Collisional evolution of dust aggregates. From compaction to catastrophic destruction

Abstract

The coagulation of dust aggregates occurs in various astrophysical environments. Each one is characterized by different conditions that influence the growth, e.g., relative velocities, composition, and size of the smallest constituents (monomers). Here we study the microphysics of collisions of dust aggregates in a four-dimensional parameter space. The parameters are the collision energy, the initial compactness of agglomerates, the mass ratio of collision partners, and the impact parameter. For this purpose we employ a state of the art molecular dynamics type of model that has been extensively and successfully tested against laboratory experiments. It simulates the motion of individual monomers interacting dynamically via van der Waals surface forces. The structure of aggregates is quantified by the filling factor that provides information about the internal structure, the packing density of monomers, and the projected surface area of aggregates. Our results show the importance of the impact parameter that causes formation of elongated particles, due to tensile forces acting in offset collisions. In head-on impacts, aggregates are compacted at lower energies. A sufficiently high energy causes restructuring to reach maximum compaction. If more energy is provided, pancake-like structures are formed. We find that the outcome of collisions can be represented in a simple way. A highly pronounced large fragment component coexists with a power-law distribution of small fragments. The structural parameter of these small fragments is described very well by a simple relation, largely independent of the initial compactness, impact energy or impact parameter. The simulations show that erosion by collisions with high mass-ratio can be significant. The ejected mass can be several orders of magnitude higher than the impactor mass. This contrasts with collisions of equal mass aggregates, where the same impact energy can lead to perfect sticking. These findings are summarized in the form of a simple collision recipe. The recipe specifies the outcome of a collision, averaged over the impact parameter. It is provided in tabular form for a range of physical parameters such as impact energy and pre-collision filling factor. The dependence on the mass ratio of impactor and target is taken into account by providing both a local and a global branch of the recipe.

Growth efficiency of dust aggregates through collisions with high mass ratios

“…This is much less than the collision velocity achieved in protoplanetary disks. Our estimation on u col,crit for silicate aggregates is consistent with several laboratory experiments and numerical simulations for silicate aggregates (e.g., Blum & Wurm 2000Langkowski et al 2008;Güttler et al 2010;Paszun & Dominik 2009;Seizinger & Kley 2013), suggesting the difficulty of silicate dust growth.…”

“…Collisions between aggregates are directly simulated by N-body codes (e.g., Dominik & Tielens 1997;Paszun & Dominik 2009;Wada et al 2009Wada et al , 2011Suyama et al 2012;Seizinger & Kley 2013). We have been investigating the feasibility of dust growth at high-velocity collisions using an N-body code.…”

“…Teiser et al (2011) report in their experiments that dust accretion efficiency decreases with increasing the impact angle θ, although their experiments are limited to θ < 45 • . From another point of view, different-sized collisions may have an opposite effect producing much ejecta due to the injection of impact energy into a local region (Paszun & Dominik 2009). Therefore collisions between different-sized BPCA aggregates, including offset collisions, are worth investigating for discussion on dust growth.…”

“…Therefore collisions between different-sized BPCA aggregates, including offset collisions, are worth investigating for discussion on dust growth. In addition, high-velocity collisions produce a large number of small aggregates as ejecta fragments (Paszun & Dominik 2009;Wada et al 2009). Since ejecta would play an important role in dust growth and the total ejecta mass is a key to determine the mass loss rate through collisional cascades , we need a model of ejecta mass at high-velocity collisions of different-sized aggregates.…”

Dust Coagulation with Porosity Evolution

“…Moreover, the mass ratio of the projectile and the target and off-center collisions has been studied, but is still under discussion. Readers should also see the papers that we did not discuss in this chapter (Paszun and Dominik 2009;Ringl et al 2012), and should be cautious about the assumptions that we and other authors have made.…”