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

今天我们知道行星是在所谓的原行星盘中围绕年轻恒星形成的。当星盘冷却下来的时候，由于与周围气体的相互作用，尘埃粒子逐渐凝结并开始相互碰撞。在低速时，由于表面力的作用，这些碰撞会导致粘连。因此，一些不断增长的尘埃团块出现了。实验还表明，大团块在碰撞中会反弹，或者在碰撞能量更高的情况下会碎裂。为了研究从单个尘埃颗粒到公里大小的星子的演化，在过去几年中，基于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，这种增长在总质量达到几克时停止。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，两个预期的生长过程将进行实验研究。首先，在碰撞中，其中一个碰撞伙伴的碎片可能导致质量转移到幸存的物体。这种质量传递在大小不同的团块之间的高速撞击中众所周知。最近的实验表明，在类似大小的尘埃聚集体之间的碰撞中，这种生长也是可能的。由于到目前为止只研究了大小相同和大小非常不同的碰撞伙伴的极端情况，我们打算在本提案中解决质量比对传质效率的影响。其次，研究近似球形和均质尘埃团块的常见假设。我们的初步实验表明，大约。0.1毫米大小的尘埃团块会形成非常开放的星团，有时还会形成链状星团。这些星系团之间的碰撞仍然会以相同质量的致密团块反弹的速度导致粘滞。这种簇的形成和演变，以及它们的碰撞行为，从而它们的生长条件，将在本提案中通过实验室和落塔实验进行研究。

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.