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

今天，我们知道行星在年轻恒星周围形成了所谓的原行星盘。当圆盘冷却时，尘埃颗粒由于与周围气体的相互作用，逐渐凝聚并开始相互碰撞。在低速时，由于表面作用力，这些碰撞会导致粘滞。因此，出现了一些不断增长的尘埃团聚体。实验还表明，大的团聚体在碰撞中反弹，或者，对于更高的碰撞能量，反弹为碎片。为了研究从单个尘埃颗粒到千米大小的行星体的演化，在过去的几年里，根据实验数据，进行了几次原行星尘埃的增长模拟。模拟表明，由于弹跳碰撞，增长在几克的总质量时停止。然而，最近的实验和模拟表明，增长仍然是可能的。在这项提议的范围内，应该对两个预期的生长过程进行实验研究。第一，碰撞，其中一个碰撞伙伴碎片可以导致质量转移到幸存的物体。这种质量传递是众所周知的，因为不同大小的凝聚体之间的高速碰撞。最近的实验表明，在大小相似的尘埃聚集体之间的碰撞中也可能出现增长。由于到目前为止只研究了完全相同和大小非常不同的碰撞伙伴的极端情况，我们打算在这个方案中讨论质量比对传质效率的影响。第二，我们将研究近似球形和均匀的尘埃团聚体的共同假设。我们的初步实验表明，粘合碰撞大约是。0.1毫米大小的尘埃凝聚体导致形成非常疏松的星团，有时甚至形成链状星团。这些星系团之间的碰撞仍然导致以相同质量的致密凝聚体反弹的速度粘在一起。这种团簇的形成和演化，以及它们的碰撞行为和生长条件，应在本建议的范围内，通过实验室和落塔实验来研究。

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.