Upload
others
View
18
Download
0
Embed Size (px)
Citation preview
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
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
Discs in the Universe
Discs in the Universe
Discs in the Universe
Discs in the Universe
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�)
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
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
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”
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”
Discs in the Universe
Disc: (continuous) medium in orbit around a massive central body
Observations: Saturn’s rings
Cassini spacecraft (2004 - )Moons creating structure
Discs in the Universe
Disc: (continuous) medium in orbit around a massive central body
Observations: protoplanetary discs
Jets: evidence for accretionGaps, possibly by satellites
PP disks: Gaps
(HL Tauri)
H Latter Discs in the Universe
Discs in the Universe
Disc: (continuous) medium in orbit around a massive central body
Observations: binary star discs
Discs in the Universe
Disc: (continuous) medium in orbit around a massive central body
Observations: Dwarf Novae
Luminosity variability (orbital time)
IY UMa
Discs in the Universe
Disc: (continuous) medium in orbit around a massive central body
Observations: Dwarf Novae
Outbursts (days-weeks)
SS Aur
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
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
Discs in the Universe
Disc: (continuous) medium in orbit around a massive central body
Observations: Galactic discs
Spiral density waves, gravitational instability
Messier 101
Discs in the Universe
•Disc-like structures are ubiquitous•Are long-lived (many orbits)•Evidence for accretion•Observations of waves•Perturbations by satellites
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)
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
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
Discs in the Universe
Disc: (continuous) medium in orbit around a massive central body
Formation:
Discs in the Universe
Disc: (continuous) medium in orbit around a massive central body
Formation:
slowly rotating
rapidly rotating centrifugallysupported
Gravitationalforce
Centrifugal force
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
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
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
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?
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
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...
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:
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”:
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