Growth of dust aggregates in protoplanetary disks beyond the "bouncing barrier"

今天，我们知道行星在所谓的原行星盘中围绕年轻恒星形成。当圆盘冷却下来时，尘埃颗粒逐渐冷凝并开始相互碰撞，这是由于它们与周围气体的相互作用。在低速下，由于表面力，这些碰撞导致粘附。因此，出现了许多不断增长的灰尘团块。实验还表明，大的团聚体在碰撞中反弹，或者在更高的碰撞能量下碎裂。为了研究从单个尘埃颗粒到千米大小的微行星的演化，在过去几年中，根据实验数据进行了几次原行星尘埃的生长模拟。模拟表明，由于弹跳碰撞，生长在几克的聚集质量处停止。然而，最近的实验和模拟表明，增长仍然是可能的。在本建议的范围内，将对两种预期的增长过程进行实验研究：第一，碰撞伙伴碎片之一可能导致质量转移到幸存物体的碰撞。这种质量转移是众所周知的高速冲击之间的大小非常不同的团聚体。最近的实验表明，在类似大小的尘埃聚集体之间的碰撞中也有可能增长。由于只有极端的情况下，相同的和非常不同的大小的碰撞伙伴已经研究到目前为止，我们打算在这个建议中解决的质量比上的传质效率的影响。第二，近球形和均匀的粉尘团聚的共同假设应进行调查。我们的初步实验表明，粘附碰撞约。0.1毫米大小的粉尘聚集体导致形成非常开放的，有时是链状的簇。这些团簇之间的碰撞仍然会导致以相同质量的致密团聚体反弹的速度粘在一起。应在本提案范围内通过实验室和落塔实验研究这种团簇的形成和演变以及它们的碰撞行为，从而研究它们的生长条件。

Today we know that planets form around young stars in so-called protoplanetary disks. While the disks cool down, dust particles progressively condense and start to collide with each other, due to their interaction with the surrounding gas. At low velocities, these collisions lead to sticking, due to surface forces. Thus, a number of growing dust agglomerates emerge. Experiments also show that large agglomerates rebound in collisions or, for even higher collision energies, fragment. To study the evolution from individual dust grains to km-sized planetesimals, several growth simulations of protoplanetary dust, based on the experimental data, were performed within the last years. The simulations show that the growth is stopped at aggregate masses of a few grams, due to bouncing collisions. However, recent experiments and simulations indicate that growth is still possible. Within the scope of this proposal, two prospective growth processes shall be studied experimentally.First, collisions in which one of the collision partners fragments can lead to a mass transfer to the surviving object. This mass transfer is well known from high-velocity impacts between very different-sized agglomerates. Recent experiments showed that growth is also possible in collisions between similar-sized dust aggregates. Since only the extreme cases of identical and very-different-sized collision partners have been studied so far, we intend to address in this proposal the influence of the mass ratio on the mass-transfer efficiency.Second, the common assumption of nearly spherical and homogenous dust agglomerates shall be investigated. Our preliminary experiments showed that sticking collisions of approx. 0.1 mm-sized dust agglomerates lead to the formation of very open and sometimes chain-like clusters. Collisions between these clusters still lead to sticking at velocities where compact agglomerates of the same mass rebound. The formation and evolution of such clusters, as well as their collision behavior and, thus, their growth conditions, shall be studied within this proposal by means of laboratory and drop-tower experiments.