Crossing barriers in planetesimal formation: The growth of mm-dust aggregates with large constituent grains
Crossing barriers in planetesimal formation: The growth of mm-dust aggregates with large constituent grains
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跨越行星形成过程中的障碍：具有大颗粒的毫米尘埃聚集体的生长
DOI：
10.1051/0004-6361/201218984
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发表时间：
2012
影响因子：
6.5
通讯作者：
I. Bertini
中科院分区：
文献类型：
--
作者：
T. Jankowski;G. Wurm;T. Kelling;J. Teiser;W. Sabolo;P. J. Guti'errez;I. Bertini
毫米大小的尘埃聚集体的碰撞在行星形成的早期阶段起着至关重要的作用。例如，目前尚不清楚是否存在反弹屏障，其中毫米聚集体不再通过粘附而生长。我们开发了一个实验室装置，使我们能够观察在mbar压力和800 K高温下悬浮的尘埃聚集体的碰撞。我们报告的玄武岩尘埃聚集体之间的碰撞，从0.3至5毫米的大小在0.1和15厘米/秒之间的速度。单个颗粒的尺寸小于25 μm。我们发现，在所研究的范围内的所有冲击能量的粘附发生的概率平均为32.1 ± 2.5%。一般情况下，粘附概率随冲击参数的增大而减小。粘附概率随着能量密度（单位接触面积的冲击能量）而增加。我们还观察到由两个较大的聚集体先前粘附形成的聚集体的碰撞。这些聚集体的伴侣可以通过第二次碰撞分离，平均概率为19.8 ± 4.0%。与其他实验结果相比，测得的吸积效率是非常高的。我们将此归因于我们实验中使用的相对较大的尘埃颗粒，这使得聚集体更容易重组和能量耗散。由于骨料已经非常致密，孔隙率仅为54 ± 1%，因此可能不会发生压实导致的碰撞硬化。在碰撞中，先前生长的聚集体的分解可能会阻止进一步的聚集体生长。然而，由于悬浮技术和有限的数据统计，还不能给出关于这方面的结论性声明。我们发现，剥离效率随速度的增加而降低，在较高的速度范围内，吸积占主导地位。对于高吸积效率，我们的实验表明，在mm范围内继续生长较大的组成颗粒将是一种可行的方式来产生较大的聚集体，这可能反过来形成种子，继续生长星子。
Collisions of mm-size dust aggregates play a crucial role in the early phases of planet formation. It is for example currently unclear whether there is a bouncing barrier where millimeter aggregates no longer grow by sticking. We developed a laboratory setup that allowed us to observe collisions of dust aggregates levitating at mbar pressures and elevated temperatures of 800 K. We report on collisions between basalt dust aggregates of from 0.3 to 5 mm in size at velocities between 0.1 and 15 cm/s. Individual grains are smaller than 25 μm in size. We find that for all impact energies in the studied range sticking occurs at a probability of 32.1 ± 2.5% on average. In general, the sticking probability decreases with increasing impact parameter. The sticking probability increases with energy density (impact energy per contact area). We also observe collisions of aggregates that were formed by a previous sticking of two larger aggregates. Partners of these aggregates can be detached by a second collision with a probability of on average 19.8 ± 4.0%. The measured accretion efficiencies are remarkably high compared to other experimental results. We attribute this to the relatively large dust grains used in our experiments, which make aggregates more susceptible to restructuring and energy dissipation. Collisional hardening by compaction might not occur as the aggregates are already very compact with only 54 ± 1% porosity. The disassembly of previously grown aggregates in collisions might stall further aggregate growth. However, owing to the levitation technique and the limited data statistics, no conclusive statement about this aspect can yet be given. We find that the detachment efficiency decreases with increasing velocities and accretion dominates in the higher velocity range. For high accretion efficiencies, our experiments suggest that continued growth in the mm-range with larger constituent grains would be a viable way to produce larger aggregates, which might in turn form the seeds to proceed to growing planetesimals.