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G. Takeda, F. Rasio (Northwestern University)
One of the most remarkable features of extrasolar planets revealed by the on-going radial-velocity surveys is their high orbital eccentricities. Compared with the planets in our solar systems, these extrasolar planets show much higher orbital eccentricities, which cannot be explained easily by our standard planet formation theory. Some additional orbital dynamics is required. Observations have shown that some of the planets with highly eccentric orbits are in wide binary star systems. The presence of a binary companion can produce significant secular perturbations of the planetary orbit. In particular, at high relative inclinations, large-amplitude eccentricity oscillations can be produced. This so-called "Kozai mechanism" is a very long-range effect, and its amplitude is purely dependent on the relative orbital inclination. In this paper, we have investigated the following question: assuming that each extrasolar planetary system contains a (possibly unseen, e.g., substellar) companion star, with reasonable distributions of companion masses and orbital properties, how well can secular perturbations reproduce the observed eccentricity distribution of planets? Our calculations show that the Kozai mechanism consistently produces more planets with very high (e~q0.5 or higher) and very low (e~q0.1 or lower) eccentricities than observations. Although the global features of the observed eccentricity distribution cannot be fully explained by these secular perturbations, we can still easily explain the abundance of highly eccentric planets. The overproduction of high eccentricities can be combined with other dissipation mechanisms to create more intermediate eccentricities (e~q0.3-0.5). On the other hand, the observed scarcity of near-circular orbits cannot be reconciled easily with our models.
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Bulletin of the American Astronomical Society, 36 #4
© 2004. The American Astronomical Soceity.