Accretion discs: from quasars to planets

从大质量的螺旋星系到土星环，盘状星云遍布整个宇宙。我们都见过滑冰运动员在将双臂拉近身体时旋转得更快。之所以会发生这种情况，是因为角动量是守恒的，而同样的过程，写得很大，会导致整个宇宙形成圆盘。大多数天体都是通过引力崩塌形成的，当重力将物质拉向内部时，它的旋转速度会越来越快，从而形成圆盘。在恒星和黑洞周围，这些圆盘起到了气体流入管道的作用，被称为“吸积盘”。吸积到黑洞上是我们所知的最有效的能量产生方式(效率比核聚变高出许多倍)，因此吸积盘导致了宇宙中一些最壮观的现象。吸积盘调节着超大质量黑洞的生长。我们现在相信，即使不是全部，也是大多数星系的中心都有超大质量黑洞。这些宇宙巨星比太阳重数百万倍，它们有巨大的引力，它们的形成似乎与它们所在星系的形成密切相关。气体通过吸积盘进入遥远星系中的超大质量黑洞，被认为是驱动类星体的引擎，类星体是宇宙中最亮的物体。在当地，我们现在知道一个超大质量的黑洞也生活在我们银河系的中心。这个黑洞的质量大约是太阳的400万倍，但它目前还没有被进食(谢天谢地！)相反，它的周围是一个由非常年轻的恒星组成的旋转圆盘，它们的运动被现代望远镜精确地绘制出来。这些年轻恒星的存在最初是一个谜，因为通常承载恒星诞生的稀薄气体云会被如此接近黑洞的重力撕裂。我们现在认为，这些恒星是在较早的进食阶段形成的，当时黑洞的吸积盘在自身引力下解体。通过研究我们本地的超大质量黑洞，我们可以了解到许多关于黑洞是如何喂养和形成恒星的知识，这反过来将增强我们对黑洞和星系如何在遥远的宇宙中形成和生长的知识。行星形成于年轻恒星周围的吸积盘中。近些年来，围绕其他恒星运行的行星已经变得很常见，目前已知的太阳系外行星有200多颗。年轻恒星周围的吸积盘是这些遥远太阳系的诞生地。在这些年轻太阳周围的圆盘内，尘埃和岩石粘在一起，最终成长为行星。然而，构成行星的固体材料只占圆盘材料的一小部分；圆盘的绝大多数是气态的。观测告诉我们，这些圆盘的寿命只有几百万年(从天体物理学的角度来看，仅仅是眨眼的时间)，所以了解它们的气体是如何被移除的，对于理解行星是如何形成的至关重要。行星形成和圆盘演化的过程是密不可分的，只有了解两者，我们才有希望全面了解行星是如何形成的，以及我们是如何存在的。我将对吸积盘进行大型计算机模拟，以研究恒星是如何在超大质量黑洞附近形成的，以及行星是如何在年轻恒星周围的盘中形成的。相似的物理学适用于这两个问题，尽管它们的规模大不相同，因此在一种情况下开发的技术可以很容易地应用于另一种情况。我的研究将向我们展示黑洞是如何形成恒星的，以及恒星形成是如何反过来调节星系中心黑洞的生长的。在更小的尺度上，我将模拟像我们的太阳这样年轻恒星周围行星的形成，了解吸积盘如何形成猛烈的“热木星”和遥远的地球。

Discs occur throughout the Universe, from massive spiral galaxies to Saturn's rings. We have all seen an ice-skater spin faster as she pulls her arms closer to her body. The reason this happens is because angular momentum is conserved, and that same process, writ large, causes discs to form throughout the Universe. Most astrophysical objects form via gravitational collapse, and as gravity pulls material inwards it rotates progressively faster and faster, resulting in discs. Around stars and black holes these discs act as a conduit for infalling gas, and are called 'accretion discs'. Accretion on to a black hole is the most efficient means of energy generation we know of (many times more efficient than nuclear fusion), and consequently accretion discs are responsible for some of the most spectacular phenomena in the Universe. Accretion discs regulate the growth of super-massive black holes. We now believe that most, if not all, galaxies have super-massive black holes at their centres. These cosmic giants are many millions of times heavier than the Sun, and they have an enormously strong gravitational pull, and their formation appears to be intimately linked to the formation of the galaxies in which they reside. The 'feeding' of gas, through accretion discs, into super-massive black holes in distant galaxies is thought to be the engine that powers quasars, the brightest objects in the Universe. Locally, we now know that a super-massive black hole also lives at the centre of our galaxy, the Milky Way. This black hole is approximately four million times more massive than the Sun, but it is not currently being fed (thankfully for us!). Instead, it is surrounded by a rotating disc of very young stars, whose motions modern telescopes map with exquisite precision. The existence of these young stars was initially a puzzle, because the tenuous gas clouds which usually host star birth would be shredded by gravity so close to the black hole. We now believe that these stars formed during an earlier feeding episode, when the black hole's accretion disc broke apart under its own gravity. By studying our local super-massive black hole we can learn a great deal about the how black holes feed and form stars, and this in turn will enhance our knowledge of how black holes and galaxies form and grow in the distant Universe. Planets form in accretion discs around young stars. In recent years it has become clear that planets around other stars are common, with over 200 'extra-solar' planets now known. Accretion discs around young stars were the birthplaces of these distant solar systems. Within the discs around these young suns, dust and rocks stick together and eventually grow into planets. However, the solid material that makes up the building blocks for planets represents only a tiny fraction of the disc material; the vast majority of the disc is gaseous. Observations tell us that these discs live only for a few million years (a mere blink of the eye, in astrophysical terms), so understanding how their gas is removed is crucial to understanding how planets form. The processes of planet formation and disc evolution are inextricably linked, and only by understanding both can we hope to gain a full understanding of how planets form, and of how we came to exist. I will conduct large computer simulations of accretion discs, to study how stars form near super-massive black holes and how planets form in discs around young stars. Similar physics applies to both of these problems, despite their widely differing scales, so techniques developed in one context can readily be applied in the other. My research will show us how black holes form stars, and how star formation in turn regulates the growth of black holes at the centres of galaxies. On smaller scales I will simulate the formation of planets around young stars like our Sun, learning how accretion discs form violent 'hot Jupiters' and distant Earths.