disks (TD) which are characterized by a large hole in the central regions, whose formation remains yet unexplained. Despite this hole, accretion rates comparable to the ones found in PPD are measured, suggesting an inward motion of matter.

A possible explanation for these high accretion rates is the presence of magnetised winds that would allow matter to fall onto the star at high radial velocity. Following previous works, the disk can be described using non ideal MHD while the ideal MHD picture is used to compute the wind.

I will show the impact of the depleted dust repartition in TDs on the ionization fraction through theoretical calculations based on a simple lattice of chemical reactions. It can be shown that the non ideal MHD effects are also affected by such a hole. I will then present the results of 2D simulations modelling winds in a TD based on the predicted non ideal effects profiles.

Axisymetry allows to explore the parameters space and to check the stability of the hole profile through time. Such a work will later on lead the way to a more accurate description of the chemistry at stake in TD and 3D simulations.

Speaker: Étienne Martel (IPAG)

12:15 → 13:30 Lunch break

13:30 → 14:45 Observational Signatures & Constraints

13:30 Predicting the Observational Signature of Migrating Neptune-sized Planets in Low-viscosity Disks

The migration of planetary cores embedded in a protoplanetary disk is an important mechanism within planet-formation theory, relevant for the architecture of planetary systems. Consequently, planet migration is actively discussed, yet often results of independent theoretical or numerical studies are unconstrained due to the lack of observational diagnostics designed in light of planet migration. In this work we follow the idea of inferring the migration behavior of embedded planets by means of the characteristic radial structures that they imprint in the disk’s dust density distribution. We run hydrodynamical multifluid simulations of gas and several dust species in a locally isothermal α-disk in the low-viscosity regime (α = 10-5) and investigate the obtained dust structures. In this framework, a planet of roughly Neptune mass can create three (or more) rings in which dust accumulates. We find that the relative spacing of these rings depends on the planet’s migration speed and direction. By performing subsequent radiative transfer calculations and image synthesis we show that—always under the condition of a near-inviscid disk—different migration scenarios are, in principle, distinguishable by long-baseline, state-of-the-art Atacama Large Millimeter/submillimeter Array observations.

Speaker: Philipp Weber (Niels Bohr Institute)

13:55 Locating the water snowline in protoplanetary disks using the chemical tracer H13CO+

Snowlines, in particular the water snowline, are important for the formation of planets in protoplanetary disks. However, locating the water snowline directly is challenging. Firstly, due to the proximity of the water snowline to the host star. But ALMA can now resolve this region for the first time. Secondly, due to the absorption of water in the Earth's atmosphere. A chemical tracer, HCO+, provides a solution to the latter problem. HCO+ is destroyed by gas-phase water, therefore no HCO+ is expected to be present when water desorps from the grains. It has already been shown by van 't Hoff et al. (2018) that the optically thin isotopologue, H13CO+, acts as a tracer of the water snowline in the envelope around a Class 0 object. We investigate whether this also works in Class 2 objects where planets form. The HCO+ abundance is modeled using our small chemical network and using the density and temperature structure from a DALI model. The expected emission is modeled for different transitions of H13CO+. I will discuss how well H13CO+ traces the water snowline in disks. We can already confirm that the HCO+ abundance drops when water desorps from the grains and I will discuss what observations are needed to locate the water snowline with ALMA.

Speaker: Ms Margot Leemker (Leiden University)

14:20 Elemental chemistry in the inner regions of protoplanetary discs

Elemental abundance ratios in the inner regions of protoplanetary discs are important for setting the composition of exoplanets, but are likely not to represent the bulk composition of the star's parental cloud. Abundance differences are expected to be driven by the differential transport of chemical species in solid and gaseous form: elements which are mainly concentrated in species with high melting points may be either over-represented in the inner disc (due to efficient inward radial drift of icy grains) or else under-represented (if such grains become trapped in the outer disc).

Here I explore a new observational window for determining the relative abundances of C, N and Si in the inner disc through examination of ultraviolet emission lines generated by material accreting onto the central star via an accretion column and shock. I use CLOUDY to calculate the resulting ratios of CIV, NV and SiIV lines and how these vary with input elemental abundance ratios. I conclude that such line ratios provide a sensitive probe of abundances in the inner disc and describe the constraints provided by existing HST spectroscopic observations of T Tauri stars.

Speaker: Pooneh Nazari (Leiden University)