Dust in disks with ALMA

Exciting abstract

Measuring scattered light emission from circumstellar disks

We investigate scattered light images of circumstellar disks for a better understanding of the dust particles which are an important ingredient for forming planets. We use data from SPHERE/ZIMPOL at the VLT to image the reflected light of bright transition disks and debris disks with high spatial resolution in the visible wavelength range. It is possible with our data to measure both the intensity and polarized intensity of the scattered light simultaneously and with high precision. For accurate measurements, it is important to take the limited resolution and the variable atmospheric conditions of the observations into account. We determine fractional polarization at different position and scattering angles in the disk and for different wavelengths and provide tighter constraints on the scattering phase function and albedo of the scattering dust.

Constraining grain size in protoplanetary disks with (sub-)/millimeter observations

Multi-wavelength observations of protoplanetary disks can be used to measure indirectly the size of dust grains through the millimeter spectral index. Detalied studies on single disks indicate that grain sizes are generally following a monotonic distribution in size, with larger grains close to the star and smaller grains outside, as we expect from the differential action of radial drift. Small scale structures that have been recently observed in the dust and gas density distribution are being associated with local pressure maxima in the gas, that could trap grains and promote their growth. Spatially resolved observations with ALMA can help in measuring the grain size across such sub-structures and investigate the timescale and mechanisms of dust growth.

Experiments with dust and pebble analogues

A particle-gas two-stream instability was recently discovered in a laboratory flow, which exhibited scale similarity to the same fluid instability predicted to operate in protoplanetary discs. Owing to its ability to aerodynamically concentrate solids, this mechanism has been singled out as a promising route to enable the gravitational collapse of planetesimals – thus overcoming the meter-size barrier problem – and is one of several ways to generate disc turbulence. I will review the experimental findings and report on the progress in developing new rarefied gas flow facilities to follow-up these results.

From dust to planetesimals — a journey along 32 orders of magnitude in mass

It is believed that planetesimals, the smallest building bricks of planets that are gravitationally bound, are needed in order to form planets. Unfortunately, planetesimals cannot be observed in other systems which is why we rely on theoretical modelling. In this review talk I will recap particle dynamics in circumstellar disks, how we think micron-sized dust grows to dm-sized pebbles, and what is limiting further growth. Pebbles can get concentrated which finally leads to a jump to roughly 100 km objects caused by gravitational instability. Recent works model planetesimal formation incorporated into dust and gas evolution codes, leading to a radial distribution where its shape affects further planetesimal-planetesimal interactions. Hence, knowledge of the spacial distribution of initial planetesimals is crucial for simulations on planet formation. I will compare different models for the planetesimal formation rate and discuss (dis-)advantages.

Timing of Rapid Core Cooling Events in the Early Solar System Revealed by the Pd-Ag Chronometer

Rapid cooling of planetesimal cores was inferred for several iron meteorite parent bodies based on fast and variable metallographic cooling rates. This was linked to the loss of insulating mantles during hit-and-run impacts. The short-lived 107Pd-107Ag decay system can constrain core crystallisation, and hence the timing of disruptive events. However, the solar system initial (SSI) 107Pd/108Pd is ambiguous. We use Pd-Ag ages for the IAB, IIAB and IIIAB iron meteorites to constrain the causes of these events.

Cooling ages for these three bodies calculated using a 107Pd/108Pd SSI of 3.5x10-5 suggest a highly energetic inner solar system between ~2.9-6.3 Myr after CAI. This corresponds with the suggested outward migration of Jupiter after ~5 Myr. However, this SSI is at odds with thermal events on the IAB parent body. Closure times relative to a 107Pd/108Pd SSI of 5.9x10-5 equate to cooling in the later timeframe ~7.8-11.2 Ma after CAI, and may instead date the dissipation of gas in the disk. After this, the damping effect of gas drag ceases and energetic collisions capable of disrupting planetesimals are predicted. This age coincides with observations of the lifetime of gas in stellar disks.

Radial Drift and Concurrent Ablation of Boulder-Sized Objects

The composition of a protoplanetary disk at a given location does not only depend on temperature and pressure but also on the time dependent transport of matter, such as radial drift of solid bodies, which could release water and other volatile species before disintegration or accretion onto a larger body with potentially large implications for the composition of planets. To study the water depletion of different sized bodies able to cross the water snowline by gas induced radial drift, we either use the analytical Hertz-Knudsen-Langmuir sublimation formula or a more involved, numerical model for the internal thermal evolution is coupled with an $\alpha$-disk model. Different properties of the disk and the embedded body are explored. Bodies with radii up to \SI{100}{\meter} drift faster towards the central star than the water snowline, hence, cross it. The region that can be reached before complete disintegration, and is therefore polluted with H$_2$O ice, can extend all the way to half of the snowline location, depending heavily on the presence and properties of a dust mantle. Hence, given a significant abundance of meter sized boulders in protoplanetary disks, the transport of water by radial drift of these bodies towards regions closer to the star than the snowline is not negligible and this flux of volatiles can be estimated for a given distribution of solid body sizes and compositions. A simple expression for surface sublimation is applicable for a homogeneous body consisting of only dust and water ice without a dust mantle.

Sublimation experiments with ice/dust mixtures

Between 2014 and 2017, we have performed a series of sublimation experiments with various ice/dust samples in the Planetary Ice Laboratory at the University of Bern. In this presentation, I will review these past experiments, summarize what we have learned and describe our plans for future work on this topic based on these previous results. This presentation will be complemented by a visit of the laboratory.

The collisional tale of minor bodies as clue to their origin

A number of small bodies such as cometary nuclei, asteroids and moons have been imaged from flyby and rendezvous spacecraft. How and when these objects acquired their peculiar characteristics has distinct implications on the origin of the solar system and its dynamics. The collisional tale of some of these bodies will be analysed and the implications discussed in the context of the formation of planetesimals, cometesimals and moonlets.

Orbital dynamics of small bodies with non-Newtonian forces

To compute the orbital dynamics of small bodies in the Solar System, non-Newtonian forces must be considered. These includes Yarkovsky effect, Poynting-Robertson drag, tidal forces and GR-corrections. It will be demonstrated how this forces can be included in an n-body integrator. As an application, an Asteroid break-up event is considered, by modelling the produced meteor streams.

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Exciting abstract

3D Modelling of the activity distribution in the inner-coma of comet 67P/Churyumov-Gerasimenko

The Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA) provided in situ measurements of the gas abundances in the coma of comet 67P/Churyumov- Gerasimenko (67P/CG), which we use to investigate the distribution of gas emissions from the surface.

We employ the 3D Direct Simulation Monte Carlo (DSMC) method to simulate the coma of 67P/CG up to 10 km above the surface. Comparing our model results with ROSINA/COPS and ROSINA/DFMS data, we can modify the activity on the surface to find the best fit possible. Our results suggest that a purely insolation-driven outgassing from a homogeneous surface distribution of H2O-ice is not enough condition to explain the data. Moreover, measurements at high cometocentric distances make difficult to distinguish between subtle differences in distributions of ice at the surface. We therefore need to use multi-instrument data to constrain the emission distribution at the nucleus.

Researching the place of birth of a cometary dust tail

The vast OSIRIS (Optical, Spectroscopic, and Infrared Remote Imaging System) dataset of the Rosetta mission to comet 67P/Churyumov-Gerasimenko provides unprecedented means of studying the dust in the innermost coma of a comet. The image data set contains information about the dynamics of dust motion and the properties of dust particles in the first few kilometres above the nucleus surface. Through studying the scattered light from the unresolved dust coma caught on the image data, we aim to learn more about the aforementioned aspects. However, interpretation of the data is often not straight forward and as a tool to help us with the interpretation we use numerical simulation of the gas and dust coma in 3D around the comet nucleus to produce synthetic results directly comparable with measurements from the Rosetta mission.

In my talk at the workshop, I will shortly introduce our method of simulating the dust coma around comet 67P. I then focus on how I use these results in combination with analysis of OSIRIS images to gain a better understanding of the physical processes governing dust dynamics in the first few kilometres above the comet surface.

How the radial pebble flux determines a terrestrial-planet or super-Earth growth mode

Around a third of all solar-like stars host super-Earths, planets with sizes between the Earth and Neptune. We argue that these planets are not scaled-up terrestrial planets that grow only through mutual accretion of planetary embryos, as previously advocated. Instead, we propose that these super-Earths form by a combination of pebble accretion and embryo migration. Our numerical integrations, which combine pebble accretion, type-I migration and N-body gravity between embryos, show the formation of systems of close-in super-Earths in the 5 to 10 Earth-mass range, bounded by the pebble isolation mass. In the subsequent dynamical evolution of these compact systems we find that, after disc dissipation, instabilities generally trigger additional merging events that dislodge the system from resonant chains. If the pebble flux is too low for the formation of super-Earths, we instead find a terrestrial-planet formation mode where Mars-mass embryos after disc dissipation form Earth-like planets. A concluding discussion will highlight the promising aspects of this model with respect to understanding our exoplanet sample, but also cover outstanding challenges.

Accretion of gas and dust onto embedded planets

Planets grow in size/mass by accreting gas and dust from protoplanetary discs that surround young stars for the first few million years of their evolution. The 3D accretion flow is complex and strongly depends on the cooling rate of the gas. We use 3D smoothed particle hydrodynamic simulations to follow isothermal and adiabatic accretion of gas onto an embedded 15 Earth-mass core at 0.25 au in a disc orbiting a solar mass star. After relaxing the system for 7--10 orbits, we add six dust phases (0.5 micron -- 5 cm) and track the resulting dynamics, accretion, and mixing in and around the planet. By simultaneously tracking multiple grain sizes in the flow we can approximate the 3D dust opacity, may have important implications for determining the thermal structure in more sophisticated cooling models.

Dusty disks dynamics is not so simple!

Since a decade, our understanding of PPDisk dynamics and of planet formation processes has done a big step foreward. However, the increasing amount of disk observations (ALMA, scattered light) show complex structures and dynamics, which are a chalenge for the models. I will show that even with simple physics, disk models can have a very broad range of outcomes. Dust and gas interaction being a simple but formidable source of complexity! When following in details the long term evolution of dusty disks, it comes out that classical, overrepresentated processes like the streaming instability or the pebble accretion are found to be ineffective. The dynamics of planet forming dusty disks is very complex and exploring this diversity is fascinating. It is also mandatory in order to make models really predictive.

Chondrules: tracers of early solar system processes and evolution

Chondrules are small (100 µm to a few mm) spherical objects found in primitive meteorites named chondrites. They formed in the early times (1-6 Myr after Calcium-, Aluminium-rich Inclusions (CAIs)) of the solar protoplanetary disk where they experienced high temperature events and fast cooling, causing processing of precursor dust grains. They can make up to 80 % of chondrites, meteorites that are believed to be the building blocks of terrestrial planets. Thus, the study of these objects is important to better understand early processes in the solar system and their link to planetary formation. However, the exact processes that led to their formation are still in debate.

We present our exploration of a model in which the shocks caused by a massive planet (Jupiter) in the disk are considered as a possible source of heat for chondrule formation. This exploration is conducted using 2D hydrodynamical simulations with the RoSSBi code.