The role of massive protoplanetary discs in the formation of stars and planets

现在众所周知，恒星是在巨型分子云(GMC)内形成的，巨型分子云主要由冷密的分子氢和尘埃组成。尽管构成恒星的分子云核旋转速度非常慢，但它们包含的角动量仍然比任何一颗恒星都要大得多。现在已经很好地理解了，最终形成恒星的大多数质量--特别是低质量恒星--必须首先通过扁平的环绕恒星的圆盘。目前尚不清楚的是角动量是如何通过圆盘向外传输的，从而允许质量向内移动到恒星上。尽管有人提出，磁流体(MHD)湍流可能提供了一种传输角动量的机制，但这些圆盘似乎不太可能被充分电离，从而有效地运行。由于形成中央恒星的大部分质量必须首先通过圆盘，所以很可能--相当于中央恒星的质量--这些圆盘可能相对较大。这表明，这些圆盘可能容易受到引力不稳定性的影响，这将导致螺旋密度波的形成--这一过程类似于盘状星系中螺旋臂的形成。这些螺旋密度波起到向外传输角动量的作用，允许质量聚集在中央恒星上。这项工作的一个主要目标将是利用解析计算和数值模拟来调查年轻恒星周围质量盘的演化，目的是了解这种引力不稳定性是否可能是年轻恒星盘的主要传输机制。特别是，这些模型将包括详细的热力学和辐射传输，因为现在很明显，引力不稳定性的演变强烈依赖于圆盘中的加热和冷却过程。如果引力不稳定性很重要，那么在恒星形成的最早阶段很可能也是如此，在这段时间里，尘埃颗粒预计会长大，形成行星的基石--小行星。行星形成过程中仍未被理解的一个方面是厘米大小的粒子是如何迅速成长为千米大小的天体的。已经有人提出，密度波在行星形成中可能发挥作用，因为它产生了密集的粒子集合，这些粒子既可以通过碰撞快速增长，也可以通过引力崩塌直接增长。除了研究引力不稳定盘的演化，我们还将研究由此产生的螺旋密度波是否能在行星体的形成中发挥作用。这将是早期工作的扩展，因为我们将详细了解螺旋波可能存在于原恒星盘中的哪里，以及它们可能有多强。我们还将致力于量化螺旋密度波对厘米大小的颗粒数量的影响，以了解它们随后的生长是通过碰撞发生的，还是通过直接引力坍塌变得足够致密而生长的。

It is now well known that stars form inside Giant Molecular Clouds (GMCs) made up primarily of cold, dense molecular hydrogen and dust. Although the molecular cloud cores from which stars form rotate very slowly, they still contain far more angular momentum than any single star. It is now well understood that most of the mass that ultimately forms a star - in particular low-mass stars - must first pass through a flattened circumstellar disc. What is still unknown is how the angular momentum is transported outwards through the disc, allowing mass to move inwards onto the star. Although, it has been suggested the magnetohydrodynamic (MHD) turbulence may provide a mechanism for transporting angular momentum, it seems unlikely that these discs are sufficiently ionised for this to operate effectively. Since most of the mass that forms the central star must first pass through the disc, it is quite likely that - comared to the mass of the central star - these discs may be relatively massive. This suggests that these discs could be susceptible to the growth of a gravitational instability which will lead to the formation of spiral density waves - a process analagous to the formation of spiral arms in disc galaxies. These spiral density waves act to transport angular momentum outwards allowing mass to accrete onto the central star. A primary goal of the work here will be to investigate - using analytic calculations and numerical simulations - the evolution of massive discs around young stars with the aim of understanding if this gravitational instability can be the primary transport mechanism in young stellar discs. In particular, these models will include detailed thermodynamics and radiation transfer as it is now clear that the evolution of the gravitationally instability depends strongly on the heating and cooling processes in the disc. If the gravitational instability is important, it is likely to be so during the earliest stages of star formation, which is also the period during which dust grains are expected to grow to form planetesimals - the building blocks of planets. One aspect of the planet formation process that is still not understood is how cm-sized particles grow quickly into kilometre-sized bodies. It has already been suggested that spitral density waves may play a role in planet formation by producing dense collections of particles that can either grow rapidly through collisions or could grow directly through gravitational collapse. Together with studying the evolution of gravitationally unstable discs we will also investigate if the resulting spiral density waves can play a role in the formation of planetesimals. This will extend earlier work in that we will have a detailed understanding of where spiral waves are likely to exist in protostellar discs and how strong they are likely to be. We will also aim to quantify the influence of spiral density waves on the population of cm-sized grains to see if their subsequent growth occurs through collisions or if they can become sufficiently dense to grow through direct gravitational collapse.