Accretion discs: from quasars to planets

Lead Research Organisation: University of Leicester
Department Name: Physics and Astronomy

Abstract

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.

Publications

10 25 50
publication icon
Alexander R (2011) Disc instability in RS Ophiuchi: a path to Type Ia supernovae RS Oph and Type Ia SNe in Monthly Notices of the Royal Astronomical Society

publication icon
Alexander R (2014) Planet formation in evolving protoplanetary discs in Proceedings of the International Astronomical Union

publication icon
Alexander R (2014) Protostars and Planets VI

publication icon
Alexander R (2015) Magnetospheres of hot Jupiters: hydrodynamic models and ultraviolet absorption in Monthly Notices of the Royal Astronomical Society

publication icon
Alexander R (2012) Deserts and pile-ups in the distribution of exoplanets due to photoevaporative disc clearing Exoplanet deserts and pile-ups in Monthly Notices of the Royal Astronomical Society: Letters

publication icon
Alexander R (2012) Galactic Centre star formation: the case of the missing gas disc The missing gas disc at the Galactic Centre in Monthly Notices of the Royal Astronomical Society

publication icon
Alexander R (2012) THE DISPERSAL OF PROTOPLANETARY DISKS AROUND BINARY STARS in The Astrophysical Journal

publication icon
Ardila D (2013) HOT GAS LINES IN T TAURI STARS in The Astrophysical Journal Supplement Series

publication icon
Dunhill A (2014) Misaligned accretion on to supermassive black hole binaries in Monthly Notices of the Royal Astronomical Society

 
Description We have gained increased understanding of how planets form around young stars, and how super-massive black holes grow.
Exploitation Route My findings are already widely used by other researchers seeking to understand planet formation and super-massive black holes.
Sectors Other

URL http://www.astro.le.ac.uk/~rda5/index.html
 
Description My findings have been published in a number of peer-reviewed international journals, and have been presented (both by me and others) at both national and international conferences.
First Year Of Impact 2009
Sector Other
Impact Types Societal

 
Description ERC Consolidator Grant
Amount € 1,945,721 (EUR)
Funding ID 681601 
Organisation European Research Council (ERC) 
Sector Public
Country Belgium
Start 06/2016 
End 05/2021
 
Description Philip Leverhulme Prize
Amount £70,000 (GBP)
Organisation The Leverhulme Trust 
Sector Charity/Non Profit
Country United Kingdom
Start 07/2014 
End 07/2017
 
Description Cafe Sci Talk 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Type Of Presentation Keynote/Invited Speaker
Geographic Reach Local
Primary Audience Public/other audiences
Results and Impact Public talk on "Hunting for Exoplanets" at Cafe Scientifique, Nottingham, February 2013.

-
Year(s) Of Engagement Activity 2013
 
Description Nottingham Public Lecture 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Type Of Presentation Keynote/Invited Speaker
Geographic Reach Regional
Primary Audience Public/other audiences
Results and Impact Public lecture at the University of Nottingham, entitled "Exoplanets: Hunting for Other Worlds".

-
Year(s) Of Engagement Activity 2013
 
Description Pile-ups PR 
Form Of Engagement Activity A press release, press conference or response to a media enquiry/interview
Part Of Official Scheme? No
Type Of Presentation Paper Presentation
Geographic Reach International
Primary Audience Media (as a channel to the public)
Results and Impact http://www2.le.ac.uk/offices/press/press-releases/2012/march/some-orbits-more-popular-than-others-in-solar-systems

The press release and our subsequent interviews received widespread coverage in the science media (New Scientist, space.com, Astronomy Now, etc.).
Year(s) Of Engagement Activity 2012
 
Description RPS talk 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Type Of Presentation Keynote/Invited Speaker
Geographic Reach Local
Primary Audience Public/other audiences
Results and Impact Public talk on planets & black holes, hosted by Rushcliffe Photographic Society (Nottinghamshire).

-
Year(s) Of Engagement Activity 2011
 
Description Sky at Night 
Form Of Engagement Activity A broadcast e.g. TV/radio/film/podcast (other than news/press)
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Public/other audiences
Results and Impact Appearance on BBC TV's "The Sky at Night", September 2015.
Year(s) Of Engagement Activity 2015
URL http://www.bbc.co.uk/programmes/b06cbmpr