Planet Formation in Realistic Protostellar Disc Models

天文学中最近最重要的发现之一是围绕太阳以外的恒星运行的行星，这使科学家们提出了这样一个问题：这些行星系统和我们的太阳系是如何形成的？这个建议正是为了解决这个问题。关于行星系统是如何形成的，有许多相互竞争的观点，但它们有一个共同点，那就是相信行星是由在年轻恒星周围观察到的气体和尘埃盘形成的。为了理解这是如何发生的，我们首先需要了解行星形成的环境，而拟议研究的第一部分是使用并行超级计算机来模拟所谓的原行星盘的复杂模型。这些模拟将考虑到一些新的物理过程，这些过程迄今为止还没有包括在这类模型中：来自中央恒星的辐射加热，化学效应，以及来自其表面的辐射对盘的冷却。一旦新的圆盘模型被计算出来，我们将使用进一步的超级计算机模拟来研究科学家们关于行星形成方式的一些悬而未决的问题。要研究的第一个问题是，在行星生长的早期阶段形成的被称为星子的小天体的运动中，圆盘中的湍流对它们的影响。这里的情况类似于一架飞机在地球大气层的湍流中飞行，而星子也同样被推挤。如果这种碰撞变得过于剧烈，那么星子成长为行星所需的时间就会变得非常长，可能会使在所需的几百万年时间内形成一个完整的行星系统变得困难，而在这段时间内，人们观察到年轻恒星周围的圆盘存在。很明显，检验我们关于行星形成的理论是否能够在规定的时间内形成行星是非常重要的。下一个问题是，当低质量行星在这些圆盘中形成时，它们的轨道是如何演变的。这些行星的质量介于地球质量和地球质量的50倍之间，目前的理论告诉我们，这些行星在形成并与圆盘相互作用时，会以非常快的速度旋转进入中心恒星。这显然是一个大问题，我们将检查这种非常快速的向内迁移是否在新的圆盘模型中停止或减缓。我们要研究的最后一个问题是，当木星或土星这样的气态巨行星在原行星盘中形成时会发生什么。像这样的大质量行星能够通过引力将圆盘附近的气体推开，从而在行星附近形成一个缺口。我们希望了解这个间隙是如何形成的，以及行星附近圆盘的结构。一颗气态巨行星通过从圆盘中吸收气体而变得巨大，然后在行星周围形成一个包络层。我们将研究这种“气体吸积”是如何发生的，这样我们就可以理解为什么行星会变得像观测到的那样大。一颗巨大的行星也可能通过与周围的圆盘相互作用，缓慢地向中心恒星旋转。我们将研究这种向内迁移发生的速度有多快，这样我们就能理解围绕其他恒星运行的行星的观测结果。

One of the most important and recent discoveries in astronomy has been of planets orbiting stars other that the Sun, and this has led scientists to ask the question: how did these planetary systems and our own Solar System form ? This proposal is to address exactly this question. There are a number of competing ideas about how planetary systems come into being, but one thing they have in common is the belief that planets form out of the discs of gas and dust that are observed around young stars. To understand how this happens, we first need to know about the environment in which planet formation occurs, and the first part of the proposed research is to use parallel super computers to simulate sophisticated models of so-called protoplanetary discs. These simulations will take into account a number of new physical processes that have not been included in models of this type so far: heating by radiation from the central star, chemical effects, and the cooling of the disc by radiation from its surface. Once the new disc models have been computed, we will use further super computer simulations to study a number of outstanding questions that scientists have about the way in which planets form. The first question to be examined is the effect that turbulence in the disc has on the movement of small bodies called planetesimals that form in the early stages of planetary growth. The situation here is analogous to an aeroplane travelling through turbulence in the Earth's atmosphere, and the planetesimals are similarly jostled about. If this jostling becomes too severe, then the time taken for planetesimals to grow into planets becomes very long, and may make it difficult to form a completed planetary system in the required time of a few million years, which is the time for which the discs around young stars are observed to exist. It is obviously very important to check that our theory of how planet formation occurs is able to form planets in the required time. The next question we will look at is how the orbits of low mass planets evolve when they form in these discs. These planets will have a mass between that of the Earth and 50 times the mass of the Earth, and current theory tells us that these planets spiral into the central star very rapidly when they form and interact with the disc. This is obviously a big problem, and we will examine whether this very fast inward migration is halted or slowed down in the new disc models. The last question we will examine is what happens when a gas giant planet like Jupiter or Saturn forms in a protoplanetary disc. A massive planet like this is able to gravitationally push the nearby gas in the disc away, thus forming a gap in the planet's vicinity. We wish to understand how this gap formation happens, and the structure of the disc near the planet. A gas giant planet becomes massive by taking gas from the disc which then forms an envelope around the planet. We will examine how this 'gas accretion' happens so that we can understand why planets become as massive as they are observed to be. A giant planet is also expected to slowly spiral in toward the central star by interacting with the surrounding disc. We will examine how quickly this inward migration happens so that we can understand the observations of planets orbiting other stars.