Rapid Coagulation of Porous Dust Aggregates Outside the Snow Line: A Pathway to Successful Icy Planetesimal Formation

Abstract

Rapid orbital drift of macroscopic dust particles is one of the major obstacles against planetesimal formation in protoplanetary disks. We reexamine this problem by considering 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 a 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 particles' aerodynamical property 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.

Free collisions in a microgravity many-particle experiment. III. The collision behavior of sub-millimeter-sized dust aggregates

“…A second point which should be addressed is the single porosity of our samples (φ ∼ 0.37). Various numerical simulations (Suyama et al 2008;Zsom et al 2011;Okuzumi et al 2012) have shown that the first dust agglomerates might have had a much lower volume filling factor (down to φ ∼ 10 −3 ). Zsom et al (2011) showed that particles with a mass of ∼ 10 −6 g can still have such a low density.…”

The Emerging Paradigm of Pebble Accretion

“…It also implies that pebbles are constantly replenished: they are lost to the inner disk, but drift in from the outer disk. Pebble accretion, in contrast to planetesimal accretion, is therefore a global phenomenon: one has to consider the evolution of the dust population throughout the entire disk (Birnstiel et al 2010;Okuzumi et al 2012;Testi et al 2014). …”

Dust Coagulation with Porosity Evolution

“…The mass and filling factor of the dust aggregates in typical protoplanetary disks are derived to be m 10 4 g and f 10 5 . However, even when collisional compression occurs, dust aggregates are not compactified as much: the filling factor remains constant or continues to decrease (Suyama et al , 2012Okuzumi et al 2012). This can be understood as follows.…”

“…In this regime, the growth time scale becomes shorter. On the pathway of the filling factor evolution, when the dust aggregates have a Stokes number of unity, the radius of the aggregates is 100 m, which is much larger than the mean free path of the gas, 1 m. More details can be found in Okuzumi et al (2012). The region where the growth time scale is shorter than the drift time scale is shown in Fig.…”

“…Whether the radial drift problem is avoided depends on the location of the protoplanetary disks. Roughly speaking, rapid coagulation to avoid the radial drift barrier occurs within 10-20 AU but outside of the water snow line (see Okuzumi et al 2012;Kataoka et al 2013a). …”