Accretion Disks By: Jennifer Delgado Version:1.0 StartHTML: EndHTML: StartFragment: EndFragment: StartSelection: EndSelection: SourceURL: 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= mr 2 ω A change in distance from the central mass will change the rotational speed. A collapsing cloud will rotate faster.

Why a disk? Gravity Pressure* Centrifugal force *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. ω θ

Why a disk? Gravity will act to collapse the material towards the center of mass. 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. θ ω

Why a disk? θ The result is that the material collapses more on the axis than at the “equator”. This flattens the material into a disk. ω ω

Why a disk? 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! What if there were absolutely no initial rotation?

So it’s a disk, now what? Well, what is the disk doing?

Viscosity From Kepler: P 2 ~a 3 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 Ω Denser objects release more energy from accretion Luminosity from the accretion of matter also depends on the accretion rate Matter that accretes onto the central mass releases energy.

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 For a white dwarf with R~ 5x10 8 cm, M~M sun, and L ~ 6.25x10 38 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~10 40 J/s Black holes radiate 10-40% of the energy gained from the mass accreted. E~ 0.1 mc 2 m/s= 10 L/c 2 ~ 1 x 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 Magnetic field of FU Orionis n%201.htm Wikipedia.com The Cosmic Perspective. 4 th Ed. Bennet, J., Donahue, M., Schneider, N., Voit, M. 2007

Radiation Temperature Bonus Slide ● Optically thick gas will have a radiation temperature T~ T blackbody ● Optically thin gas T~ T th