RAPID COAGULATION OF POROUS DUST AGGREGATES OUTSIDE THE SNOW LINE: A PATHWAY TO SUCCESSFUL ICY PLANETESIMAL FORMATION

By S. Okuzumi, Hidekazu Tanaka, H. Kobayashi, K. Wada. 2012

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Rapid orbital drift of macroscopic dust particles is one of the major obstacles to planetesimal formation in protoplanetary disks. We re-examine this problem by considering the porosity evolution of dust aggregates. We apply a porosity model based on recent N-body simulations of aggregate collisions, which allows us to study the porosity change upon collision for a wide range of impact energies. As a first step, we neglect collisional fragmentation and instead focus on dust evolution outside the snow line, where the fragmentation has been suggested to be less significant than inside the snow line because of the high sticking efficiency of icy particles. We show that dust particles can evolve into highly porous aggregates (with internal densities of much less than 0.1 g cm−3) even if collisional compression is taken into account. We also show that the high porosity triggers significant acceleration in collisional growth. This acceleration is a natural consequence of the particles’ aerodynamical properties at low Knudsen numbers, i.e., at particle radii larger than the mean free path of the gas molecules. Thanks to this rapid growth, the highly porous aggregates are found to overcome the radial drift barrier at orbital radii less than 10 AU (assuming the minimum-mass solar nebula model). This suggests that, if collisional fragmentation is truly insignificant, formation of icy planetesimals is possible via direct collisional growth of submicron-sized icy particles.

Subfields: Astronomy and Astrophysics, Instrumentation

View Paper: https://www.semanticscholar.org/paper/a767fb07e68b2d89a248aaed2c9e1cca2eaf952f

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Citations in the corpus

Planetesimal formation starts at the snow line (2017, background): “…that the icy aggregates are significantly more sticky than silicate grains (Wada et al. 2011; Gundlach et al. 2011) and can thus grow to larger sizes before they fragment, or even grow directly to planetesimal sizes if they are sufficiently porous (Okuzumi et al. 2012; Kataoka et al. 2013).”
Dust Evolution and the Formation of Planetesimals (2016, background): “At the crucial regime around Stokes numbers of unity, extremely fluffy particles (internal densities of 10−3 g cm−3) have a very large size resulting in very short growth time scales, short enough to overcome the radial drift barrier (Okuzumi et al. 2012; Kataoka et al. 2013).”
The Disk Substructures at High Angular Resolution Project (DSHARP). I. Motivation, Sample, Calibration, and Overview (2018, background): “…2015; Dipierro et al. 2015; Béthune et al. 2017; Dullemond & Penzlin 2018; Suriano et al. 2018) or by strong gradients in material properties (e.g., Okuzumi et al. 2012; Estrada et al. 2016; Armitage et al. 2016; Stammler et al. 2017; Pinilla et al. 2017; but see van Terwisga et al. 2018; Long et…”
Close-in planetesimal formation by pile-up of drifting pebbles (2016, methodology): “If growth happens even at high impact speeds, for example in the case of very porous dust aggregates, planetesimal formation via direct growth may be possible (Okuzumi et al. 2012; Kataoka et al. 2013).”
The Disk Substructures at High Angular Resolution Project (DSHARP). VI. Dust Trapping in Thin-ringed Protoplanetary Disks (2018, background): “D effects, full radiative transfer (e.g. Bitsch et al. 2013; Flock et al. 2013), dust growth and fragmentation (e.g. Birnstiel et al. 2010; Okuzumi et al. 2012), and many other things.”
Forming the cores of giant planets from the radial pebble flux in protoplanetary discs (2014, background): “However, advanced coagulation codes modelling compact particle growth yield very similar results as the analytic calculation (Brauer et al. 2008; Birnstiel et al. 2012; Okuzumi et al. 2012).”
Fluffy dust forms icy planetesimals by static compression (2013, background): “We also plot the collisional growth path without static compression (Okuzumi et al. 2012); the dust aggregates initially grow with a fractal dimension of 2, and when the impact energy of dust-dust collisions reaches the rolling energy, the internal density becomes almost constant.”
THE MASS AND SIZE DISTRIBUTION OF PLANETESIMALS FORMED BY THE STREAMING INSTABILITY. I. THE ROLE OF SELF-GRAVITY (2015, background): “Whether this is also the case for icy particles is less clear (e.g., Blum & Wurm 2008; Okuzumi et al. 2012; Kataoka et al. 2013; Wada et al. 2013; Krijt et al. 2015; Musiolik et al. 2016).”
Multiwavelength analysis for interferometric (sub-)mm observations of protoplanetary disks - Radial constraints on the dust properties and the disk structure (2015, background): “…within 1 Myr (see also Laibe 2014) unless it is halted by the occurrence of dust traps (Whipple 1972; Weidenschilling 1977; Klahr & Henning 1997; Pinilla et al. 2012) or a different aerodynamic behavior when the grains are extremely fluffy aggregates (Okuzumi et al. 2012; Kataoka et al. 2013).”
THE STICKINESS OF MICROMETER-SIZED WATER-ICE PARTICLES (2014, background): “…threshold of water-ice particles found in our experiments is not in contradiction to the idea that icy planetesimals form by direct sticking of subμm-sized water-ice particles as suggested by Wada et al. (2008, 2009), Suyama et al. (2008, 2012), Okuzumi et al. (2012), and Kataoka et al. (2013).”