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Kozai Migration Yanqin Wu Mike Ramsahai

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The distribution of orbital periods P(T) increases from 120 to 2000 days Incomplete for longer periods Clear excess at 3-4 days

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Evidence for tidal evolution Maximum e declines with a: tidal circularization The two highest e planets are in binary star systems –(plot stolen from former CITA postdoc Phil Armitage)

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Migration: Disk-Planet Interactions A planet embedded in a gas disk excites spiral density waves via gravitational interactions; like the wake from a boat, these waves exert a torque on the planets In type I migration (low mass planets) the gas occupies orbits coincident with the planet In type II migration (Jupiter mass planets) the gas is pushed away from the planet, leaving a gap. This slows the migration rate to match the viscous evolution time scale of the disk

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Why Kozai Migration? The masses of close in planets tend to be smaller than those at larger a (disk migration- see plot) However, the pile up of planets at three days, where tidal effects are strong, is very suggestive. Similarly, the finding that the most massive planets in 3-4 day orbits are in binary systems is suggestive, and consistent with Kozai Finally, the fact that the highest e planets are in binary systems is evidence that Kozai is operating

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Why Kozai Migration? The pile up of planets at three days, where tidal effects are strong, is very suggestive. The masses of close in planets tend to be smaller than those at larger a (see plot) Similarly, the finding that the most massive planets in 3-4 day orbits are in binary systems is also suggestive The fact that the highest e planets are in binary systems is evidence that Kozai is operating, at least in those (relatively large a) systems

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Celestial Mechanics a semi major axis; e eccentricity; I inclination f ~ nt with n 2 =GM/a 3 ; longitude of periapse; longitude of node r = a(1-e 2 )/[1+e cos(f- )] E/m p = 1/2 v 2 - GM/r = GM/2a L/m p = [Gma(1 -e 2 )] 1/2 r 2 df/dt = L/m p r p = a(1-e)

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K K 1 is the line of nodes I, mutual inclination

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Celestial Mechanics a semi-major axis; e eccentricity; I inclination f ~ nt; longitude of periapse; longitude of node r = a(1-e 2 )/[1+e cos(f- )] E p /m p = 1/2 v 2 - GM/r = GM/2a L p /m p = [Gma(1 -e 2 )] 1/2 r 2 df/dt = L/m p r peri = a(1-e)

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How does Kozai work? The effect works on times much longer than the orbital period of either object, so imagine that the mass of both planet and secondary star is distributed in a ring around the primary. If the rings have a mutual inclination i, they will exert a torque on each other; T is perpendicular to L, so the orbits exchange angular momentum, but total L=const., as is the component of L p and L B along L. These are called Kozai constants.

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Kozai constants LpLp L total

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How does Kozai work? For low i, the apsidal line (from star to periapse) undergoes a prograde precession, while the nodal line (where the two orbit planes intersect) undergoes a retrograde precession with the same frequency As a result there are only small oscillations of i and e

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For high enough i, the apsidal line precession slows and eventually reverses, becoming prograde. A resonance occurs when the precession rate of the apsidal line equals that of the nodal line. As noted above, the mutual torque of the two rings is always along the nodal line, so it cannot affect the z component of L p ; the projection L pz of L p along L is fixed, but |L p | will oscillate, as angular momentum is drained out of and back into the orbit of the planet How does Kozai work? LpLp L total

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As L p = [a(1-e 2 )] 1/2, a decrease in i corresponds to a decrease in L p, which in turn corresponds to an increase in e. However, L pz is constant, so L p will not go to zero; when L p =L pz the angular moment will begin to flow from the outer orbit back into the planet’s orbit How does Kozai work? LpLp L total

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Tides The Kozai mechanism reduces e, but does not affect the semimajor axis a. However, the periapse r p =a(1-e) (the closest approach to the star) does shrink For I large enough, r p can approach the stellar radius When it does, the star raises a substantial tidal bulge on the planet; since the planet is no longer spherical, the mutual gravitational attraction of the planet and star is no longer given by just 1/r 2. The extra force induces a rapid precesion of the apsidal line, halting the Kozai-induced reduction of the periapse

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Tides While the periapse is held at a small value, tidal dissipation removes energy but not angular momentum from the orbit of the planet; hence a is reduced but r p is held fixed. This is effectively migration.

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Binary Star Model Ingredients The model assumes that the mutual inclination of proto-planetary disk and binary orbit are random (there is weak evidence for this) P(a B ) and P(q) are take from observations The frequency of planets in binary systems is assumed to be similar to that around single stars (some evidence for this from current surveys)

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Numerical Results

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Numerical Results (different Q)

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Predictions Kozai migration implies that many short period planets will be in binary star systems; the frequency of binarity will be higher for short period systems than for long period systems Kozai planets will inhabit dynamically empty systems Kozai migration leaves planets with a substantial inclination to the spin of the primary star (Rossiter-McLaughlin effect) Transiting Kozai planets will have secondary stars orbiting near the plane of the sky.

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Kozai In Single Star Systems The Kozai mechanism does not require a stellar companion to work: a second planet can also do the job The trick is to get a large mutual inclination Several groups have studied this

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Kozai Combined with Scattering

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Why should you believe a theorist? “An observational result should not be believed until it is confirmed by theory” A theoretical result should not be believed until it is confirmed by observation A numerical result should not be believed until it is confirmed by both

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Rossiter-McLaughlin Effect

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Fabrycky & Winn 2009

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HAT-P-7b retrograde orbit Winn et al

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Conclusions The Kozai mechanism must operate in binary star systems with single planets It will produce highly inclined (including retrograde) orbits It can also operate in multiplanet systems, possibly with mutual inclinations generated by planet-planet scattering. Such highly inclined orbits are now seen (4/14, 2 retrograde)

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