1108.4910 (Jim Fuller et al.)
Jim Fuller, Dong Lai
In compact white dwarf (WD) binary systems (with periods ranging from minutes
to hours), dynamical tides involving the excitation and dissipation of gravity
waves play a dominant role in determining the physical conditions of the WDs
prior to mass transfer or binary merger. We calculate the amplitude of the
tidally excited gravity waves as a function of the tidal forcing frequency
\omega=2(\Omega-\Omega_s) (where \Omega is the orbital frequency and \Omega_s
is the spin frequency) for several realistic carbon-oxygen WD models, assuming
that the waves are efficiently dissipated in the outer layer of the star by
nonlinear effects or radiative damping. The mechanism of wave excitation in WDs
is complex due to the sharp features associated with composition changes inside
the WD, and in our WD models gravity waves are launched just below the
helium-carbon boundary. We find that the tidal torque on the WD and the related
tidal energy transfer rate, \dot E_{\rm tide}, depend on \omega in an erratic
way. On average, \dot E_{\rm tide} scales approximately as \Omega^5\omega^5 for
a large range of tidal frequencies. We also study the effects of dynamical
tides on the long-term evolution of WD binaries. Above a critical orbital
frequency \Omega_c, corresponding to an orbital period of order one hour
(depending on WD models), dynamical tides efficiently drive \Omega_s toward
\Omega, although a small, almost constant degree of asynchronization
(\Omega-\Omega_s\sim {\rm constant}) is maintained even at the smallest binary
periods. While the orbital decay is always dominated by gravitational
radiation, the tidal energy transfer can induce significant phase error in the
low-frequency gravitational waveforms, detectable by the planned LISA project.
Tidal dissipation may also lead to significant heating of the WD envelope and
brightening of the system long before binary merger.
View original:
http://arxiv.org/abs/1108.4910
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