Dust coagulation in protoplanetary disks: porosity matters

By C. Ormel, M. Spaans, A. Tielens. 2006

199 citations recorded by the source corpus.

Context. Sticking of colliding dust particles through van der Waals forces is the first stage in the grain growth process in protoplanetary disks, eventually leading to the formation of comets, asteroids and planets. A key aspect of the collisional evolution is the coupling between dust and gas motions, which depends on the internal structure (porosity) of aggregates. Aims. To quantify the importance of the internal structure on the collisional evolution of particles, and to create a new coagulation model to investigate the difference between porous and compact coagulation in the context of a turbulent protoplanetary disk. Methods. We have developed simple prescriptions for the collisional evolution of porosity of grain-aggregates in grain-grain collisions. Three regimes can then be distinguished: "hit-and-stick" at low velocities, with an increase in porosity; compaction at intermediate velocities, with a decrease of porosity; and fragmentation at high velocities. This study has been restricted to physical regimes where fragmentation is unimportant. The temporal evolution has been followed using a Monte Carlo coagulation code. Results. This collision model is applied to the conditions of the (gas dominated) protoplanetary disk, with an alpha(T) parameter characterising the turbulent viscosity. We can discern three different stages in the particle growth process. Initially, growth is driven by Brownian motion and the relatively low velocities involved lead to a rapid increase in porosity of the growing aggregate. The subsequent second stage is characterised by much higher, turbulent driven velocities and the particles compact. As they compact, their mass-to-surface area increases and eventually they enter the third stage, the settling out to the mid-plane. We find that when compared to standard, compact models of coagulation, porous growth delays the onset of settling, because the surface area-to-mass ratio is higher, a consequence of the build-up of porosity during the initial stages. As a result, particles grow orders of magnitudes larger in mass before they rain-out to the mid-plane. Depending on the precise value of alpha(T) and on the position in the nebula, aggregates can grow to (porous) sizes of similar to 10 cm in a few thousand years. We also find that collisional energies are higher than in the limited PCA/CCA fractal models, thereby allowing aggregates to restructure. It is concluded that the microphysics of collisions plays a key role in the growth process.

Subfields: Astronomy and Astrophysics, Computational Mechanics, Fluid Flow and Transfer Processes

View Paper: https://www.semanticscholar.org/paper/3695235292f6a3026be91533cab2f1d8be27677d

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In Astronomy and Astrophysics

1. Anders Johansen
2. T. Birnstiel
3. C. Dullemond
4. C. Ormel
5. A. Youdin
6. Michiel Lambrechts
7. E. Chiang
8. J. Blum

In Computational Mechanics

1. J. Blum
2. C. Ormel
3. Anders Johansen
4. C. Guttler
5. J. Cuzzi
6. E. Chiang
7. R. Weidling
8. T. Birnstiel

In Fluid Flow and Transfer Processes

1. C. Ormel
2. T. Birnstiel
3. C. Dullemond
4. A. Youdin
5. Anders Johansen
6. D. Schoonenberg
7. J. Blum
8. E. Chiang

Citations in the corpus

• Numerical Simulation of Dust Aggregate Collisions. I. Compression and Disruption of Two-Dimensional Aggregates (2007, background): “Some studies of dust growth took into account structure evolution (e.g., Ossenkopf 1993; Weidenschilling & Ruzmaikina 1994; Kempf et al. 1999; Blum 2004; Ormel et al. 2007).”
• Numerical Simulation of Dust Aggregate Collisions. II. Compression and Disruption of Three-Dimensional Aggregates in Head-on Collisions (2008, methodology): “Recently, Ormel et al. (2007) modeled a change in the volume of aggregates at collisions.”
• The primordial nucleus of comet 67P/Churyumov-Gerasimenko (2015, background): “The 15–30 AU region was rich in µm-sized grains of ice, organics, silicates, sulfides, and metal that coagulated to form cm-sized porous pebbles (e.g., Blum et al. 2006; Ormel et al. 2007; Blum & Wurm 2008; Zsom et al. 2010).”
• Probing dust grain evolution in IM Lupi's circumstellar disc. Multi-wavelength observations and mo (2008, background): “Models of dust coagulation (Dullemond & Dominik 2005; Ormel et al. 2007) show that, at least during the first stages of the process, the slope of the grain size distribution does not vary much in the regime that we are interested in.”
• The outcome of protoplanetary dust growth: pebbles, boulders or planetesimals? (2009, methodology): “In Paper II, we will also use the enlargement parameter Ψ as introduced by Ormel et al. (2007), which is the reciprocal quantity Ψ = φ−1.”
• Complex organic molecules in protoplanetary disks (2013, background): “Grain coagulation models suggest that the growing dust grains retain a ‘fluffy’ morphology (with a low filling factor,≪ 1) such that the volume grain-surface area may not significantly decrease before compression occurs (see,e.g., Ossenkopf 1993; Ormel et al. 2007; Kataoka et al. 2013).”
• Closed-form expressions for particle relative velocities induced by turbulence (2007, methodology): “…closedform solution to the Völk et al. (1980) and MMV expressions for ∆V12 has not previously been available, many dust coagulation models (e.g., Suttner & Yorke 2001; Dullemond & Dominik 2005; Ormel et al. 2007) have relied on the Weidenschilling (1984) fits to calculate relative velocities.”
• Planetesimal formation near the snowline: in or out? (2017, background): “Although micron-sized grains can quite easily coagulate to mm-sized particles (Dominik & Tielens 1997; Ormel et al. 2007), typical relative velocities quickly become so large that particles fragment or bounce rather than stick (Blum & Wurm 2000; Zsom et al. 2010).”
• Dust coagulation and fragmentation in molecular clouds. I. How collisions between dust aggregates alter the dust size distribution (2009, methodology): “The first term converges to CCA in the limit of V 2 = V 1 , and is the same as in Ormel et al. (2007).”
• Dust Evolution and the Formation of Planetesimals (2016, background): “…of particles k. Detailed descriptions of different recent astrophysical coagulation codes can be found in Brauer et al. (2008), Laibe et al. (2008), Ormel et al. (2007), Ormel & Spaans (2008), Zsom & Dullemond (2008), Okuzumi et al. (2009), Birnstiel et al. (2010), Okuzumi et al. (2012), and…”
• Gas- and dust evolution in protoplanetary disks (2010, background): “However, there are several aspects we do not include such as back-reactions by the dust through opacity or collective effectsWeidenschilling(1997), porosity effects (Ormel et al. 2007), grain charging (Okuzumi 2009) or the “bouncing barrier” (Zsom et al., in press).”
• Forming Planetesimals in Solar and Extrasolar Nebulae (2009, background): “Ormel et al. (2007) calculate that in turbulent disks, porous bodies attain equivalent compact sizes of several cm, for α = 10−2 at r = 1 and 5 AU.”
• Free collisions in a microgravity many-particle experiment. I. Dust aggregate sticking at low velocities (2011, methodology): “An exact value for the fractal dimension of dust aggregates in this evolutionary phase remains highly speculative at this point (Ormel et al., 2007; Suyama et al., 2008; Okuzumi et al., 2009) but it is likely that dust aggregates crossing the threshold from sticking to bouncing for the first time po”