标题： 考虑在通过引力坍缩形成星子时的接触力：相互轨道、自转状态和形状 显示英文标题

标题： Considering contact forces during the formation of planetesimals by gravitational collapse: mutual orbits, spin states, and shapes

摘要： 在本工作中，我们将在PKDGRAV N体积分器中应用软球离散元方法（SSDEM）来研究通过超粒子云的引力坍缩形成星子系统的机制。 先前发布的数值模型已经表明，尘埃粒子云的引力坍缩是产生双星星子系统的一种有效途径。 然而，这些研究受到其使用完美合并和膨胀半径超粒子方法的限制，这使得无法分析星子的形状和自转状态，排除了最紧密双星轨道的形成，并导致星子显著密度不足。 SSDEM使超粒子能够通过相互表面渗透而彼此接触，并通过模拟接触物理过程。 超粒子不需要膨胀，碰撞也不被处理为完美合并；因此，我们可以跟踪星子形状、自转和紧密双星轨道的演化。 我们证明SSDEM是一种建模坍缩过程的优秀方法，并能够从一个云中产生许多双星星子系统。 我们的结果确认了先前发布模型的发现，同时也关于星子自转和形状特性产生了新的结果。 新形成的星子平均具有10小时的自转周期，可以被描述为各种各样的形状（球形、扁球形、顶部形、扁平形、蛋形或长球形），其中质量最大的星子主要形成球形和扁球形。 显示英文摘要

摘要： In this work, we apply a soft-sphere discrete element method (SSDEM) within the PKDGRAV N-body integrator to investigate the formation of planetesimal systems through the gravitational collapse of clouds of super-particles. Previously published numerical models have demonstrated that the gravitational collapse of pebble clouds is an efficient pathway to produce binary planetesimal systems. However, such investigations were limited by their use of a perfect-merger and inflated-radii super-particle approach, which inhibits any analysis of planetesimal shapes and spin states, precludes the formation of the tightest binary orbits, and produces significantly under-dense planetesimals. The SSDEM enables super-particles to rest upon each other through mutual surface penetration and by simulating contact physics. Super-particles do not need to be inflated and collisions are not treated as perfect mergers; we can thus track the evolution of planetesimal shapes, spins, and tight binary orbits. We demonstrate that the SSDEM is an excellent method to model the collapse process, and is capable of producing many binary planetesimal systems from a single cloud. Our results confirm the findings of previously published perfect-merging models while also producing novel results about planetesimal spin and shape properties. Newly-formed planetesimals exhibit 10-hr rotation periods on average and can be characterized by a wide variety of shapes (spherical, oblate, top-shaped, flattened, egg-shaped, or prolate), with the most-massive planetesimals primarily forming as spheres and oblate-spheroids.