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，即大约毫米到厘米大小的颗粒，如何凝聚形成最初的行星构建块（星子）。一种名为“卵石吸积”的新模型或许能够解决这个问题。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卵石减速。减速的小卵石缓慢地附着在星子上，而不是高速撞击并碎裂。然而，在这个模型中有许多未知数，没有人知道它会工作得多好。该项目将通过研究卵石在吸积盘中穿过气体时发生的化学变化，并通过解释微行星的热力学，来完善卵石吸积模型。这项工作的成功将使我们更好地了解行星系统的形成，包括我们自己的太阳系。研究小组将把他们的结果与太阳系天体进行比较，并与阿塔卡马大型毫米/亚毫米阵列（阿尔马）设施对其他太阳系（行星正在形成的地方）行星吸积盘的观测结果进行比较。该项目侧重于卵石吸积过程的数值流体动力学模拟，最初是两项独立的工作，后来合并。其中一项工作将包括卵石在吸积盘中穿过气体时的化学演化。另一个将结合实际的热力学为星子和周围的原大气，以及大气阻力的影响，下落的鹅卵石。该团队将单独制作卵石吸积的全球数值模拟，跟踪行星吸积的所有固体的历史和化学演变。最后，该小组将联合收割机两种方法结合成一个自洽的模型，并使用结果来开发合成观测，他们可以直接比较阿尔马观测吸积盘和子盘结构。该奖项反映了NSF的法定使命，并已被认为是值得通过使用基金会的智力价值和更广泛的影响审查标准进行评估的支持。

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.