dynamics of astrophysical discs - damtp
TRANSCRIPT
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Dynamics of Astrophysical Discs
Henrik Lattere-mail: [email protected]
16 lectures, 3 example classes
http://www.damtp.cam.ac.uk/user/hl278/DAD.html
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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
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Discs in the Universe
Disc: (continuous) medium in orbit around a massive central body
• Usually circular, thin
• Usually Keplerian
• Celestial mechanics
• Fluid mechanics
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Discs in the Universe
Disc: (continuous) medium in orbit around a massive central body
• Usually circular, thin
• Usually Keplerian
• Celestial mechanics
• Fluid mechanics
• It accretes!
Angular momentum
Mass
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Discs in the Universe
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Discs in the Universe
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Discs in the Universe
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Discs in the Universe
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Discs in the Universe
Disc: (continuous) medium in orbit around a massive central body
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�)
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Discs in the Universe
Disc: (continuous) medium in orbit around a massive central body
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
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Discs in the Universe
Disc: (continuous) medium in orbit around a massive central body
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
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Discs in the Universe
Disc: (continuous) medium in orbit around a massive central body
Observations: Saturn’s rings
Galileo (1610): “I have seen the most distant planet to have a triple
form”
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Discs in the Universe
Disc: (continuous) medium in orbit around a massive central body
Observations: Saturn’s rings
Huygens (1656): “It is surrounded by a thin flat ring, nowhere touching, and
inclined to the ecliptic”
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Discs in the Universe
Disc: (continuous) medium in orbit around a massive central body
Observations: Saturn’s rings
Cassini spacecraft (2004 - )Moons creating structure
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Discs in the Universe
Disc: (continuous) medium in orbit around a massive central body
Observations: protoplanetary discs
Jets: evidence for accretionGaps, possibly by satellites
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PP disks: Gaps
(HL Tauri)
H Latter Discs in the Universe
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Discs in the Universe
Disc: (continuous) medium in orbit around a massive central body
Observations: binary star discs
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Discs in the Universe
Disc: (continuous) medium in orbit around a massive central body
Observations: Dwarf Novae
Luminosity variability (orbital time)
IY UMa
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Discs in the Universe
Disc: (continuous) medium in orbit around a massive central body
Observations: Dwarf Novae
Outbursts (days-weeks)
SS Aur
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Discs in the Universe
Disc: (continuous) medium in orbit around a massive central body
Observations: X-ray binaries
Complicated variability
Cyg X-1
1996 2008
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Discs in the Universe
Disc: (continuous) medium in orbit around a massive central body
Observations: AGN & galactic discs
Active galaxies: jets, accretion onto supermassive black holes
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Discs in the Universe
Disc: (continuous) medium in orbit around a massive central body
Observations: Galactic discs
Spiral density waves, gravitational instability
Messier 101
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Discs in the Universe
•Disc-like structures are ubiquitous•Are long-lived (many orbits)•Evidence for accretion•Observations of waves•Perturbations by satellites
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Discs in the Universe
•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)
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Discs in the Universe
Disc: (continuous) medium in orbit around a massive central body
Formation:
•Consequence of angular momentum conservation•Disc is much smaller than original structure•Example: collapse of molecular cloud
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Discs in the Universe
Disc: (continuous) medium in orbit around a massive central body
Formation:
•Molecular cloud (1018 m) collapses to disc (1013 m)•Material speeds up•Has to become flatter
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Discs in the Universe
Disc: (continuous) medium in orbit around a massive central body
Formation:
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Discs in the Universe
Disc: (continuous) medium in orbit around a massive central body
Formation:
slowly rotating
rapidly rotating centrifugallysupported
Gravitationalforce
Centrifugal force
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Discs in the Universe
Disc: (continuous) medium in orbit around a massive central body
Formation:
•Orbital ang. mom. binary much larger than ang. mom. gas close to black hole
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Discs in the Universe
Disc: (continuous) medium in orbit around a massive central body
•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
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Discs in the Universe
Disc: (continuous) medium in orbit around a massive central body
• Discs are strange objects
• Balance between gravity and centrifugal force
• Circular velocity increases towards center
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Discs in the Universe
Disc: (continuous) medium in orbit around a massive central body
• Consider circular orbits only
• 2 spaceships on same orbit
• What do I have to do to overtake?
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Discs in the Universe
Disc: (continuous) medium in orbit around a massive central body
• 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
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Discs in the Universe
Disc: (continuous) medium in orbit around a massive central body
• Breaking = accelerating
• Bonus question: what if the spaceships are connected by a spring?
• Related to MHD turbulence in discs...
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Discs in the Universe
Disc: (continuous) medium in orbit around a massive central body
• 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:
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Discs in the Universe
Disc: (continuous) medium in orbit around a massive central body
• Beautiful Cassini images (www.ciclops.org)
• Formation of extrasolar planets
• Quasars: most distant galaxies
Discs are “hot”:
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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