Growth efficiency of dust aggregates through collisions with high mass ratios

Abstract

Context. Collisional growth of dust aggregates is an essential process in forming planetesimals in protoplanetary disks, but disruption through high-velocity collisions (disruption barrier) could prohibit the dust growth. Mass transfer through very different-sized collisions has been suggested as a way to circumvent the disruption barrier. Aims. We examine how the collisional growth efficiency of dust aggregates with different impact parameters depends on the size and the mass ratio of colliding aggregates. Methods. We used an N-body code to numerically simulate the collisions of different-sized aggregates. Results. Our results show that high values for the impact parameter are important and that the growth efficiency averaged over the impact parameter does not depend on the aggregate size, although the growth efficiency for nearly head-on collisions increases with size. We also find that the averaged growth efficiency tends to increase with increasing mass ratio of colliding aggregates. However, the critical collision velocity, above which the growth efficiency becomes negative, does not strongly depend on the mass ratio. These results indicate that icy dust can grow through high-velocity offset collisions at several tens of m s −1 , the maximum collision velocity experienced in protoplanetary disks, whereas it is still difficult for silicate dust to grow in protoplanetary disks.

Dust Coagulation with Porosity Evolution

“…Enormous efforts have been made with laboratory experiments that show that the threshold velocity of silicate grains is a few m s 1 (e.g., Blum and Münch 1993) and a few tens of m s 1 for ice (Gundlach and Blum 2015). In the series of numerical simulations (Wada et al 2007(Wada et al , 2011 where r 0 is the monomer radius (Wada et al 2013). This is in excellent agreement with the laboratory experiments.…”

“…If the collisional energy of dust grains is high enough to disrupt the grains, dust grains cannot grow further. The threshold velocity of fragmentation is a few meters per second for silicate-like aggregates, much smaller than the typical highest collision velocity of 50 m s 1 in disks (Blum and Münch 1993;Blum and Wurm 2008;Wada et al 2007Wada et al , 2009Wada et al , 2013. Dust grains of some size ranges do not stick, but rather bounce Zsom et al 2010).…”

“…Another possibility is to introduce high mass ratio fragmentation, which could lead to many fragments being produced Krijt et al 2015). However, we should also mention that Wada et al (2013) argue that the growth efficiency via fragmentation does not depend on the mass ratio of the colliding aggregates.…”

“…One way of solving this problem is to introduce partial fragmentations. The fragmentation threshold velocity in the series of numerical simulations (e.g., Wada et al 2013) is defined by whether the target aggregates gain mass from the projectile, called net growth. Therefore, even if the collision velocity is lower than the fragmentation velocity, the dust aggregates still produce small fragments, and this may explain the observations.…”

Formation of pebble-pile planetesimals

“…One difference is that ice particles could be very fluffy and survive collisions at higher speeds compared to silicates (e.g. Wada et al 2009Wada et al , 2013. However, even in our most massive clouds the collision speeds do not reach values over ∼10 m s −1 and the speed decreases rapidly as the collapse progresses (red line in Fig.…”