Kuiper Belt objects (KBOs) show an unexpected trend, whereby large bodies have increasingly higher densities, up to five times greater than their smaller counterparts. Current explanations for this trend assume formation at constant composition, with the increasing density resulting from gravitational compaction. However, this scenario poses a timing problem to avoid early melting by decay of ²⁶Al. We aim to explain the density trend in the context of streaming instability and pebble accretion. Small pebbles experience lofting into the atmosphere of the disk, being exposed to UV and partially losing their ice via desorption. Conversely, larger pebbles are shielded and remain icier. We use a shearing box model including gas and solids, the latter split into ices and silicate pebbles. Self-gravity is included, allowing dense clumps to collapse into planetesimals. We find that the streaming instability leads to the formation of mostly icy planetesimals, albeit with an unexpected trend that the lighter ones are more silicate-rich than the heavier ones. We feed the resulting planetesimals into a pebble accretion integrator with a continuous size distribution, finding that they undergo drastic changes in composition as they preferentially accrete silicate pebbles. The density and masses of large KBOs are best reproduced if they form between 15 and 22 au. Our solution avoids the timing problem because the first planetesimals are primarily icy and ²⁶Al is mostly incorporated in the slow phase of silicate pebble accretion. Our results lend further credibility to the streaming instability and pebble accretion as formation and growth mechanisms.

中文翻译：

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柯伊伯带天体多物种流动不稳定性和卵石吸积的密度二分问题的求解

柯伊伯带天体 (KBO) 显示出一种意想不到的趋势，即大型天体的密度越来越高，比较小天体高出五倍。目前对这一趋势的解释假设地层成分恒定，重力压实导致密度增加。然而，这种情况提出了一个计时问题，以避免²⁶ Al衰变而过早熔化。我们的目的是解释流动不稳定和卵石堆积背景下的密度趋势。小卵石会被抛入圆盘的大气中，暴露在紫外线下，并通过解吸部分失去冰。相反，较大的卵石被屏蔽并保持较冷。我们使用包括气体和固体的剪切盒模型，后者分裂成冰和硅酸盐卵石。包括自重力，使致密的团块塌缩成星子。我们发现，流动的不稳定性导致了大部分冰态星子的形成，尽管有一个意想不到的趋势，即较轻的星子比较重的星子更富含硅酸盐。我们将所得的星子输入具有连续尺寸分布的卵石吸积积分器，发现它们在成分上发生了巨大的变化，因为它们优先吸积硅酸盐卵石。如果大型柯伊伯带天体的密度和质量在 15 到 22 个天文单位之间形成，则可以最好地再现它们。我们的解决方案避免了时间问题，因为第一批星子主要是冰的，并且²⁶ Al 大部分包含在硅酸盐卵石吸积的缓慢阶段。我们的结果进一步证明了流动不稳定和卵石增生作为形成和增长机制的可信度。