Shrinking Binary and Planetary Orbits by Kozai Cycles with Tidal Friction
Abstract
At least two arguments suggest that the orbits of a large fraction of binary stars and extrasolar planets shrank by 1-2 orders of magnitude after formation: (1) the physical radius of a star shrinks by a large factor from birth to the main sequence, yet many main-sequence stars have companions orbiting only a few stellar radii away, and (2) in current theories of planet formation, the region within ~0.1 AU of a protostar is too hot and rarefied for a Jupiter-mass planet to form, yet many ``hot Jupiters'' are observed at such distances. We investigate orbital shrinkage by the combined effects of secular perturbations from a distant companion star (Kozai oscillations) and tidal friction. We integrate the relevant equations of motion to predict the distribution of orbital elements produced by this process. Binary stars with orbital periods of 0.1-10 days, with a median of ~2 days, are produced from binaries with much longer periods (10 to ~105 days), consistent with observations indicating that most or all short-period binaries have distant companions (tertiaries). We also make two new testable predictions: (1) For periods between 3 and 10 days, the distribution of the mutual inclination between the inner binary and the tertiary orbit should peak strongly near 40° and 140°. (2) Extrasolar planets whose host stars have a distant binary companion may also undergo this process, in which case the orbit of the resulting hot Jupiter will typically be misaligned with the equator of its host star.
This work is dedicated to the late Bohdan Paczyński, who inspired it.- Publication:
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The Astrophysical Journal
- Pub Date:
- November 2007
- DOI:
- 10.1086/521702
- arXiv:
- arXiv:0705.4285
- Bibcode:
- 2007ApJ...669.1298F
- Keywords:
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- Stars: Binaries: Close;
- Celestial Mechanics;
- Methods: Statistical;
- Stars: Planetary Systems;
- Astrophysics
- E-Print:
- Submitted to ApJ