The pebble accretion model for planet formation: understanding thermal and chemical histories of astrophysical pebbles

在行星形成理论中，最大的未知之一是小鹅卵石，即大约毫米到厘米大小的颗粒，如何凝结形成最初的行星基石（星子）。一种名为“卵石增生”的新模型可能能够解决这个问题。在这个模型中，原行星盘中的气体通过摩擦阻力使小行星周围的鹅卵石减速。速度变慢的鹅卵石会慢慢地吸积到小行星上，而不是高速撞击并碎裂。然而，模型中有许多未知数，没有人知道它会有多好。这个项目将通过研究鹅卵石在吸积盘中的气体中移动时发生的化学变化，以及通过计算星子的热力学，来完善鹅卵石的吸积模型。这项工作的成功将使我们更好地理解行星系统是如何形成的，包括我们自己的太阳系。该团队将把他们的结果与太阳系天体进行比较，以及阿塔卡马大型毫米/亚毫米阵列（ALMA）设备对其他太阳系（行星目前正在形成的地方）行星吸积盘的观测结果。该计划侧重于卵石吸积过程的数值流体动力学模拟，最初有两个单独的努力，后来结合起来。其中一项工作将包括鹅卵石在吸积盘气体中移动时的化学演化。另一个将结合实际的热力学，为星子和周围的原始大气，以及大气阻力对下落的鹅卵石的影响。该团队将分别制作全球鹅卵石堆积的数值模拟，追踪地球上所有固体堆积的历史和化学演变。最后，该团队将把这两种方法结合成一个自洽模型，并利用结果开发合成观测结果，他们可以直接与ALMA对吸积盘和子盘结构的观测结果进行比较。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

In the theory of planet formation, one of the biggest unknowns is how small pebbles, that is, particles of roughly millimeter to centimeter size, coagulate to form the initial planetary building blocks (planetesimals). A new model called "pebble accretion" might be able to solve this problem. In the model, the gas in the protoplanetary disk slows the pebbles around a planetesimal by frictional drag. The slowed pebbles gently accrete onto the planetesimal instead of hitting at high speed and fragmenting. However, there many unknowns in the model, and nobody knows how well it will work. This project will refine the pebble accretion model by studying the chemical changes that occur in pebbles as they move through the gas in the accretion disk, and by accounting for the thermodynamics of the planetesimal. Success of the proposed work will give a better understanding of how planetary systems form, including our own Solar System. The team will compare their results to Solar System bodies, as well as observations of planetary accretion disks in other solar systems (where planets are currently forming) by the Atacama Large Millimeter/Sub-Millimeter Array (ALMA) facility.This program focuses on numerical hydrodynamics simulations of the pebble accretion process, initially with two separate efforts that are later combined. One effort will incorporate the chemical evolution of pebbles as they move through the gas in the accretion disk. The other will incorporate realistic thermodynamics for the planetesimal and surrounding proto-atmosphere, as well as the effects of atmospheric drag on the infalling pebbles. The team will separately produce global numerical simulations of pebble accretion which track the history and chemical evolution of all solids accreted by the planet. Finally, the team will combine both approaches into one self-consistent model and use the results to develop synthetic observations that they can directly compare to ALMA observations of accretion disks and sub-disk structures.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.