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

在行星形成理论中，最大的未知数之一是小鹅卵石，也就是大约毫米到厘米大小的粒子如何凝结成最初的行星积木(行星体)。一种名为“卵石吸积”的新模型或许能解决这个问题。在该模型中，原行星盘中的气体通过摩擦阻力减缓了小行星周围的鹅卵石速度。减慢的鹅卵石轻轻地附着在行星上，而不是高速撞击和碎裂。然而，该模型中有许多未知数，没有人知道它会运行得多好。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，以及考虑到小行星的热力学，来完善鹅卵石吸积模型。拟议工作的成功将使我们更好地了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以及阿塔卡马大毫米/亚毫米阵列(ALMA)设备对其他太阳系(行星目前正在形成的)行星吸积盘的观测结果进行比较。这个项目专注于鹅卵石吸积过程的数值流体力学模拟，最初有两个单独的工作，后来结合起来。其中一项工作将包括鹅卵石在吸积盘中的气体中移动时的化学演化。另一项研究将包括小行星和周围原始大气的真实热力学，以及大气阻力对落下的鹅卵石的影响。该团队将分别对鹅卵石吸积进行全球数值模拟，跟踪该行星吸积的所有固体的历史和化学演化。最后，该团队将把这两种方法结合成一个自洽的模型，并使用结果来开发综合观测，它们可以直接与ALMA对吸积盘和亚盘结构的观测进行比较。这一奖项反映了NSF的法定使命，并通过使用基金会的智力优势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.