dynamics of astrophysical discs - damtp

Dynamics of Astrophysical Discs

Henrik Lattere-mail: [email protected]

16 lectures, 3 example classes

http://www.damtp.cam.ac.uk/user/hl278/DAD.html

Course Outline

• Introduction: Discs in the Universe

• Orbital dynamics

• Viscous accretion discs

• Vertical disc structure

• Local disc models (shearing sheet)

• Inertial waves, vortices, particles

• Density waves and gravitational instability

• Planet-disc interactions

• Magneto-rotational instability (MRI)

16 lecturesTu. Th. 10

Office: F1.19hl278@cam.

ac.uk

Discs in the Universe

Disc: (continuous) medium in orbit around a massive central body

• Usually circular, thin

• Usually Keplerian

• Celestial mechanics

• Fluid mechanics



• Usually circular, thin

• Usually Keplerian

• Celestial mechanics

• Fluid mechanics

• It accretes!

Angular momentum

Mass



Discs come in different sizes:

• Galactic discs: ~1020 m (~10000 ly)• AGN disc: ~1014 m (~1000 AU)• Protoplanetary disc: ~1013 m (~100 AU)• X-ray binary star: ~109 m (~R⊙)• Planetary ring: ~108 m (~R�)



Discs have different compositions:

• Galactic discs: stars, gas, dark matter • Protoplanetary disc: weakly ionised gas, solids

• X-ray binary star: dense H/ He plasma

• Planetary ring: metre-sized iceballs



Relevant descriptions:

•Gravitational collisionless dynamics (stars, dark matter, solids)•Gas dynamics (neutral, ideal gas)•Magnetohydrodynamics (ionised, ideal gas)•Kinetic theory (low density gas, particle gas)

+ relativity, radiation forces where needed



Observations: Saturn’s rings

Galileo (1610): “I have seen the most distant planet to have a triple

form”




Huygens (1656): “It is surrounded by a thin flat ring, nowhere touching, and

inclined to the ecliptic”




Cassini spacecraft (2004 - )Moons creating structure



Observations: protoplanetary discs

Jets: evidence for accretionGaps, possibly by satellites

PP disks: Gaps

(HL Tauri)

H Latter Discs in the Universe



Observations: binary star discs



Observations: Dwarf Novae

Luminosity variability (orbital time)

IY UMa



Observations: Dwarf Novae

Outbursts (days-weeks)

SS Aur



Observations: X-ray binaries

Complicated variability

Cyg X-1

1996 2008



Observations: AGN & galactic discs

Active galaxies: jets, accretion onto supermassive black holes



Observations: Galactic discs

Spiral density waves, gravitational instability

Messier 101


•Disc-like structures are ubiquitous•Are long-lived (many orbits)•Evidence for accretion•Observations of waves•Perturbations by satellites


•Disc-like structures are ubiquitous•Are long-lived (many orbits)•Evidence for accretion•Observations of waves•Perturbations by satellites

•Implicated in •star formation•planet formation •galactic structure•ICM structure (AGN feedback)



Formation:

•Consequence of angular momentum conservation•Disc is much smaller than original structure•Example: collapse of molecular cloud



Formation:

•Molecular cloud (1018 m) collapses to disc (1013 m)•Material speeds up•Has to become flatter



Formation:



Formation:

slowly rotating

rapidly rotating centrifugallysupported

Gravitationalforce

Centrifugal force



Formation:

•Orbital ang. mom. binary much larger than ang. mom. gas close to black hole



•Discs are all about angular momentum•Ubiquity of disks because of

•gravitational collapse (accretion) •Angular momentum conservation

•Subsequent accretion because of transport (how?)•Importance of instabilities and turbulence



• Discs are strange objects

• Balance between gravity and centrifugal force

• Circular velocity increases towards center



• Consider circular orbits only

• 2 spaceships on same orbit

• What do I have to do to overtake?



• Breaking = accelerating (!)

• All because of balance between gravity and centrifugal force

• Energy of acceleration is more than compensated for by climbing out of potential well of star



• Breaking = accelerating

• Bonus question: what if the spaceships are connected by a spring?

• Related to MHD turbulence in discs...



• What is the structure of astrophysical discs?

• How do they evolve?

• What is driving accretion?

• What happens to embedded objects? (particles, moons, planets, black holes)

Main questions:



• Beautiful Cassini images (www.ciclops.org)

• Formation of extrasolar planets

• Quasars: most distant galaxies

Discs are “hot”:

Course Outline

• Introduction: Discs in the Universe

• Orbital dynamics

• Viscous accretion discs

• Vertical disc structure

• Local disc models (shearing sheet)

• Inertial waves, vortices, particles

• Density waves and gravitational instability

• Planet-disc interactions

• Magneto-rotational instability (MRI)

16 lecturesTu. Th. 10

Office: F1.19hl278@cam.

ac.uk

dynamics of astrophysical discs - damtp

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