EXO1

How to cite: Kluska, J.: Zooming in on the place of rocky planet formation: infrared interferometric observations of protoplanetary disks, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-861, https://doi.org/10.5194/epsc2021-861, 2021.

Short-period super-Earths and mini-Neptunes have been shown to be common, yet it is still not understood how and where inside protoplanetary discs they could have formed. To form these planets at the short periods at which they are detected, the inner regions of protoplanetary discs must be enriched in dust. Dust could accumulate in the inner disc if the innermost regions accrete via the magneto-rotational instability (MRI). We developed a model of the inner disc which includes MRI-driven accretion, disc heating by both accretion and stellar irradiation, vertical energy transport, dust opacities, dust effects on disc ionization, thermal and non-thermal sources of ionization. The inner disc is assumed to be in steady state, and the dust is assumed to be well-mixed with the gas. Using this model, we explore how various disc and stellar parameters affect the structure of the inner disc and the possibility of dust accumulation. We show that properties of dust strongly affect the size of the MRI-accreting region and whether this region exists at all. Increasing the dust-to-gas ratio increases the size of this region, suggesting that dust may accumulate in the inner disc without suppressing the MRI. Overall, conditions in the inner disc may be more favourable to planet formation earlier in the disc lifetime, while the disc accretion rate is higher.

How to cite: Jankovic, M., Mohanty, S., Owen, J., and Tan, J.: MRI-accreting inner regions of protoplanetary discs, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-70, https://doi.org/10.5194/epsc2021-70, 2021.

Sticking properties are of great importance for planetesimal formation in protoplanetary disks. During the early phases of pebble growth, grains regularly travel from cold, water-rich regions to the warm inner part of the disk, experiencing ever higher temperatures. This drift leads to changes in composition, grain size, morphology, and water content. To follow the evolution that the dust might go through, we milled a meteorite to micrometer dust and tempered it under vacuum at increasing temperatures up to 1400 K. For each tempered sample we measured the splitting tensile strength of millimeter-sized dust aggregates using a well-established method known as the Brazilian test. To link the evolution of surface energy with temperature to compositional changes, we additionally measured the (ferric) composition of the samples by Mössbauer spectroscopy. 

At the low pressure of protoplanetary disks and at moderate temperatures, grains presumably hold only a monolayer of surface water and can thus be considered as dry. As the dust drifts further inwards towards higher temperature regions even this monolayer of water may evaporate completely and grains are super dry. In the laboratory, contacts between grains in cylinders are dry and remain dry for measurement if they are prepared for the Brazilian test before tempering. If cylinders are prepared after tempering but from the tempered dust, contacts are wet contacts but the influence of the composition can be traced.

For wet samples, we measured an effective surface energy which monotonously decreases by a factor of 5 from room temperature to about 1300 K due to compositional changes [1]. This reduction of surface energy still holds for dry samples but with an increase in the sticking force by a factor of ~ 10 over the wet samples. Super dry samples deviate strongly from this starting at about 900 K. Above this temperature, the surface energy is boosted and increases exponentially up to another factor of ~ 100 at about 1200 K [2].

Exceeding temperatures of 1300 K, grain sizes microscopically change (increase) for all samples, leading to an instability of aggregates and therefore making growth challenging. Beyond 1400 K no classical collisional growth is possible. 

As consequence, there is a spatial region in protoplanetary disks with temperatures around 1200 K which favors aggregation and, therefore, will likely be a sweet spot for planetesimal formation as indicated in fig.1.

[1] Bogdan, T., Pillich, C., Landers, J., Wende, H., & Wurm, G. (2020). Drifting inwards in protoplanetary discs I Sticking of chondritic dust at increasing temperatures. Astronomy & Astrophysics, 638, A151.

[2] Pillich, C., Bogdan, T., Landers, J., Wurm, G., Wende, H. (2021). Drifting inwards in protoplanetary discs II The influence of water on sticking properties at increasing temperatures. Astronomy & Astrophysics, (accepted).

How to cite: Bogdan, T., Pillich, C., Landers, J., Wende, H., and Wurm, G.: The influence of water and mineralogy on sticking properties at increasing temperatures, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-514, https://doi.org/10.5194/epsc2021-514, 2021.

Here we discuss terrestrial planet formation by using Earth and our knowledge from various isotope data such as ¹⁸²Hf-¹⁸²W, U-Pb, lithophile-siderophile elements, atmospheric ³⁶Ar/³⁸Ar, ²⁰Ne/²²Ne, ³⁶Ar/²²Ne isotope ratios, the expected solar ³He abundance in Earth’s deep mantle and Earth’s D/H sea water ratios as an example. By analyzing the available isotopic data one finds that, the bulk of Earth’s mass most likely accreted within 10 to 30 million years after the formation of the solar system. Proto-Earth most likely accreted a mass of 0.5 to 0.6 M_Earth during the disk lifetime of 3 to 4.5 million years and the rest after the disk evaporated (see also Lammer et al. 2021; DOI: 10.1007/s11214-020-00778-4). We also show that particular accretion scenarios of involved planetary building blocks, large planetesimals and planetary embryos that lose also volatiles and moderate volatile rock-forming elements such as the radioactive decaying isotope ⁴⁰K determine if a terrestrial planet in a habitable zone of a Sun-like star later evolves to an Earth-like habitat or not. Our findings indicate that one can expect a large diversity of exoplanets with the size and mass of Earth inside habitable zones of their host stars but only a tiny number may have formed to the right conditions that they could potentially evolve to an Earth-like habitat. Finally, we also discuss how future ground- and space-based telescopes that can characterize atmospheres of terrestrial exoplanets can be used to validate this hypothesis.   

How to cite: Lammer, H., Scherf, M., and Erkaev, N. V.: Terrestrial planet accretion constrained by isotopes: Implications for Earth-like habitats, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-721, https://doi.org/10.5194/epsc2021-721, 2021.

How to cite: Lichtenberg, T. and Krijt, S.: System-level fractionation of carbon from disk and planetesimal processing, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-130, https://doi.org/10.5194/epsc2021-130, 2021.

In early phases of planet formation, bouncing and fragmentation barriers still represent major obstacles. Beginning at micrometer, dust can readily grow to sub-millimeter size in collisions due to cohesion before bouncing prevails. Later, streaming instabilities trigger further growth which might finally results into planetesimal formation by gravitational collapse. However, for streaming instabilities sub-millimeter grains might be too small, therefore there is gap of at least 1 order of magnitude in size which needs to be bridged.

Here, we present our ongoing work how to bridge this gap by charge moderated aggregation [1]. When two (dielectric) grains collide they charge. This tribocharging or collisional charging is omnipresent in nature. We designed drop tower experiments in which we generated charges on glass and basalt grains by collisions in a shaker. In microgravity, the particle trajectories and collisions were observed, and charges were measured by applying an electric field.

In early work, we analyzed millimeter-sized glass grain collisions with a copper plate. The coefficient of restitution increased with the charge on a single grain due to mirror charge forces. That means highly charged grains tend to stick more easily to surfaces than uncharged grains. The velocity where sticking is possible was increased by a factor of 100 up to several dm/s [2].
 
More recently, we used half millimeter basalt spheres and observed sticking events at several cm/s among grains themselves [3]. This is also way higher than predicted by adhesion. In a number of cases, we could observe the sequential formation of aggregates of up to ten single grains. During approach the grains are accelerated due to net charge Coulomb forces but likely also due to higher order charges on the surfaces in agreement to earlier measurements of strong permanent dipole moments [4]. Attraction increases collision cross-sections and the growth is sped up. Growth only stopped by the end of microgravity [3]. 

To observe the formation of still larger aggregates we developed a new setup, in which a dense cloud of 150 µm diameter basalt grains was continuously agitated slightly under microgravity and in vacuum. Here, the growth of a giant aggregate of centimeter size was observed collecting nearly all material in one cluster [5].

To conclude, in experiments under various conditions, we see strong evidence that electrostatic charges on grains are able to conquer the bouncing barrier. We observed the bottom-up growth tracking individual particles, stable clusters emerging from dense regions and the formation of giant clusters during agitation. These are all bricks in the wall giving evidence that collisional charging might play a crucial role in planet formation.

References:

[1] Steinpilz, T.; Joeris, K.; Jungmann, F.; Wolf, D.; Brendel, L.; Teiser, J.; Shinbrot, T.; Wurm, G. Nature Physics 2020a, 16, 225-229.

[2] Jungmann, F.; Steinpilz, T.; Teiser, J.; Wurm, G. Journal of Physics Communications 2018, 2 095009, 095009.

[3] Jungmann, F.;Wurm, G. Astronomy and Astrophysics 2021, DOI: https://doi.org/10.1051/0004-6361/202039430.

[4] Steinpilz, T.; Jungmann, F.; Joeris, K.; Teiser, J.; Wurm, G. New Journal of Physics 2020b, 22, 093025.

[5] Teiser, J.; Kruss, M.; Jungmann, F.; Wurm, G. The Astrophysical Journal Letters 2021, 908, L22.

How to cite: Jungmann, F., Teiser, J., Kruss, M., Steinpilz, T., Joeris, K., and Wurm, G.: Charge moderated preplanetary growth from single grains to giant aggregates, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-265, https://doi.org/10.5194/epsc2021-265, 2021.

Protoplanetary disks are thought to harbor deadzones, regions in their interior, where the ionization fraction is too low to trigger MRI turbulence. This has quite severe consequences for accretion and planet formation. 
So far, the dominant source considered for ionization is high energy radiation from external cosmic rays over stellar radiation to radioactive decay. Especially, external radiation might indeed be attenuated too much before reaching parts of the midplane in disks. Radiation might not be the only source of ionization though.

In recent years, we carried out a number of laboratory experiments on collisional charging (tribocharging) of grains related to planet formation. Being mm-size, in a range where fragmentation and bouncing dominate, these particles are large and, although charged, might not be relevant in the context of MRI themselves directly. 

An underlying assumption though is that triboelectric charge generated in collisions among grains stays on the grains. In this case, grain charging and aggregation as part of early particle growth on one side and MRI building on gas ionization on the other side would be distinct processes. However, what we recently found in a number of different experiments, is that tribocharging of grains also charges the ambient gas.

In one set of microgravity experiments, we observed that the charge on two particles before and after a collision is not conserved. On the order of 20\% of the pre-charge on a grain is discharged into the environment. To mention a second experiment, the number of ions detected in a gas flow after passing a granular medium increases strongly if the particles undergo collisions, i.e. are charged. There are experiments with further evidence but these two clearly show that charge is leaking into the ambient atmosphere during collisions.

In detail, the charging depends on the collision frequencies and particle sizes, which are model dependent. We also caution that these are only first experiments in this direction, but, in any case, estimates suggest that ionization rates within the gas might regularly be way larger than e.g. provided by radiation at the surface of a disk. If this holds, the charge balance in the midplane of protoplanetary disks might be quite different than previously assumed. The obvious and severe consequence might simply be that there are just no deadzones.

How to cite: Wurm, G., Jungmann, F., van Unen, H., Voellings, N., Schoenau, L., and Teiser, J.: Does Leakage of Charge in Particle Collisions Kill the Concept of Deadzones?, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-219, https://doi.org/10.5194/epsc2021-219, 2021.

A notable challenge of planet formation is to find a path to directly form planetesimals from small particles. Small particles are, in general, challenged to grow beyond meter size due to the “growth barrier” and the “drift barrier”.

Over the last few decades, magnetohydrodynamic (MHD) simulations have shown that a region where ionization is too low for the magneto-rotational instability (MRI) to operate (i.e., a “dead zone”) might ubiquitously exist in the inner part of the disk midplane, and that only surface layers are magnetically active, supporting accretion. These indicate a nonuniform turbulence structure.

In this study, we aim to understand how drifting pebbles pile up in a protoplanetary disk with a nonuniform turbulence structure (Figure 1). We consider a disk structure in which the midplane turbulence viscosity (i.e., diffusion of pebbles is characterized by a distinct non-dimensional parameter (α_mid)) increases with the radius in protoplanetary disks, such as in the outer region of a dead zone. We consider that the gas accretion toward the central star is characterized by a distinct non-dimensional parameter (α_acc). We perform 1D diffusion-advection simulations of pebbles that include back-reaction (the inertia) to the radial drift and the vertical and radial diffusions of pebbles for a given pebble-to-gas mass flux (F_p/g).

We report a new mechanism, the “no-drift” runaway pile-up (i.e., instability), that leads to a runaway accumulation of pebbles in disks, thus favoring the formation of planetesimals by streaming and/or gravitational instabilities. This occurs when pebbles drifting in from the outer disk and entering a dead zone experience a decrease in vertical turbulence. Consequently, the scale height of the pebble then decreases, and, for small enough values of the turbulence in the dead zone and high values of the pebble-to-gas flux ratio, the back-reaction of pebbles on gas leads to a significant decrease in their drift velocity and thus their non-steady-state accumulation (i.e., a local runaway pile-up of pebbles). This process is independent of the existence of a pressure bump and/or pebble growth.

Figure 1: Schematic illustration of pebble drift and its pile-up within a protoplanetary disk with a dead zone. The disk gas accretion is characterized by alpha-parameter (α_acc), while the midplane diffusivity, being a dead zone (α_dead << α_acc) in the inner region, is characterized by α_mid. During the inward drift of pebbles, the pebble scale height H_p decreases as H_p is proportional to α_acc^1/2 until a KH instability prevents it from becoming smaller. A smaller H_p leads to an elevated local midplane concentration of pebbles within a thinner midplane layer. The elevated midplane pebble-to-gas ratio causes the back-reaction to be more effective in reducing the radial drift velocity of pebbles. Such a physical interplay with a sufficiently large pebble-to-gas mass flux results in a continuous accumulation of pebbles in a runaway fashion (i.e., the “no-drift” runaway pile-up). Redrawn from Hyodo, Ida, Guillot (2021) A&A, 645, L9.

How to cite: Hyodo, R., Ida, S., and Guillot, T.: Planetesimal formation by the “no-drift” mechanism, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-161, https://doi.org/10.5194/epsc2021-161, 2021.

    Large scale vortices are thought to be natural outcomes of hydrodynamic instabilities in protoplanetary disks, as for instance the Rossby Wave Instability [1] or Baroclinic instability. Analytical and numerical studies showed that they can be long-lived and catalyze efficiently dust material concentration (e.g) [2], [3] and encourage to think they could play a role in protoplanetary-disk evolution and planetesimal formation. Their presence in the outer regions of circumstellar disks is possibly betrayed by recent observations of lopsided structures with ALMA and VLT [4], [5] and raises also a question: what is the importance of disk self-gravity on their structure and evolution.

    We present 2D hydrodynamical simulations of steady vortices evolving under the effect of their own gravity. The goal is to study their evolution and the possibility of gravitational collapse. Are particularly addressed the structure, migration, mass, and physical variables characterizing these self-gravitating structures. We also pay special attention to the case of very massive vortices.

Figure 1. From left to right: Rossby number and density (in log scale) perturbations induced by a large scale vortex after 171 orbital periods at 7.5 AU.

[1] Lovelace, R. V. E., Li, H., Colgate, S. A., & Nelson, A. F. 1999, ApJ, 513, 805

[2] Barge, P. & Sommeria, J. 1995, A&A, 295, L1

[3] Raettig N, Klahr H Lyra W, 2015, ApJ804

[4] Tsukagoshi, T., Muto, T., Nomura, H., et al. 2019, ApJ, 878, L8

[5] Dong, R., Liu, S.-y., Eisner, J., et al. 2018, ApJ, 860, 124

How to cite: Rendon Restrepo, S. and Barge, P.: Selfgravitating vortices in protoplanetary disks, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-697, https://doi.org/10.5194/epsc2021-697, 2021.

    Large scale vortices are thought to be natural outcomes of hydrodynamic instabilities in protoplanetary disks, as for instance the Rossby Wave Instability [1] or Baroclinic instability. Analytical and numerical studies showed that they can be long-lived and catalyze efficiently dust material concentration [2], [3] what encourages to think they could play a role in protoplanetary-disk evolution and planetesimal formation. Their presence in the outer regions of circumstellar disks is possibly betrayed by recent observations of lopsided structures with ALMA and VLT [4], [5].

    We present our latest results of two-phase 3D numerical simulations in which the solid particles are embedded in the gas and described as a second pressureless fluid.  The goal is to discuss the sedimentation of the dust phase in the protoplanetary disk midplane, and the dust trapping mechanism induced by 3D vortices. We will also address the question if 3D vortices are more stable to the streaming instability than 2D vortices.