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Astronomy 340 Fall 2007

Astronomy 340 Fall 2007. 11 December 2007 Class #29. Announcements. Final Exam - Tues Dec 18 at 10:50am in 6515 HW#6 due Thursday in class You will not be penalized for not doing the HW (though I do recommend you try them!) – points will be award after dealing with the class curve

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Astronomy 340 Fall 2007

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  1. Astronomy 340Fall 2007 11 December 2007 Class #29

  2. Announcements • Final Exam - Tues Dec 18 at 10:50am in 6515 • HW#6 due Thursday in class • You will not be penalized for not doing the HW (though I do recommend you try them!) – points will be award after dealing with the class curve • Project due on Thurs Dec 13

  3. Second Half Review • Atmospheres of giant planets – know the basic compositions and we’ve figured that out • Interiors of the giant planets • Chapter 6.1 (eqn 6.27) • Figure from page 24 of Lecture 18 (Fig 6.23) • What are the J terms all about? • What is the underlying physics behind the derivation of the maximum size of a planet • Satellites of the outer planets • Figure 6.21

  4. Second Half Review – cont’d • Satellites of the outer planets • Chapters 5.5.5, 5.5.6, 5.5.7, 5.5.8, 5.5.9 • Know why Io, Europa, Titan, Encelaedus, Triton are interesting – what’s the role of tidal forces in all this? • Lecture 20 & 21!!! • Rings – what accounts for the variation in structure? • Chapter 11 (through 11.4) • Comets – equation 10.5 • Chapter 10.3, 10.6, 10.7

  5. Second Half Review – cont’d • Dwarf Planets and KBOs • You should be able to summarize the results of your project • How do you estimate the mass/size of a KBO? • Why is Pluto considered a dwarf planet? • What are the advantages of near-IR spectroscopy? • Recreate the HW question on why asteroids are brighter in the mid-IR than in the optical – do you think the same is true for KBOs? • Extrasolar Planets • Detection techniques (how do they work and what are the limitations?) • Chapter 13 • Star/Planet Formation • Chapter 12 • Eqn 12.7, 12.22 • What are the effects of planetary migration and why do people think it happens?

  6. Review • What are the primary techniques for detecting extrasolar planets and how do they work? • Given the radial velocity curve for a star would you be able to identify the period, mass, orbital eccentricity of the orbiting planet? • How do you detect atmospheres around exoplanets?

  7. Fraction of Stars with Planets Lineweather & Grether

  8. Star Formation

  9. Star Formation Feigelson & Montmerle

  10. Star Formation

  11. Key Observations of the Solar System • Coplanar/prograde orbits – angular momentum • Orbital spacing • Comets • 0.2% of mass in planets, 98% of the angular momentum • composition • Asteroid belt  power law size distribution • Age  4.5Gyr • Consistent isotopic ratios • Rapid heating/cooling • Cratering record  bombardment

  12. Key Physical Characteristics • Angular momentum  disk formation a must • Key properties of disk • Same abundance as the star • Spins in the same direction as the star • Temperature/density gradient (T(r) ~ r-0.5) • Other characteristics • Size: 25-500 AU observed • Total mass ~ 0.04 MEarth • R ~ 150 AU • Lifetime: 105-107 years

  13. 1st Phase - Condensation • Grains can survive in ISM conditions • Condensation • Nebula/disk cools  solids condense • “refractory” elements go 1st • Fe, silicates condense at 1400-1700 K • Meteoritic ages  condensation ~4.5 Gyr ago • Meteorites sample asteroid belt

  14. 2nd Phase – Collisional Accretion • Sticky collisions • Vi = (V2 + Ve2)1/2 = impact velocity • Ve = [2G(M1+M2)/(R1+R2)]1/2 • If Vi < Ve bodies remain bound  accretion • Growth rate • dM/dt = ρvπR2Fg or R2ΣΩFg/(2π) • Fg = cross-section = 1+(Ve/V)2 • dR/dt = (ρdv/ρp)(1+[8πGρpRp2]/3v2) • ρd = mass density in disk • ρp = mass density of planetesimals • V = average relative velocity • Rp= radius of planetesimals

  15. Collisional Accretion continued • If Ve >> V, then dR/dt goes as R2 big things grow rapidly • Can evaluate growth rate using R1=R2 (same assumption for V) • Formation of rocky/solid cores  next step is accretion • Raccretion = GMp/c2 (c = speed of sound)

  16. Collisional Accretion III • Predicted timescales • Accretion of dust  1 km sized bodies (104 yrs) • “runaway growth”  1 km to planetesimals (105 yrs) • Impacts finalize terrestrial planets (~108 yrs) • Lifetimes • Disk lifetimes: 105-107 yrs so process must be complete by then! • Earth timescales ~ 108 yrs • Much larger for Neptune

  17. Formation Scenarios • Core Accretion vs Gravitational Collapse • Q = κc/πGΣ gravity vs thermal pressure • Surface mass density • Local velocity (dispersion, sound speed) • Κ = R-3(d/dr(R4Ω2)) • Timescales ~ freefall time • One simulation with Md ~ 0.1 Earth masses, T~100K, Rd~20 AU  make J in 6Myr • Benefit • Can make planets on eccentric orbits • Timescales are short • Minuses • Hard to explain rocky cores

  18. Core Accretion Alibert, Mordasini, Benz 2004

  19. Formation Issues • Minimum Mass  r—1.5 • Core-Accretion • Timescales too long for Uranus, Neptune • What makes cm-size things stick? • How come things don’t spiral into Sun? • Gravitational Collapse • Faster, but is it plausible?

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