The Formation and Evolution of Planets and Their Discs 19.06.-21.06.2017, LRZ, Garching Abstracts Booklet
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The Formation and Evolution of Planets and Their Discs 19.06.-21.06.2017, LRZ, Garching Abstracts booklet MONDAY, 19.06.2017 Transition Disks: Observations vs Models Cornelis DULLEMOND With the enormous progress in high-resolution and high-contrast imaging of protoplanetary disks at optical/NIR and (sub-)millimeter wavelengths, protoplanetary disks have revealed themselves to be highly structured, yet highly “organized”: Rather than displaying a clumpy mess, we see mathematically well-defined shapes: rings, m=2 spirals, shadows, one-sided blobs. This offers a unique chance to study the physics of protoplanetary disks. I will talk about a few lines of thought how we might connect these observations with simple physical mechanisms. An opening criterion for dust gaps in protoplanetary discs Giovanni DIPIERRO Recent spectacular spatially resolved observations of gaps and ring-like structures in nearby dusty protoplanetary discs have revived interest in studying gap-opening mechanisms. In this talk I’ll describe the two distinct physical mechanisms for dust gap opening by embedded planets in protoplanetary discs: I) A mechanism where low mass planets, that do not disturb the gas, open gaps in dust by tidal torques assisted by drag in the inner disc, but resisted by drag in the outer disc; and II) The usual, drag assisted, mechanism where higher mass planets create pressure maxima in the gas disc which the drag torque then acts to evacuate further in the dust. Starting from numerical evidences, we derive a grain size-dependent criterion for dust gap opening in viscous protoplanetary discs by revisiting the theory of dust drift to include disc-planet tidal interactions and viscous forces. From this formalism, we derive i) a grain size-dependent criterion for dust gap opening in discs, ii) an estimate of the location of the outer edge of the dust gap and iii) an estimate of the minimum Stokes number above which low-mass planets are able to carve gaps in the dust only. These analytical estimates are particularly helpful to appraise the minimum mass of an hypothetical planet carving gaps in discs observed at long wavelengths and high resolution. We validate the theory against 3D SPH simulations of a broad range of dusty discs hosting an embedded planet. We find a remarkable agreement between the theoretical model and the numerical experiments. Shadows and spirals in the protoplanetary disk HD100453 Myriam BENISTY In this talk, I will present observations of the Herbig Ae star HD100453 in polarized scattered light with SPHERE/VLT at optical and near infrared wavelengths. In the observations, we detect a cavity, a rim with azimuthal brightness variations, two shadows and two symmetric spiral arms. I will present our radiative transfer model that accounts for the main characteristics of the features, with an inner and outer disk misaligned by 72 degrees. Finally, motivated by the close proximity of the spirals with the shadows, I will discuss the hydrodynamical consequences of the shadows on the disk. Warm gas inside the dust cavities of transition disks, planets, Andres CARMONA GONZALEZ and accretion. Transition disks are protoplanetary disks that have inner dust cavities with sizes of a few to tens of AU. To measure the gas content inside those cavities is important to establish the mechanisms responsible of the transitional disk shape. In this talk, I will review our current knowledge of the warm gas inside the dust cavities of transition disks, and discuss the links to planet formation, disk dissipation, and accretion. I will illustrate the talk with examples of transition disks in which detailed modeling of 4.7 micron CO ro-vibrational emission, [OI] 63 micron emission, and sub-mm CO rotational lines has been performed. I will argue that although dust cavities contain molecular gas, there is significant reduction of the gas density inside them and that the distribution of the gas is compatible with a flat or an increasing surface density profile with radius. I will show that if the gas density drop and dust gaps and cavities are due to a planet, its mass should be lower than a few Jupiter masses. Finally, I will discuss that the measured inner gas surface densities are not compatible with the accretion rates reported if the disks are assumed to be standard alpha disks. Probing dust trapping in transition disks with (sub-)millimeter Adriana POHL polarization observations There have been several mechanisms proposed to produce millimeter-wave polarization in the context of evolved planet-forming disks: dust grain alignment with magnetic fields or radiative flux, and self-scattering of thermal dust emission. We claim that, apart from the spectral index, the detection of polarization due to self-scattering is an independent method to constrain the disk grain size distribution. In this context we study the dust trapping scenario proposed for transition disks by predicting the polarization at ALMA bands due to scattered thermal emission, based on planet-disk interaction and self-consistent dust growth models. With radiative transfer calculations we determine the polarization degree in the disk and investigate the polarization map, which shows a characteristic ring pattern. We compare different dust evolution timescales and analyze the wavelength dependence of the polarization degree. Self-induced dust traps: overcoming planet formation barriers Jean-François GONZALEZ Planet formation is thought to occur in discs around young stars by the aggregation of small dust grains into much larger objects. The growth from grains to pebbles and from planetesimals to planets is now fairly well understood. The intermediate stage has however been found to be hindered by the radial-drift and fragmentation barriers. We identify a powerful mechanism in which dust overcomes both barriers. Its key ingredients are (i) backreaction from the dust on to the gas, (ii) grain growth and fragmentation and (iii) large- scale gradients. The pile-up of growing and fragmenting grains modifies the gas structure on large scales and triggers the formation of pressure maxima, in which particles are trapped. We show that these self-induced dust traps are robust: they develop for a wide range of disc structures, fragmentation thresholds and initial dust-to-gas ratios. They are favored locations for the formation of pebble-sized solids and their subsequent growth into planetesimals, thus opening new paths towards the formation of planets. Rings, rings, rings: what does CN tell us? Paolo CAZZOLETTI In the last few years ALMA has shown that ring-like structures are common in protoplanetary disks. They can be found in both gas and dust, and in both full and transitional disks, but the rings differ depending on tracer. CN, one of the brightest molecules in protoplanetary disk, is a particularly interesting case. High-resolution ALMA CN observations often show ring-like structures. We investigate if such structures are due to the morphology of the disk itself or if they are instead an intrinsic feature of the emission of CN. Using the 2D thermochemical code DALI (Bruderer 2013), we run a set of disk models for different stellar spectra, disk masses and disk structures, and by using an updated chemical network accounting for the most relevant CN reactions. We find that ring-shaped emission is a common feature of all adopted models: the highest abundance is found in the outer regions, the column density always peaks at 50-70 AU (for T Tauri stars), and the emission profile follows the column density. Higher mass disks therefore generally show brighter CN. We also find a strong dependence of the ring brightness and location on the UV field, and in general higher incident UV fluxes result in brighter and larger rings. This happens when the UV radiation impinging on the disk increases because of a Herbig rather than T Tauri star, or due to higher disk flaring. These trends are caused by the fact the main formation route of CN is thrpugh reactions of N with excited H2*, which is formed through FUV pumping of the H2 molecules. The strong link between FUV flux and CN emission and morphology could therefore provide critical information on the physical structure of the outer part of the disk and on the distribution of dust grains (which affects the UV penetration), and could help to break some degeneracies in the SED fitting. Finally, CN emission is optically thin and comes from the upper layers of the disk, making it an ideal molecule for probing the vertical structure of disks. The effect of radiative feedback on disc fragmentation Anthony MERCER Protostellar discs may become massive enough to fragment and produce planets, brown dwarfs or low-mass stars. Accretion of material onto these objects would result in an increase in temperature and heat the surrounding disc. Similar to their higher mass stellar counterparts, we assume that the accretion onto such formed objects could be episodic, occurring in short intermittent bursts. We have simulated massive discs using radiative hydrodynamics to study the effect of radiative feedback from formed objects. We compare the results from cases where the secondary objects provide: no radiative feedback, continuous radiative feedback and episodic radiative feedback. We find that the most objects form when radiative feedback is neglected, and the least when radiative feedback is continuous. Episodic radiative feedback does not suppress further disc fragmentation as the disc cools sufficiently between episodes. Radiative feedback mildly suppresses consequent mass accretion, but the mass accretion history ultimately depends on where the object is formed within the disc, and on further interactions. We find that the masses of secondary objects formed by disc fragmentation are between a few Jupiter masses to a few tenths of a solar mass. Planets formed by fragmentation are typically ejected from the disc. We conclude that wide-orbit planets are unlikely to form by disc fragmentation.