Triggered planet formation in young stellar clusters

了解行星的起源是最古老的天体物理学研究领域之一，至今仍有一些主要问题没有得到解答。其中之一是天王星和海王星是如何在原行星盘存在的有限时间内形成的，从行星形成的物质库中。根据观测和理论数据，这样一个圆盘的寿命估计不到1000万年。另一方面，被广泛接受的行星形成模型--微行星的碰撞，然后周围物质的引力吸积--将需要超过1亿年，特别是对于最外层的巨行星海王星，对于像塞德娜或最近发现的2003 UB 313（媒体中通常称为Xena）这样的大型海王星外天体（TNO）甚至更长。另一个问题是关于已发现的太阳系外行星（系外行星）的轨道，与太阳系中接近圆形的轨道相比，这些行星具有很高的离心率和倾角。这里概述的项目解决了这两个基本问题。其中心思想假设非常年轻的太阳与太阳系原生星团中的一颗（很可能）低质量星星发生了近距离相遇。这样的遭遇可能会导致局部的引力不稳定性，碎片和崩溃，形成足够质量的原行星核心，以进行快速的吸积过程。其目的是通过对盘内的整体物理过程进行流体动力学计算，直到引力坍缩和行星核心形成以及随后的吸积过程，来研究这种情况。

Understanding the origin of the planets is one of the oldest astrophysical research fields, and there are still unanswered principal questions. One of them is how Uranus and Neptune formed within the limited available time during the existence of the protoplanetary disk, from the reservoir of material out of which the planets formed. According to observational and theoretical data the lifetime of such a disk has been estimated to be less than about ten million years. On the other hand, the widely accepted model of planet formation - collisions of planetesimals followed by gravitational accretion of surrounding material - would take more than 100 million years especially for the outermost giant planet Neptune and even longer for the large trans-Neptunian objects (TNOs) like Sedna or the recently discovered 2003 UB313 (often referred to as Xena in the media). Another question concerns the orbits of the discovered extrasolar planets (exoplanets), which have high eccentricities and inclinations in contrast to the nearly circular orbits in the Solar System. The project outlined here addresses both these fundamental questions. The central idea postulates a close encounter between the very young Sun and a (most likely) low-mass star within the native stellar cluster of our Solar System. Such an encounter could cause local gravitational instabilities that fragment and collapse, forming protoplanetary cores of sufficient mass for a quick following accretion process. The aim is to investigate this scenario by hydrodynamical computations of the overall physics within the disk down to the process of gravitational collapse and planet core formation, and subsequent accretion.