How cores grow by pebble accretion I. Direct core growth
How cores grow by pebble accretion I. Direct core growth
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DOI：
10.1051/0004-6361/201731824
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发表时间：
2018-03-26
影响因子：
6.5
通讯作者：
Ormel, C. W.
中科院分区：
文献类型：
--
作者：
Brouwers, M. G.;Vazan, A.;Ormel, C. W.
语境。通过卵石吸积形成行星是星子驱动的核心吸积的替代方案。在这种情况下，行星是通过厘米到米大小的卵石的吸积而不是公里大小的星子来生长的。与星子驱动的核心吸积的主要区别之一是卵石经历的热烧蚀增加。这可以为行星的包层提供早期富集，从而影响其随后的演化并改变核心生长的过程。我们的目标是预测由卵石吸积形成的行星的核心质量和包络成分，并将卵石的质量沉积与星子进行比较。具体来说，我们计算了卵石在到达核心之前完全蒸发并被吸收的核心质量，这标志着核心直接生长的结束。我们通过计算原行星的包层结构来模拟原行星的早期生长，同时考虑到撞击卵石或星子的命运。高Z材料可以蒸气形式存在的区域由与温度相关的蒸气压决定。我们通过局部修改包膜的平均分子量来包括富集效应。结果。在卵石情况下，可以确定核心生长的三个阶段。在第一阶段（M-核心 < 0.23-0.39 M-圆+），卵石撞击核心而没有明显的烧蚀。在第二阶段（M-核心 < 0.5 M-圆+），消融变得越来越严重。一层高Z蒸气开始在核心周围形成，吸收一小部分烧蚀质量。其余的物质要么落入核心，要么向外混合，从而减缓核心的生长。在第三阶段（M-核心 > 0.5 M-圆+），高 Z 内部区域向外膨胀，以蒸气形式吸收越来越多的烧蚀材料。降雨在核心质量达到 0.6 M 圆以上之前结束，终止直接核心生长。对于冰冷的 H2O 卵石，这种情况发生在 0.1 M 圆加上之前。结论。我们的结果表明，卵石增生只能直接形成最多只有 0.6 M 圆以上的岩石核心，并且无法形成类似大小的冰核。当行星冷却时，只要它能够保留其高Z材料，后续的核心生长就可以间接进行。
Context. Planet formation by pebble accretion is an alternative to planetesimal-driven core accretion. In this scenario, planets grow by the accretion of cm- to m-sized pebbles instead of km-sized planetesimals. One of the main differences with planetesimal-driven core accretion is the increased thermal ablation experienced by pebbles. This can provide early enrichment to the planet's envelope, which influences its subsequent evolution and changes the process of core growth.Aims. We aim to predict core masses and envelope compositions of planets that form by pebble accretion and compare mass deposition of pebbles to planetesimals. Specifically, we calculate the core mass where pebbles completely evaporate and are absorbed before reaching the core, which signifies the end of direct core growth.Methods. We model the early growth of a protoplanet by calculating the structure of its envelope, taking into account the fate of impacting pebbles or planetesimals. The region where high-Z material can exist in vapor form is determined by the temperature-dependent vapor pressure. We include enrichment effects by locally modifying the mean molecular weight of the envelope.Results. In the pebble case, three phases of core growth can be identified. In the first phase (M-core < 0.23-0.39 M-circle plus), pebbles impact the core without significant ablation. During the second phase (M-core < 0.5 M-circle plus), ablation becomes increasingly severe. A layer of high-Z vapor starts to form around the core that absorbs a small fraction of the ablated mass. The rest of the material either rains out to the core or instead mixes outwards, slowing core growth. In the third phase (M-core > 0.5 M-circle plus), the high-Z inner region expands outwards, absorbing an increasing fraction of the ablated material as vapor. Rainout ends before the core mass reaches 0.6 M-circle plus, terminating direct core growth. In the case of icy H2O pebbles, this happens before 0.1 M-circle plus.Conclusions. Our results indicate that pebble accretion can directly form rocky cores up to only 0.6 M-circle plus, and is unable to form similarly sized icy cores. Subsequent core growth can proceed indirectly when the planet cools, provided it is able to retain its high-Z material.