Dynamics of Astrophysical Discs

Transcription

1 Dynamics of Astrophysical Discs 16 lectures, 3 example classes Henrik Latter hl278@cam.ac.uk

2 16 lectures Tu. Th. 10 Course Outline Office: F1.19 ac.uk Introduction: 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)

3 Usually circular, thin Usually Keplerian Celestial mechanics Fluid mechanics

4 Angular momentum Mass Usually circular, thin Usually Keplerian Celestial mechanics Fluid mechanics It accretes!

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9 Discs come in different sizes: Galactic discs: AGN disc: Protoplanetary disc: X-ray binary star: Planetary ring: ~1020 m ~1014 m ~1013 m ~109 m ~108 m (~10000 ly) (~1000 AU) (~100 AU) (~R ) (~R )

10 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

11 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

12 Observations: Saturn s rings Galileo (1610): I have seen the most distant planet to have a triple form

13 Observations: Saturn s rings Huygens (1656): It is surrounded by a thin flat ring, nowhere touching, and inclined to the ecliptic

14 Observations: Saturn s rings Cassini spacecraft ( ) Moons creating structure

15 Observations: protoplanetary discs Jets: evidence for accretion Gaps, possibly by satellites

16 PP disks: Gaps (HL Tauri) H Latter

17 Observations: binary star discs

18 Observations: Dwarf Novae IY UMa Luminosity variability (orbital time)

19 Observations: Dwarf Novae SS Aur Outbursts (days-weeks)

20 Observations: X-ray binaries Cyg X Complicated variability

21 Observations: AGN & galactic discs Active galaxies: jets, accretion onto supermassive black holes

22 Observations: Galactic discs Messier 101 Spiral density waves, gravitational instability

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

24 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)

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

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

27 Formation:

28 Formation: Centrifugal force Gravitational force rapidly rotating centrifugally supported slowly rotating

29 Formation: Orbital ang. mom. binary much larger than ang. mom. gas close to black hole

30 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

31 Discs are strange objects Balance between gravity and centrifugal force Circular velocity increases towards center

32 Consider circular orbits only 2 spaceships on same orbit What do I have to do to overtake?

33 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

34 Breaking = accelerating Bonus question: what if the spaceships are connected by a spring? Related to MHD turbulence in discs...

35 Main questions: 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)

36 Discs are hot : Beautiful Cassini images ( Formation of extrasolar planets Quasars: most distant galaxies

37 16 lectures Tu. Th. 10 Course Outline Office: F1.19 ac.uk Introduction: 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)