Accretion discs: from quasars to planets

吸积盘：从类星体到行星

• 批准号: ST/G00711X/1
• 负责人: Richard Alexander
• 金额: $ 50.74万
• 依托单位: University of Leicester
• 依托单位国家: 英国
• 项目类别: Fellowship
• 财政年份: 2009
• 资助国家: 英国
• 起止时间: 2009 至 无数据
• 项目状态: 已结题
• 来源: https://gtr.ukri.org/projects?ref=ST%2FG00711X%2F1
• 关键词: Accretion; discs; quasars; planets

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.

圆盘出现在整个宇宙中，从巨大的螺旋星系到土星环。我们都见过滑冰运动员把手臂拉得更靠近身体时旋转得更快。发生这种情况的原因是因为角动量是守恒的，同样的过程，放大，导致整个宇宙形成圆盘。大多数天体都是通过引力坍缩形成的，当引力把物质向内拉时，它会越来越快地旋转，从而形成圆盘。在恒星和黑洞周围，这些圆盘充当着下降气体的管道，被称为“吸积盘”。黑洞的吸积是我们所知的最有效的能量产生方式（比核聚变效率高出许多倍），因此吸积盘是宇宙中一些最壮观现象的原因。吸积盘控制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，它们具有非常强大的引力，它们的形成似乎与它们所在的星系的形成密切相关。通过吸积盘将气体“喂入”遥远星系中的超大质量黑洞，被认为是类星体（宇宙中最明亮的物体）的动力引擎。在本地，我们现在知道一个超大质量黑洞也生活在我们的银河系中心。这个黑洞的质量大约是太阳的400万倍，但它目前还没有被喂养（谢天谢地！）。相反，它被一个由非常年轻的恒星组成的旋转圆盘所包围，现代望远镜以极其精确的精度绘制了这些恒星的运动。这些年轻恒星的存在最初是一个谜，因为通常孕育星星的稀薄气体云在离黑洞如此近的地方会被引力撕碎。我们现在相信，这些恒星是在较早的一次进食过程中形成的，当时黑洞的吸积盘在自身引力的作用下破裂。通过研究我们本地的超大质量黑洞，我们可以了解很多关于黑洞如何喂养和形成恒星的信息，这反过来又将增强我们对黑洞和星系如何在遥远宇宙中形成和生长的知识。行星在年轻恒星周围的吸积盘中形成。近年来，围绕其他恒星的行星变得越来越普遍，现在已知的“太阳系外”行星超过200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中，尘埃和岩石粘在一起，最终成长为行星。然而，构成行星基石的固体物质只占圆盘物质的一小部分;圆盘的绝大多数是气体。观察告诉我们，这些圆盘只能存活几百万年（从天体物理学的角度来看，这仅仅是一眨眼的功夫），因此了解它们的气体是如何被移除的对于了解行星如何形成至关重要。行星的形成和圆盘的演化过程是密不可分的，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，以研究恒星如何在超大质量黑洞附近形成，以及行星如何在年轻恒星周围的盘中形成。这两个问题都适用于类似的物理学原理，尽管它们的规模差异很大，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如何形成恒星，以及星星的形成如何反过来调节星系中心黑洞的生长。在较小的尺度上，我将模拟像我们的太阳这样的年轻恒星周围行星的形成，学习吸积盘如何形成猛烈的“热彗星”和遥远的地球。

项目成果

Galactic Centre star formation: the case of the missing gas disc The missing gas disc at the Galactic Centre

银河中心恒星形成：失踪气盘案例 银河中心失踪气盘

• DOI: 10.1111/j.1365-2966.2011.19849.x
• 发表时间: 2012
• 期刊: Monthly Notices of the Royal Astronomical Society
• 影响因子: 4.8
• 作者: Alexander R

Planet formation in evolving protoplanetary discs

演化中的原行星盘中的行星形成

• DOI: 10.48550/arxiv.1308.1791
• 发表时间: 2013
• 作者: Alexander R

Disc instability in RS Ophiuchi: a path to Type Ia supernovae RS Oph and Type Ia SNe

RS Ophiuchi 中的盘不稳定性：通向 Ia 型超新星 RS Oph 和 Ia SNe 的路径

• DOI: 10.1111/j.1365-2966.2011.19647.x
• 发表时间: 2011
• 期刊: Monthly Notices of the Royal Astronomical Society
• 影响因子: 4.8
• 作者: Alexander R

Disc instability in RS Ophiuchi: a path to Type Ia supernovae?

RS Ophiuchi 盘不稳定性：通往 Ia 型超新星的路径？

• DOI: 10.48550/arxiv.1108.3837
• 发表时间: 2011
• 作者: Alexander R

Deserts and pile-ups in the distribution of exoplanets due to photoevaporative disc clearing

• DOI: 10.1111/j.1745-3933.2012.01243.x
• 发表时间: 2012-02
• 作者: Richard Alexander;I. Pascucci