1107.4594 (Jim Fuller et al.)
Jim Fuller, Dong Lai
Recent observation of the tidally-excited stellar oscillations in the
main-sequence binary KOI-54 by the KEPLER satellite provides a unique
opportunity for studying dynamical tides in eccentric binary systems. We
develop a general theory of tidal excitation of oscillation modes of rotating
binary stars, and apply our theory to tidally excited gravity modes (g-modes)
in KOI-54. The strongest observed oscillations, which occur at 90 and 91 times
the orbital frequency, are likely due to prograde m=2 modes (relative to the
stellar spin axis) locked in resonance with the orbit. The remaining flux
oscillations with frequencies that are integer multiples of the orbital
frequency are likely due to nearly resonant m=0 g-modes; such axisymmetric
modes generate larger flux variations compared to the m=2 modes, assuming that
the spin inclination angle of the star is comparable to the orbital inclination
angle. We examine the process of resonance mode locking under the combined
effects of dynamical tides on the stellar spin and orbit and the intrinsic
stellar spindown. We show that KOI-54 can naturally evolve into a state in
which at least one m=2 mode is locked in resonance with the orbital frequency.
Our analysis provides an explanation for the fact that only oscillations with
frequencies less than 90-100 times the orbital frequency are observed. We have
also found evidence from the published KEPLER result that three-mode nonlinear
coupling occurs in the KOI-54 system. We suggest that such nonlinear mode
coupling may explain the observed oscillations that are not harmonics of the
orbital frequency.
View original:
http://arxiv.org/abs/1107.4594
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