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Accretion Disks
By: Jennifer Delgado
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SourceURL:http://www.ast.obs-mip.fr/users/donati/press/fuori_eng.html Magnetic field of FU Orionis
Artist view of a protostellar accretion disc. (©David Darling)
Outline
Definition
Formation
Radiation
Types
Binary Stars
Black Holes
Planetary
Summary
What is an accretion disk?
Accretion disk: Rotating disk of material spiraling
inward as it orbits a starlike object.
Starlike
object
Material
spiraling
inward
Why is it an accretion disk and not an accretion sphere?
Why a disk?
Conservation of Angular Momentum
L= mr2ω
A change in distance from the central mass will change the
rotational speed. A collapsing cloud will rotate faster.
Why a disk?
Gravity

GMm
FG  
r2
ω
θ
Pressure*

P  RxT

Centrifugal force

Fcf  m 2 r sin(  )
*Gas pressure is not a vector, but in this case it acts to oppose the acceleration due to
gravity, which is why I have an arrow indicating a direction for pressure.
Why a disk?
Gravity will act to collapse the material
towards the center of mass.
ω
θ
Pressure will oppose this according to
density, temperature and mean molecular
weight of the material.
Centrifugal force acts more along the
“equator” than along the axis of rotation.

Fcf  m 2 r sin(  )
Why a disk?
The result is that the material collapses
more on the axis than at the “equator”.
This flattens the material into a disk.
ω
θ
ω

2
Fcf  m r sin(  )
Why a disk?
What if there were absolutely no initial rotation?
If the roughly spherical
blob that we start with is
not spinning …
Then gravity will
collapse the blob into a
smaller sphere and not a
disk.
Initial rotation is necessary for disk rotation!
So it’s a disk, now what?
Well, what is the disk doing?
Viscosity
From Kepler: P2 ~a3
Assuming circular orbits, v ~ a/P
The orbital velocity is then v~a-1/2
Matter closer to the center of the disk is traveling faster
than matter further from the disk…
This causes viscous drag between the inner and outer parts
of the disk. The inner disk to loses angular momentum
and falls into the central mass. To conserve the total
angular momentum it is believed that the outer portions
of the disk speed up.
Energy from Accretion disks
Matter that accretes onto the central mass releases energy.
Ω
Denser objects release more
energy from accretion
Luminosity from the
accretion of matter also
depends on the
accretion rate
Black holes and Binaries and Planets, Oh My!
Binaries
White dwarf paired with less evolved star
overflowing it’s Roche lobe.
Material from the
companion does not
travel directly to the
star. Angular
momentum must be
conserved, the matter
must spiral in, like
water draining from a
bathtub.
Radiation from Accretion onto a White Dwarf
Assume hν ~ kT and then define an upper and lower limit for T

Tb  Lacc / 4R 
2

1/ 4
Tb  Trad  Tth Tth 
GMmp
3kR
For a white dwarf with R~ 5x108 cm, M~Msun,
and L ~ 6.25x1038 MeV/s
200 nm ≤ ν ≤ ~.1 Å
This ranges from the UV to X-Rays!
However the disk itself, because of a smaller L will emit at longer
wavelengths.
Black Holes
Black holes in
binary pairs may
should also
accrete matter
and form an
accretion disk.
SS443 a black hole in a binary
pair expels jets of material.
Active Galactic Nuclei
The energy produced from
AGN and quasars can be
explained by emission
from accretion disks
around super massive
black holes in the centers
of galaxies. The gas
accreting onto the black
hole would produce X-ray
emission
Mass needed for Quasar Luminosity
Bright Quasar
L~1040 J/s
Black holes radiate 10-40% of the energy gained
from the mass accreted.
E~ 0.1 mc2
m/s= 10 L/c2 ~ 1 x 1024 kg/s
This is 17 Solar masses per year of material accreted.
Why the Jets?
Jets are thought to be
charged particles
following the twisted
magnetic field lines
of the black hole.
It is also argued that these magnetic field lines could
transport angular momentum (and account for the lost
AM from accretion onto the central mass)
Planetary Accretion Disk
Stars and planets are thought to
form from clouds that
collapse into accretion disks.
Protoplanetary disks will emit
in the infared as dust in the
system reemits absorbed
stellar light.
Protoplanetary disk HH-30
Summary
●
●
●
Accretion disks are all over the place
They emit radiation, the energy of this radiation
can give us clues about the objects creating the
disk
Rotation is key
Works Cited
http://www.ast.obs-mip.fr/users/donati/press/fuori_eng.html Magnetic field of FU Orionis
http://phobos.physics.uiowa.edu/rlm/mathcad/addendum%204%20chap%2017%20stellar%20evolutio
n%201.htm
Wikipedia.com
www.mssl.ucl.ac.uk/www_astro/lecturenotes/hea/HEA_Accretion_2005-06.ppt
http://www.answers.com/topic/accretion-disc
http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/001106a.html
http://jilawww.colorado.edu/research/images/accretion.jpg
http://www.pas.rochester.edu/~afrank/A105/LectureXII/FG21_002_PCT.jpg
http://blackholes.stardate.org/directory/factsheet.php?id=33
http://science.nasa.gov/headlines/y2001/ast05sep_1.htm
The Cosmic Perspective. 4th Ed. Bennet, J., Donahue, M., Schneider, N., Voit, M. 2007
Radiation Temperature Bonus Slide
●
●
Optically thick gas will have a radiation
temperature T~ Tblackbody
Optically thin gas T~ Tth