Astronomy and Astrophysics – Astrophysics
Scientific paper
Aug 2007
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007iaus..240..334f&link_type=abstract
Binary Stars as Critical Tools and Tests in Contemporary Astrophysics, International Astronomical Union. Symposium no. 240, he
Astronomy and Astrophysics
Astrophysics
Scientific paper
Because of the large radii of pre-main-sequence stars, the current separation of close binaries was not likely established at their formation. The secular perturbation of a third star orbiting a binary at high inclination can cause its eccentricity to grow close to unity as angular momentum is gained by the third star. Close pericenter passages at high eccentricity cause dissipative tides, and the loss of orbital energy can result in a close binary. A population of triple stars is considered, of which some will undergo this evolution. The distribution of binary period and of relative inclination of the third star's orbit to the binary plane is derived using a Monte-Carlo method. The averaged (secular) equations of motion are integrated, including a model for tidal dissipation and extra precession due to the tidal and spin distortion of the stars and general relativity. The predicted period distribution matches reasonably to a set of spectroscopic binaries with known companions: there is an excess of such binaries with periods below about 5 days relative to the population of true binary systems. Reasonable statistics for the relative inclinations of such triple stars will be built up by optical interferometers in the near future, and these observations can check the predicted distribution. This mechanism is also applicable to extrasolar planets hosted by binary stars, causing them to migrate to short periods. For planets with periods less than 20 days, there is a paucity of massive (minimum mass>2 Jupiter mass) planets orbiting single stars, yet there are three such planets (Tau Boo b, HD 195019b, Gl 86b) in binaries. Therefore the single-star migration mechanism is apparently sensitive to planetary mass, whereas the binary-star migration mechanism is not; the latter matches the present study. Furthermore, HD 195019b and Gl 86b have circularized orbits, whereas single-star planets with comparable periods show a range of eccentricities; planetary radius inflation resulting from the epoch of high eccentricity may have led to this efficient circularization.
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