Planetesimal Accretion at Short Orbital Periods

Formation models in which terrestrial bodies grow via the pairwise accretion of planetesimals have been reasonably successful at reproducing the general properties of the solar system, including small body populations. However, planetesimal accretion has not yet been fully explored in the context of the wide variety of recently discovered extrasolar planetary systems, particularly those that host short-period terrestrial planets. In this work, we use direct N-body simulations to explore and understand the growth of planetary embryos from planetesimals in disks extending down to ~1 day orbital periods. We show that planetesimal accretion becomes nearly 100 percent efficient at short orbital periods, leading to embryo masses that are much larger than the classical isolation mass. For rocky bodies, the physical size of the object begins to occupy a significant fraction of its Hill sphere towards the inner edge of the disk. In this regime, most close encounters result in collisions, rather than scattering, and the system does not develop a bimodal population of dynamically hot planetesimals and dynamically cold oligarchs, like is seen in previous studies. The highly efficient accretion seen at short orbital periods implies that systems of tightly-packed inner planets should be almost completely devoid of any residual small bodies. We demonstrate the robustness of our results to assumptions about the initial disk model, and also investigate the effects that our simplified collision model has on the emergence of this non-oligarchic growth mode in a planet forming disk.以星子（planetesimals）两两吸积的方式形成类地天体（terrestrial bodies）的形成模型，在重现太阳系的整体属性（包括小天体族群特征）方面已取得较为理想的成功。然而，星子吸积过程尚未在近年来发现的各类系外行星系统中得到充分探索，尤其是那些拥有短周期类地行星的系统。本研究采用直接N体（N-body）模拟，对延伸至约1日轨道周期的星子盘中，星子演化形成行星胚胎的过程展开探究与解析。我们发现，在短轨道周期环境下，星子吸积效率几近100%，所形成的胚胎质量远大于经典孤立质量。对于岩质天体而言，随着系统趋近星盘的内边缘，天体的物理尺寸开始占据其希尔球（Hill sphere）的可观份额。在此状态下，绝大多数近距离交会事件都会引发碰撞而非散射，且系统不会形成此前研究中所见的、由动力学热星子与动力学冷寡头星子构成的双峰族群。短轨道周期下观测到的高效吸积现象表明，紧密排布的内行星系统几乎不会残留任何小天体。我们验证了研究结果对初始星盘模型假设的鲁棒性，同时还探究了简化碰撞模型对行星形成盘中非寡头生长模式出现的影响。