Turbulence is a key ingredient in planet formation and protoplanetary disk evolution. It generates an effective viscosity that regulates the global viscous evolution of the disk. Turbulence also controls local processes important for planet formation, for example, dust evolution, gas and solids accretion into planetary cores, the formation of planet-driven structures, among many others. However, the mechanism responsible for the low level of turbulence recently constrained by gas molecular line observations is not yet well understood. In the weakly ionized outer regions of the disk, purely hydrodynamical instabilities are expected to dominate as a source of turbulence. Among the candidates, The Vertical Shear Instability (VSI) is a promising mechanisms to operate in the outer regions. Nevertheless, empirical evidence of the instability from observations is still missing. We study the velocity perturbations produced by the VSI in the gas via 3D global hydrodynamical simulations of protoplanetary disks. Further, post-processing the hydro-simulations with a radiative transfer code, we investigate the observability of the VSI kinematic signatures in CO rotational emission lines with ALMA. We have found that VSI-signatures are observable in the line of sight velocity maps as deviations from a sub-Keplerian disk equilibrium solution. The deviations appear as axis-symmetric rings of meridional flows of gas, moving upwards and downwards through the disk. We explore the effect of disk inclination and emitting CO isotopologue in the observability of  the recovered structures. While a standard spatial resolution is enough to resolve the structure with ALMA, the highest spectral resolution available is needed. Observations of a close to face-on protoplanetary disk is the best option to detect VSI-signatures in CO kinematics.

The temperature structure of protoplanetary disks is a key property for interpreting observations, the physical and chemical evolution of the disk, and provides insight into planet formation. In this study, we constrain the two-dimensional thermal structure of the disk around Herbig Ae star HD 163296. Utilizing the thermo-chemical code RAC2D, we derive a thermal structure that reproduces spatially resolved ALMA observations (~0.12  - 0.25 arcsec) of CO J = 2-1, 13CO J = 1-0, 2-1, C18O J = 1-0, 2-1, and C17O J = 1-0, an HD J = 1-0 flux within the observed upper limit, and the spectral energy distribution (SED). The final model incorporates a radial depletion profile of CO (roughly by 0.5 inside of 50 au and .1 outside of 50 au) and additional heating above z/r = 0.21 within a radius of 100 au, which we speculate to be from PAH heating not accounted for in our model. Additionally, it reproduces the empirically derived temperatures and observed emitting surfaces. We find an upper limit for the disk mass of 0.41 Msun, using the upper limit of the HD J = 1-0 flux. With our final thermal structure, we explore the impact that gaps have on the temperature structure. Our model suggests that the temperature increases inside the gap, both in models with a gap only in the large dust grain distribution, and models with corresponding gaps in the gas and small dust populations. A large gap in the gas and small dust increases gap temperature by 5-10\%. We also find that when adding a gap in the gas surface density, the gas-phase CO abundance is enhanced within the gaps compared to a smooth gas surface density model by at most a factor of two.<

We perform a linear analysis of the stability of isothermal, rotating, magnetic, self-gravitating sheets that are weakly ionized. We include the nonideal magnetohydrodynamic (MHD) effects of Ohmic dissipation and ambipolar diffusion and focus on their influence on the properties of gravitational instability (GI). Our results show that there is always a preferred lengthscale and associated minimum timescale for GI depending on important free parameters of our model: the initial normalized mass-to-flux ratio mu_0, the rotational Toomre parameter Q, the Ohmic diffusivity eta_OD,0, and the neutral-ion collision time tau_ni,0 that is a measure of the ambipolar diffusivity (eta_AD,0).  Under flux freezing, there is a generalized Toomre criterion (that includes a magnetic dependence) and modified lengthscales and timescales for collapse. When nonideal MHD effects are also included, the Toomre criterion reverts back to the hydrodynamic value. We apply our results to protostellar disk properties in the early embedded phase and find that the maximum preferred scale of instability occurs in the transcritical (mu_0 >~ 1) regime, and can significantly exceed the thermal (Jeans) scale. A number density of about 10^12 cm^-3 is the approximate dividing line such that the Ohmic diffusivity eta_OD,0 becomes stronger than the ambipolar diffusivity eta_AD,0. We apply our model to the results of a simulation of a protostellar disk and find that ambipolar diffusion dominates Ohmic dissipation in most of the area of the disk and that nonideal MHD effects will modify the normal expectations of GI in the disk.

As protoplanetary disks are weakly ionised, the magnetorotational instability (MRI) cannot fully develop everywhere and lead to a fully turbulent disk. Instead, a dead zone naturally occurs where the ionisation level is too low for the magnetic field to efficiently couple to the charged particles, leading to low turbulence in that region. It is generally assumed that the turbulence can be encoded into a single parameter called the alpha-parameter. This parameter is crucial for understanding how gas and dust evolve because it somehow constrains the gas viscosity as well as the main dust transport processes. The presence of a dead zone leads to a non-constant alpha-parameter across the disk: low values in low ionised regions, high values in high ionised regions and a sharp transition from low to high ionised regions. It then promotes regions in the disk where dust can easily grow as well as regions where dust growth is limited by effective fragmentation. In this talk, I will present some of the results from the code mhdpy we have developed. This new tool is a bridge between the disk ionisation level and MRI-driven turbulence that can be combined to the dust evolution code dustpy to self-consistently study the impact of disk ionisation on the radial distribution of dust particles across the disk.

Turbulence within protoplanetary disks plays a crucial role in the formation and evolution of planets, through its influence on processes ranging from the collisional velocity of small dust grains to the ability of gas-giant planets to open gaps in the disk. Despite this importance, few direct observational constraints on its strength exist. I will report on our ongoing effort to constrain turbulence using ALMA observations of CO emission from protoplanetary disks. I will discuss recent additions to our collection of targets with upper limits (V4046 Sgr, MWC 480, IM Lup, HD 169142), as well as the prospect of constraining the physical mechanism driving turbulence based on our detection around DM Tau.

The dynamics of the gas and dust play a fundamental role in the formation of planets. To understand the detailed kinematics we still have to rely on advanced numerical simulations including the complex physics of magneto-hydrodynamics, radiation transfer and dust and gas interactions. In the recent years it became clear that the ionization state in protoplanetary disks suggest the existence of large areas with low ionization and weak coupling between the gas and magnetic fields. In this regime hydrodynamical instabilities may become important. In this talk I will present recent results from global 3D radiation hydrodynamics simulations covering all 360° of azimuth with embedded particles of 0.1 and 1 mm size. Stellar irradiation heating is included. The vertical shear instability turbulence produces a stress-to-pressure ratio of  α≃10−4  . The value of α is lowest within 30 au of the star, where thermal relaxation is slower relative to the orbital period and approaches the rate below which VSI is cut off. The rise in α from 20 to 30 au causes a dip in the surface density near 35 au, leading to Rossby wave instability and the generation of a stationary, long-lived vortex. Our results confirm previous findings that millimeter-sized grains are strongly vertically mixed by the VSI. The scale height aspect ratio for 1 mm grains is determined to be 0.037, much higher than the value H/r = 0.007 obtained from millimeter-wave observations of the HL Tau system. The measured aspect ratio is better fit by non-ideal MHD models. In our VSI turbulence model, the millimeter grains drift radially inwards and many are trapped and concentrated inside the vortex. Finally I will talk which constraints could be tested and compared with current and future ALMA observations.

The dispersion of protoplanetary disks is determined by several properties of the central star, the disk itself, and the surrounding environment. In particular, the metallicity of disks may impact disks evolution, even if to date controversial results exist: in low-metallicity clusters disks seem to rapidly disperse, while in the Magellanic Clouds some evidence support the existence of accreting disks few tens of Myrs old. In this talk I will discuss the dispersal timescale of disks in Dolidze 25, the low-metallicity young cluster closest to the Sun, with the aim of understanding whether disks evolution is impacted by the low-metallicity of the cluster. The study is based on new Chandra/ACIS-I data of the cluster and archival optical and infrared catalogs of the region. I will show marginal evidence supporting a faster evolution of disks in Dolidze25 compared with clusters and star-forming regions.

Magnetic fields likely play an important role in driving the evolution of protoplanetary disks through angular momentum transport, but observational evidence of magnetic fields has so far only been found in a small number of disks. Observations of circular polarization in molecular lines produced by Zeeman splitting offer a direct measure of the line-of-sight magnetic field strength in disks. We present upper limits on the net toroidal and vertical magnetic field strengths in the protoplanetary disk AS 209 derived from Zeeman splitting observations of the CN 2-1 line. The 3 sigma upper limit on the net line-of-sight magnetic field strength in AS 209 is 5.0 mG on the redshifted side of the disk and 4.2 mG on the blueshifted side of the disk. Given the disk's inclination angle, we set a 3 sigma upper limit on the net toroidal magnetic field strength of 8.7 and 7.3 mG for the red and blue sides of the disk, respectively, and 6.2 and 5.2 mG on the net vertical magnetic field on the red and blue sides of the disk. If magnetic disk winds are a significant mechanism of angular momentum transport in the disk, magnetic fields of a strength close to the upper limits would be sufficient to drive accretion at the rate previously inferred for regions near the protostar. Additionally, we find that the previously estimated accretion rate is more consistent with accretion driven mainly by magnetic disk winds than with accretion driven mainly by the magnetorotational instability.

In stellar clusters, the UV radiation of massive stars can very quickly truncate and deplete protoplanetary discs of material. This certainly affects the discs, but there is a broader question about whether or not this affects the resulting planets. Given the increasing evidence for very early planet formation (e.g.  Segura-Cox et al. 2020) perhaps by the time the discs are no longer embedded planet formation is already well established...
In this contribution I will introduce recently discovered proplyds in the extremely young (0.2-0.5Myr) sub-region of NGC 2024 (The flame nebula). As the star forming cloud gets evaporated by a nearby O star, discs are sequentially being exposed to the radiation and rapidly dispersed. With a disc fraction of only about 50% in this region (van Terwisga et al. 2020) we demonstrate that radiation environment can threaten even the very early evolution of planet forming discs.

Luminosity bursts hint at late accretion events. Such events can explain arc- and tail-like structures associated with disks around Herbig stars. In particular, an encounter event of gas with an existing star can lead to the formation of a second-generation disk significantly after the initial protostellar collapse phase. Additionally, observations of shadows in disks can be well described by a configuration of misaligned inner and outer disk, such that the inner disk casts a shadow on the outer disk. Carrying out altogether nine 3D hydrodynamical models with the moving mesh code arepo, we test whether a late encounter of an existing star-disk system with a cloudlet of gas can lead to the formation of an outer disk that is misaligned with respect to the primordial inner disk. Our models demonstrate that a second-generation disk with large misalignment with respect to an existing primordial disk can easily form if the infall angle is large. The second-generation outer disk is more eccentric, though the asymmetric infall also triggers eccentricity of the inner disk of e ≈ 0.05 to 0.1. Retrograde infall can lead to the formation of counter-rotating disks and enhanced accretion. As the angular momentum of the inner disk is reduced, the inner disk shrinks and a gap forms between the two disks. The resulting misaligned disk system can survive for ∼ 100 kyr or longer without aligning each other even for low primordial disk masses given an infall mass of ∼ 10−4 M. Synthetic images of our models reveal shadows in the outer disk similar to the ones observed in multiple transition disks that are caused by the misaligned inner disk. We conclude that late infall events can be responsible for observations of shadows in at least some transition disks. Finally, I briefly present the prospects in accounting for the protostellar environment in zoom-in models.

In this talk we focus on the long-term evolution and dispersal of protoplanetary disks. Although much effort has been made to understand the disk evolution, there still remain unresolved problems. In this talk we specifically focus on the following two questions: (i) How does stellar evolution affect disk evolution? (ii) How do disks with weak turbulence disperse? On the first question, since photoevaporation is driven by stellar high-energy photons, we first derived the evolution of stellar XUV luminosities by combining stellar evolution simulations, stellar atmospheric models and empirical relations from observations and theoretical considerations. From disk evolution simulations including a time-dependent photoevaporation model, we found that photoevaporation rates around low-mass stars are almost constant with time. On the other hand, those around intermediate-mass stars change dramatically: the X-ray photoevaporation rate decreases with time due to the stellar structure evolution (i.e., convective to radiative), whereas the FUV increases due to the increase of the stellar effective temperature. We conclude that stellar evolution is crucially important for the disk evolution around intermediate-mass stars. Our results show that the disk lifetime decreases with stellar mass, which has been suggested by observations. On the second question, both recent observations and theoretical studies have suggested protoplanetary disks are less turbulent. However, previous studies have suggested that a low viscosity results in a long (> 10 Myr) disk lifetime if the disk evolves with only viscous accretion and photoevaporation. In this study we investigated the effects of MHD winds and wind-driven accretion. First we investigated the disk evolution with these MHD processes and (inefficient) viscous accretion. We found that although these MHD processes significantly change the inner disk structure, the disks with last long. On the other hand, if we also consider photoevaporation, the disk lifetime can be less than 6 Myr. We conclude that all the three processes (i.e., photoevaporation, MHD winds and wind-driven accretion) should be considered in a realistic disk evolutionary model.

Understanding the magnetic field strength and morphology of proto-planetary disks is of great importance in understanding their dynamics. For (sub-)millimeter observations, it has been usual to probe the magnetic field morphology using dust or molecular line polarization. Recently, it has become clear that dust polarization does not always faithfully trace the magnetic field structure, but is instead affected by polarization mechanisms such as self-scattering or radiative alignment. Line polarization observations are not affected by such processes, and they therefore likely trace the magnetic field structure. Recently, we developed a three-dimesional polarized radiative transfer code adapted to line polarization (PORTAL). With it, we explore the tendancy for a range of spectral lines of different molecules to polarize under the influence of a variety of magnetic field structures. We speculate about the possibility of directly detecting ambipolar diffusion in disks through the polarization of molecular ions via collisional polarization.

We present a study of the evolution of the inner few astronomical units of protoplanetary disks around low-mass stars. We consider nearby stellar groups with ages spanning from 1 to 11 Myr, distributed into four age bins. Combining PANSTARSS photometry with spectral types, we derive the reddening consistently for each star, which we use (1) to measure the excess emission above the photosphere with a new indicator of IR excess and (2) to estimate the mass accretion rate ( Mdot) from the equivalent width of the Hα line. Using the observed decay of Mdot as a constraint to fix the initial conditions and the viscosity parameter of viscous evolutionary models, we use approximate Bayesian modeling to infer the dust properties that produce the observed decrease of the IR excess
with age, in the range between 4.5 and 24 μm. We calculate an extensive grid of irradiated disk models with a two-layered wall to emulate a curved dust inner edge and obtain the vertical structure consistent with the surface density predicted by viscous evolution. We find that the median dust depletion in the disk upper layers is epsilon ~ 3x10^-3 at 1.5Myr, consistent with previous studies, and it decreases to epsilon ~ 3x10^-4 by 7.5Myr. We include photoevaporation in a simple model of the disk evolution and find that a photoevaporative wind mass-loss rate of ~1–3x10^-9 Msun/yr agrees with the decrease of the disk fraction with age reasonably well. Our models show the inward evolution of the H2O and CO snowlines.

The connection between the nature of a protoplanetary disk and that of a debris disk is not well understood. We aim to reconcile both manifestations of dusty circumstellar disks through a study of optically thin Class III disks and how they correlate to younger and older disks. In this work, we collect literature and ALMA archival sub-mm/mm fluxes for 91 disks (15%) of all Class III disks across nearby star-forming regions. We derive millimeter-dust masses and compare these with Class II and debris disk samples in the context of excess infrared luminosity, accretion rate, and age. We propose a new evolutionary scenario wherein radial drift is very efficient for non-structured disks during the Class II phase resulting in a rapid decrease of Mdust, whereas disk evolution/dissipation is a more gradual process. We find long infrared protoplanetary disk timescales of ~7-9 Myr, which are also consistent with slow disk evolution. Finally, in structured disks, the presence of dust traps allow for the formation of planetesimal belts at large radii, such as those observed in debris disks. We propose that structured disks are thus directly connected to debris disks in one evolutionary pathway, in contrast to radial drift dominated disks which evolve to diskless stars. These results set the scene for a novel view of disk evolution.

Transition discs are expected to be a natural outcome of the interplay between photoevaporation and giant planet formation. Massive planets reduce the inflow of material from the outer to the inner disc, therefore triggering the earlier onset of disc dispersal due to photoevaporation through a process known as Planet Induced PhotoEvaporation (PIPE). In this case, a cavity is formed as material inside the planetary orbit is removed by photoevaporation, leaving only the outer disc to drive the migration of the giant planet. We investigate the impact of photoevaporation on giant planet migration and focus specifically on the case of transition discs with an evacuated cavity inside the planet location. This is important to determine under what circumstances photoevaporation is efficient at halting the migration of giant planets, thus affecting the final orbital distribution of a population of planets. For this purpose we use 2D FARGO simulations to model the migration of giant planets in a range of primordial and transition discs subject to photoevaporation. The results are then compared to the standard prescriptions used to calculate the migration tracks of planets in 1D planet population synthesis models. The FARGO simulations show that once the disc inside the planet location gets depleted of gas, planet migration ceases. This contradicts the results obtained by the impulse approximation, that predicts the accelerated inward migration of planets in discs that have been cleared inside the planetary orbit.These results suggest that the impulse approximation may not be suitable for planets embedded in transition discs. A better approximation which could be used in one-dimensional models would involve halting planet migration, once the material inside the planetary orbit is depleted of gas and the surface density at the 3:2 mean motion resonance location in the outer disc reaches a threshold  value of 0.01 g/cm^2.

Snow-lines are thought to play a vital role in the evolution of protoplanetary discs and planet formation at all scales. Snow-lines occur in regions of the protoplanetary discs where the temperature reaches the sublimation temperature and volatiles transition from the solid phase to the vapour phase (or vice-versa). However, in the outer region of protoplanetary discs (beyond a few AU), the temperature is set by the distribution of solids and their ability to absorb stellar light. Thus, the thermodynamics of the disc and the volatile phases are inextricably linked. In this talk, I will show this coupling is thermally unstable, and snow-lines continually evolve in regions of the disc that are marginally optically thick. Patches of the disc proceeding through a limit cycle, where volatiles in a region of the disc continually condense and then sublimate. Using numerical simulations of the CO snow-line I will show it can move 10s AU over 10,000 years, repeatedly. I will use these simulations to discuss how this new process may effect measured Carbon abundances, solid evolution and ultimately planet formation, making connections to high-resolution images of protoplanetary discs.

Direct imaging observations have revealed a population of massive planets (10 times the mass of Jupiter) that orbit around M dwarfs. Such massive planets cannot form by core accretion. We investigate whether disc fragmentation may form these planets. We perform radiative hydrodynamic simulations to study the conditions for disc fragmentation in M dwarf discs and the properties of planets formed by this mechanism. We find that a high disc-to-star mass ratio is required (0.3-0.6) for fragmentation to happen, which implies that if fragmentation is responsible for forming these planets it should happen at a very early stage. These protoplanets form fast and are initially hot, as expected from fragmenting collapsing gas (Mercer & Stamatellos 2020).

Studying how protoplanetary disks are affected by their environment (apart from the immediate vicinity of O-stars) is an important question which has so far been out of reach of observers. It requires the homogeneous analysis of a large number of Class II YSOs in a single region and with similar ages to test how this works for the weaker radiation fields that prevail in these environments. It is now well-established that strong radiation fields, such as those found near the O-stars in the Trapezium cluster, can have a dramatic impact on the millimeter-sized dust flux of protoplanetary disks. External photoevaporation leads to significant mass loss in these environments. This effect has been observed in ALMA observations of disks in the Trapezium and NGC 2024 clusters. We present new results from the Survey of Orion Disks with ALMA (SODA), based on observations of 872 disks in the southern part of the Orion A molecular cloud. This survey is the largest of its kind so far. Using it, we can study the impact of less massive stars on disk properties, as well as other parameters of the cloud, such as local stellar density.

Powerful atomic jets and molecular outflows are observed in young protostars at all stages of active accretion, from the young embedded Class 0 and Class 1 phases to the later optically revealed T Tauri or Class 2 phase. The origin of the ejection, its role in angular momentum extraction and impact on protoplanetary disk evolution remain as fundamental open questions in star formation. Studies at high angular and spectral resolution of molecular outflows are now providing important new clues to these questions. We will report recent band 6 (continuum and 12CO) ALMA observations of the Class I disk/outflow source DG Tau B. We reconstruct the full 3D geometry and kinematics (including rotation) of the CO outflow. We will discuss the implications brought by these observations for the origin of the CO cavities/outflows and their potential impact on the disk evolution.

Accretion is known to remove significant mass from the disks of young stars and thus to contribute strongly to disk dispersal. How the disk gas sheds angular momentum to accrete onto the central star is a critical but yet unanswered question. Recently, theorists have placed renewed emphasis on the role of outflows in disk evolution. Simulations have found that winds can drive accretion of disk gas onto the star. These winds are present over most of the planet forming region, can cause significant mass loss and in combination with the classic magneto-centrifugal winds extending out to a few au and thought to re-collimate into jets, could drive accretion. This has important implications for planet formation and migration.  This talk deals with identifying these disk winds observationally. Optical forbidden emission lines have long been used to trace outflow material from young stars. Early studies identified two distinct kinematic components. The so-called high velocity component (HVC) and a low velocity component (LVC) whose origin is less certain. Recently it has become clear that the LVC can be modelled by a combination of two Gaussian components referred to as a broad component and a narrow component (BC,NC). While studies have strongly suggested that the BC has aa disk wind origin the origin of the NC is less clear. Here we present the results of a spectro-astrometric study of a sample of 7 young stars the aim of which was to recover spatial information on the LVC. We show that kinematical information alone is not enough to determine the origin of the different components. In general, results agree with previous conclusions for the BC and in one case we determine that the narrow component of the LVC is indeed tracing a disk wind.

Among the YSOs and disks that are actively accreting material onto the central protostar, exists a class of objects that exhibit extreme outbursts, likely due to periods of enhanced accretion. These objects, called FUors and EXors, may represent a relatively common stage of stellar and disk evolution. The significantly enhanced luminosity from the outburst can impact the circumstellar environment that may eventually go on to form planets. We present ALMA and VLA observations of EX Lupi, the prototypical EXor outburst system. We use these data, along with archival ALMA data, to fit radiative transfer models to EX Lupi's circumstellar disk in its quiescent state following an extreme outburst in 2008. The best fit models show a compact disk with a characteristic dust radius of 45 au and a total mass of 0.01 solar masses. Our modeling suggests grain growth to sizes of at least 3 mm in the disk, possibly spurred by the recent outburst, and an ice line that has migrated inward to 0.2-0.3 au post-outburst. At low frequencies, we detected significant emission over the expected thermal disk emission which we attribute primarily to stellar (gyro)synchrotron and free-free disk emission. The presence of this low frequency emission is consistent with a > 3 kG stellar magnetic field. Altogether, these results highlight what may be a common impact of outbursts on the circumstellar dust.