Abstract The timing of formation of 100-300 km size planetesimals in the protoplanetary disk remains largely unconstrained. Recent models show that gravitational collapse of boulders in overdense regions of a dusty accretion disk can overcome the meter-sized barrier and lead to rapid formation of planetesimals with size of several km that further grow by pebble accretion. Hf/W ages indicate that the first large planetesimals to form could be the parent bodies of magmatic iron meteorites. These ages have been so far used to constrain timing of accretion considering (i) instantaneous accretion, and (ii) purely conductive heat transfer in the planetesimal. To relax these hypotheses we model the thermal evolution of a planetesimal in course of accretion and we take into account the possibility of convection onset. Our model is further based on considering the possibility of a common thermal evolution for all the parent bodies of iron meteorites. Within that framework we show that the planetesimals could have grown following a universal accretion law starting at the very beginning of the history of the disk by a nearly instantaneous formation of 60 ± 30 km size nuclei, followed by a growth via pebble accretion at a much slower pace to reach final sizes of 150–300 km in about 3 Myr. In this universal scenario, complete melting and total differentiation are not bound to happen in the parent body due to the continuous accretion of cold pebbles. The model, though calibrated here on iron meteorites, is general and can in principle be applied to other types of planetesimals such as for instance the parent bodies of CV chondrites.

中文翻译：

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由岩浆铁陨石母体的热演化解开的星子的早期吸积

摘要 原行星盘中 100-300 公里大小的星子形成的时间在很大程度上仍然不受限制。最近的模型表明，在尘土飞扬的吸积盘的过密区域中，巨石的重力坍塌可以克服米大小的障碍，并导致数公里大小的星子快速形成，并通过卵石吸积进一步生长。Hf/W 年龄表明，第一个形成的大型星子可能是岩浆铁陨石的母体。到目前为止，考虑到（i）瞬时吸积和（ii）小行星中的纯传导热传递，这些年龄已被用于限制吸积的时间。为了放宽这些假设，我们模拟了吸积过程中小行星的热演化，并考虑了对流开始的可能性。我们的模型进一步基于对所有铁陨石母体共同热演化的可能性的考虑。在该框架内，我们表明星子可以按照普遍吸积定律生长，从盘的历史开始，几乎瞬时形成 60 ± 30 公里大小的核，然后通过卵石吸积以大量在大约 3 Myr 内达到 150-300 公里的最终尺寸的速度较慢。在这种普遍情况下，由于冷卵石的不断堆积，母体中不一定会发生完全融化和完全分化。该模型虽然是在铁陨石上校准的，但它是通用的，原则上可以应用于其他类型的星子，例如 CV 球粒陨石的母体。