Dust growth in protoplanetary disks — a comprehensive experimental/theoretical approach

Abstract

More than a decade of dedicated experimental work on the collisional physics of protoplanetary dust has brought us to a point at which the growth of dust aggregates canfor the first time -be self-consistently and reliably modelled. In this article, the emergent collision model for protoplanetery dust aggregates as well as the numerical model for the evolution of dust aggregates in protoplanetary disks are reviewed. It turns out that, after a brief period of rapid collisional growth of fluffy dust aggregates to sizes of a few centimeters, the protoplanetary dust particles are subject to bouncing collisions, in which their porosity is considerably decreased. The model results also show that low-velocity fragmentation can reduce the final mass of the dust aggregates but that it does not trigger a new growth mode as discussed previously. According to the current stage of our model, the direct formation of kilometer-sized planetesimals by collisional sticking seems impossible so that collective effects, such as the streaming instability and the gravitational instability in dust-enhanced regions of the protoplanetary disk, are the best candidates for the processes leading to planetesimals.

Dust-aggregate impact into granular matter: A systematic study of the influence of projectile velocity and size on crater formation and grain ejection

“…Such collisions are typical in protoplanetary disks and may occur at high relative velocities, since the large aggregate may already have decoupled from the motion of the gas corotating with the disk (Birnstiel et al 2016). Impact size and speed then decide on the growth (or erosion) of the collision partners (Blum 2010;Birnstiel et al 2016).…”

Low-velocity collisions of chondrules: How a thin dust cover helps enhance the sticking probability

“…These collisions are of particular importance in protoplanetary disks, where such collisions ultimately lead -by aggregation processes -to the formation of planets and moons (Blum 2010). While many aspects of such collisions have by now been investigated, the role of collisions between mm-to m-sized objects still poses many questions (Birnstiel et al 2016).…”

Collisions of small ice particles under microgravity conditions

“…Collisions of silicate dust particles have been studied extensively, in the laboratory, under microgravity conditions and in simulations, and sticking has been experimentally demonstrated with small particles at low collision velocities due to van der Waals forces (see reviews by Blum &Wurm 2008 andGüttler et al 2010; and the more recent work by Weidling et al 2012 andKothe et al 2013). There appears to be a critical velocity above which dust particles will no longer stick.…”

“…Various mechanisms have been proposed to overcome this problem. Collisional grain charging is one such mechanism (Poppe et al 2000a,b); however, this is only possible for small particles, so it is unlikely that planetesimals can form by this mechanism alone (Blum 2010). Growth has also been demonstrated through mass transfer (Wurm et al 2005;Teiser & Wurm 2009b,a;Kothe et al 2010;Teiser et al 2011;Windmark et al 2012a,b;Garaud et al 2013;Meisner et al 2013), but owing to the increasing relative velocities of larger particles mentioned earlier and the importance of erosion (Schräpler & Blum 2011;Seizinger et al 2013), it is unclear whether this can account for growth beyond centimetre sizes.…”