Planetesimal Formation with Charged Grains

在目前的行星形成模型中，行星是从不同阶段的尘埃发展到更大的天体。在毫米和米之间的尺寸范围内，生长机制从碰撞撞击转变为气体原行星盘中流体动力学主导的机制，其中粒子集中在稠密的云中，最终变得引力不稳定。这个过渡阶段没有被很好地理解，是关键的，并且形成了这个项目的重点。更具体地说，虽然由于共同的表面力，尘埃颗粒很容易粘在一起，但对于毫米大小的颗粒来说，这不再适用。它们在原行星盘的条件下有规律地相互反弹。虽然弹跳最初对进一步的生长没有建设性，但它会导致摩擦电或碰撞电荷分离。最近的研究表明，随着电荷在颗粒上积聚，并最终将颗粒强烈地结合在聚集体内，这有可能成为一个有效的额外生长阶段。这可能主导了过渡演化阶段，实际上可能是解释行星形成所必需的。本项目将研究晶粒的摩擦电荷或碰撞电荷以及形成团簇的稳定性。在实验室实验中，带电颗粒团将被储存在声阱中。从晶粒上的净电荷到偶极矩，分析了它们的电荷状态，并测量了它们的机械强度。强度也将进行数值模拟，并与实验进行比较，以估计高阶多极的影响或复杂电荷模式对晶粒表面的影响。然后，这些结果将被用于模拟一个粒子集合的生长，以显示这个带电粒子演化的潜在关键机制在多大程度上可能导致行星的形成。

In current models of planet formation, planets form from dust growing in different phases to ever larger bodies. In the size range between millimeter and meter, the growth mechanism changes from collisional hit-and-stick to a mechanism dominated by hydrodynamics within the gaseous protoplanetary disk, where particles are concentrated in dense clouds, which become gravitational instable, eventually. This transition phase is not well understood, is critical and forms the focus of this project. More specific, while dust grains easily stick together due to common surface forces, this no longer works for particles of millimeter size. They regularly bounce off each other under the conditions of protoplanetary disks. While bouncing is not constructive for further growth initially, it leads to triboelectric or collisional charge separation. Recent work showed that this has the potential for an efficient additional growth phase as charges builds up on grains and eventually bind particles within aggregates strongly. This might dominate the transitional evolution phase and in fact might be necessary to explain planet formation at all. The triboelectric or collisional charging of grains and the stability of forming clusters will be studied in this project. In laboratory experiments, clusters of charged grains will be stored in acoustic traps. Their charge state will be analyzed from net charges on grains to dipole moments and the mechanical strength will be measured. The strength will also be modeled numerically and compared to the experiments to estimate the effect of higher order multipoles or the influence of complex charge patterns on grains surfaces. These results will then be used to model the growth of a particle ensemble self-consistently to show how far this potentially crucial mechanism of charged particle evolution might lead in planet formation.